摘要:

~ Preparation and electrical conductivity of neodymium-europium oxide fluorides Masayuki Takashima, Susumu Yonezawa, Kiyoshi Horita, Kouichi Ohwaki and Hiroshi Takahashi Department of Material Science and Engineering, Faculty of Engineering, Fukui University, 3-9-1 Bunkyo, Fukui-shi 910, Japan Neodymium-europium oxide fluoride, Nd-EuO,F,, was prepared by heating appropriate mixtures of Nd203 and EuF, at temperatures ranging from 1273 to 1673 K for 2 h in argon. Two monophases, rhombohedral and tetragonal were identified. The rhombohedral phase was obtained at an EuF, composition of 50-55 mol%, and the tetragonal one was obtained in the composition range between 65 and 75 mol%. Nd2Eu2O3F6 gave a high electrical conductivity of 5 x lov2S cm-' at 923 K under an oxygen partial pressure of 0.4 Pa.The charge carrier was determined to be mainly the oxide ion. The transport numbers of the oxide ion and the electron were measured to be ca. 0.9 and (0.05, respectively, at temperatures between 723 and 923 K. Resulting from XPS measurements, it was found that: (1) the valence of Nd in Nd2Eu20,F6 was higher than in Nd203 and that of Eu was lower than in EuF,, i.e. the variance of the valency state of Nd (+ 3-+ 4) and Eu (+ 2-+ 3) affects the oxide ion conducting structure; (2) the covalency in the bonds between metal ions and oxide ions in Nd2Eu2O3F6 was weaker than that in Eu203 or Nd203; (3) the bond between the fluoride ion and the rare-earth-metal ion in Nd2Ln203F, was stronger than in the individual rare-earth-metal fluorides.These might result in increased oxide ion mobility. The complex oxides of zirconia or ceria have been widely investigated as oxide-ion-conducting solid electrolytes used for solid-state fuel cells and/or an oxygen sensors.1-6 These solid electrolytes must be heated to ca. 1273 K to reach the required conductivity, > 1 S cm-' for a fuel cell.7 In order to utilize the oxide-ion-conducting solid electrolyte for practical devices, it is desirable to lower the operating temperature to at least ca. 800 K. Many attempts to develop a new compound having a higher oxide-ion conductivity at even lower temperatures have been made.'-'' Neodymium-containing binary rare-earth-metal oxide fluorides (Nd&n,O,F,), which can be obtained as a solid solution of Nd203 and LnF,, have an oxide-ion conductivity more than ten times higher than that found for a stabilized zirconia, (Zr02)o~89(Y203)o~11, provided commer-cially as YSZ-1 1.I2-l6 Among these compounds, Nd2Eu2O3F6 was found to have the highest conductivity.In this paper we report the preparation and electrical conducting properties of NdzEu203F, and the results of our investigation of the valency state of each element in Nd2Eu203F6 in order to clarify the ionic conduction mechanism. Experimenta1 Nd203 and EuF, were commercially obtained from Shin-Etsu Chemical Ltd (purity 99.99%). Nd203 and EuF,, which were powdered (passed 400 mesh sieve), were mixed thoroughly together in appropriate molar ratios in a glove box filled with dry argon.The mixtures were compacted into an alumina boat made of 99.5% pure A120,, and fired for 2-3 h at temperatures ranging from 473 to 1673 K in argon. The products were analysed by X-ray diffractometry (XRD; Shimadzu XD-3As; Cu-Ka, 30 kV, 20 mA), X-ray photoelectron spectroscopy (XPS; Shimadzu ESCA-750; Mg target, 8 kV, 30 mA) and electrical conductivity measurements. The disk samples (diam- eter 20 mm, thickness 2 mm) for the electrical conductivity measurements were prepared by hot-pressing at 1473 K under 25 MPa cm-2 for 3 h in argon. Fig. 1 shows the cell assembly designed specially to measure the electrical conductivity and the transport number of a sintered disk sample. The impedance was measured by an ac method scanning from 10MHz to 20kHz at temperatures between 500 and 1000K under a partial pressure of oxygen ranging from 1 x to 2 x 104 Pa.The ionic and electronic transport numbers were examined by the emf method using an oxygen gas concentration cell17 and the dc polarization method," respectively. The charge-carrying species in NdzEu20,F, was identified by an electrolysis method using a cell as shown in Fig. 2. After electrolysis by a constant current of 1 mA for 12 h at 923 K in argon, the anode mixture (equimolar mixture of NiO and Ni) was analysed by means of X-ray diffractometry and a fluoride-ion-selective electrode method. Details of the electrical conductivity measurements have been reported previo~s1y.l~ Fig. 1 Cell assembly used for the electrical conductivity measurements.(1)Sintered sample; (2) Pt electrode; (3) Pt lead wire; (4)thermocouple (CA); (5) quartz mantle; (6) electrode holder; (7) quartz spacer; (8) guard electrode; (9) brass rod with spring to set electrode; (10) gas inlet/outlet; ( 11)furnace; ( 12) ground. Fig. 2 Cell used to examine the charge carrier via electrolysis. (1) Sintered sample; (2) Pt electrode; (3) compacted powder layer of Ni +NiO mixture; (4) quartz tubing; (5) thermocouple (CA). J. Muter. Chem., 1996, 6(5), 795-799 795 Results and Discussion Fig 3 shows the XRD profiles of powder samples, prepared from the Nd20,-EuF, system in a molar ratio of 1 2 by firing for 2 h at various temperatures In profile (2) of the product at 473 K, a small new diffraction line appeared around 28= 32" as shown in the inset This diffraction line became distinct as the reaction temperature was increased Profile (3) at 873 K could be separated into two patterns assigned to rhombohedral and tetragonal phases Compared with the XRD profiles of Nd and Eu oxide fluorides prepared separately, the rhombo- hedral product (lattice parameters a, =0 71 nm, a=34 2") was identified as NdOF and the tetragonal phase (ao=O 557 nm, c, =0 562 nm) corresponded to EU403F6 Diffraction profile (5) showed that the product at 1273K was a monophase which was identified as the tetragonal structure, as indicated by the Miller indices shown During the sample firing at 1273 K, the mass of the sample was found to decrease by <10 mass% Therefore, Nd,Eu,O,F, was produced stoichio-metrically from the mixture of 1mol Nd203 and 2 mol EuF, at 1273K Fig 4 shows the phases of the products from Nd20,-EuF, mixtures at various molar ratios The boundary lines shown are drawn on the basis of the intensity of the main peak of the XRD profile for Nd203, rhombohedral product, tetragonal product and EuF, The x axis shows the nominal compositions of EuF, The rhombohedral monophase was obtained at 50-55 mol% EuF,, and Nd,EuO,F, was formed from the equimolar mixture of Nd203 and EuF, The tetra- gonal monophase showed a wide homogeneous composition range from 65 to 75molYo EuF, The stoichiometric com- pound, Nd2Eu2O3F6, was obtained from a mixture of 1 mol Nd203 and 2 mol EuF, Fig 5 shows the relationship between the electrical conductivity and the nominal composition of EuF, in the products from the Nd203-EuF3 system The 1 I 1 I 1 20 30 40 50 60 2eldegrees (CU-KU) Fig.3 XRD profiles of the products prepared from the mixture of 1 mol Nd20, and 2 mol EuF, by finng at various temperatures for 2 h in argon Reaction temperature (1) starting mixture, (2) 473 K, (3) 873 K, (4) 1173 K, (5) 1273K 0,Tetragonal, 0,rhombohedral 796 J Muter Chem , 1996, 6(5), 795-799 1 v) a,.!= v)c 0, r Y8L1 + 0 4-2 02 04 06 08 EuF, (mol%) Fig.4 Phases in the products from the Nd,O,-EuF, system (a) Rhombohedra1 phase, (b) tetragonal phase TetragonalP 0.5 0.6 0.7 0.8 LnF3(mol%) Fig.5 Relationship between the electrical conductivity and the nom- inal composition of LnF, in products prepared from Nd,O,-LnF, (Ln=Y, Eu) systems Electncal conductivities measured at 923 K under an oxygen partial pressure of 1 33 x 10 Pa (1) Nd203-YF3 system," (2) Nd,O,-EuF, system electrical conductivity of the tetragonal phase is ca 1OOx higher than that of the rhombohedral phase The charge carrier in Nd2Eu20,F6 was investigated by an electrolysis method If the fluoride ion is mobile within N~,Eu,~~F~,the anode mixture (Ni+NiO) should be con- verted by the electrolysis into fluorine-containing compounds such as nickel fluoride and/or nickel oxide fluorides However, no diffraction peaks due to fluorine-containing nickel com- pounds could be found in the X-ray diffraction profile of the anode mixture after electrolysis at 923 K The Ni peaks disap- peared and only those due to Ni0 remained There were no changes in the diffraction profiles of the anode mixtures after heat treatment at 923 K Fluoride ion could not be detected using a fluoride-ion selective electrode in the solution of the anode mixture after electrolysis at 923 K and dissolving in aqueous HC1 solution These facts indicate that the oxide ion is the charge carrier in Nd2Eu203F6 Fig 7(a) and (b) show the Arrhenius plots of the electrical conductivities measured under an oxygen partial pressure of 0 4 Pa and the oxide ion transport numbers for Nd2Eu2o3F6, Nd,Y,O,F, and YSZ-11 Nd2Eu20,F, and Nd2Y2O3F6 showed higher conductivities than YSZ-11 The conductivity at 873 K of Nd,Eu203F6 was ca 3 5 x lop2S cm-I which corresponds to the value for YSZ-11 at 1273 K The activation energy of Nd,Eu,O,F, was calculated to be 70 kJ mol-l, which is lower than those of Nd2Yzo3F6 and YSZ-11 (100 kJ mol-') This seems to reflect I NiO(200) 1058 K\ NiO( 1 1 1)L(1) 40 50 2eldegrees (Cu-Ka) Fig.6 XRD profiles of the Ni +NiO anode mixture. ( 1)After electroly- sis at 1 mA for 12 h at 923 K; (2) after heat treatment for 12 h at 923 K without electrolysis. TIK 973 873 773 723 1.1 1.2 1.3 1.4 l@ WT 0.9l'*/ g! 0.8 3 Uc& 0.7 5 A o-lLdJ0.0 673 773 873 973 TJK Fig. 7 (a) Arrhenius plots of the electrical conductivity of Nd2Eu203F6, Nd,Y,O,F, and (Zr02)o.89(Y203)o.11 under an oxygen partial pressure of 0.4Pa.(1) Nd,Eu,O,F,; (2) Nd,Y,O,F,; (3) YSZ-11. (b) Oxide ion transport numbers for Nd,Eu,O,F, (4) and Nd,Y,O3F, (5); electron transport number for Nd,Eu,O,F, (6). that Nd2Eu203F6 may have a crystal structure in which there is an easy path for the rapid migration of oxide ions. The oxide ion transport number for Nd2Y2O3F6 decreased with decreasing temperature [Fig. 7( b), curve (5)].I9 The oxide ion transport number of NdzEuz03F6 [Fig. 7( b), curve (4)] remains at ca. 0.9 between 723 and 923 K. Moreover, the electron transport number of this compound, measured by means of the dc polarization method, is <0.05, as shown in Fig. 7(b), curve (6).The TG curves of Nd2Eu20,F6 (Fig. 8), show that this compound is completely stable up to ca. 1000 K I I I I I I I I I 473 673 873 1073 1273 TIK Fig. 8 TG curves of Nd2Eu2O3F, in (1) air and (2) argon at 10°Cmin-' 0 Nd 0 Eu F @o t, Fig. 9 The double layered fluorite structure of Nd,Eu,O,F,. uo= 0.5641, co= 1.1227 nm. in argon and/or air. This result indicates that Nd,Eu2O3F6 is applicable as the solid electrolyte in a fuel cell and/or an oxygen sensor, which can be operated at most at 900K, a temperature which seems sufficiently low for practical uses. In order to obtain good ionic conduction, a special arrange- ment of the cations and anions is required, in order to produce an easy path for the rapid migration of ionic species.In oxide- ion-conducting ceramics such as stabilized zirconia, the oxide ion conduction has been explained by a mechanism based on the oxide ion defect ~tructure.~As a result of the crystal structure analyses by computer simulation,j- which shows a structure in which the oxide ion is mobile, we proposed a special model for the crystal structure, the 'double-layered fluorite structure', as shown in Fig. 9.20 It is supposed that the variable valences of Nd (+ 3-+4) and Eu (+2-+3) result in an anionic defect structure in which the migration of only oxide ions is possible. Fig. 10 shows the X-ray photoelectron t Details of the crystal structure analysis of Nd2Eu2O3F6 by Rietveld refinement are available as supplementary data (SUP No.57130)from the British Library. For details of the Supplementary Publications Scheme, see Information for Authors, Issue 1 of J. Muter. Chem. J. Muter. Chem., 1996, 6(5),795-799 797 990 9854 980 150 14&178 0 984 2 139 2 binding energy/eV Fig. 10 XP spectra for (a) Nd 3d,,, and (b) Eu 4d,,, electrons of NdzEu,03F, ( 1 ), Nd,O, (2)and EuF, (3) spectra (XPS) of the Nd 3d5,2 and Eu 4d5,2 electrons in Nd203, EuF, and Nd2Eu203F6 and the binding energy of the Nd 3d,,, electron in Nd2Eu20,F6 is higher than that in Nd203 In contrast, the binding energy of the Eu 4d5/2 electron (138 0 eV) showed a downward shift of 12 eV with respect to that in EuF, These facts indicate that the valence of Nd in Nd2Eu2O3F6 is higher than that in the pure oxide, and that of Eu is lower than that in its fluoride Fig 11 shows XP spectra of the 0 1s electrons in Nd2Eu203F6, Eu203 and Nd203 Each spectrum can be separated into two peaks as shown by the dashed lines The peak at higher binding energy is assumed to be due to a rare-earth-metal hydroxide formed on the sample surface We have confirmed that the rare-earth-metal oxide is easily hydrolysed on its surface by exposure to an ordinary atmosphere The peak at lower binding energy corre- sponds to the 0 1s electron involved in metal-oxygen bonding The peak for Nd2Eu203F6 is at a slightly lower binding energy than those of Eu203 and Nd203, ie the oxide ion in Nd2Eu20,F6 tends to be more negative than in the separate oxides This shows that the covalency in the bond between the 529.7 (1) 531 9A 535 530 525 535 530 525 binding energylev Fig.11 XP spectra for the 0 1s electrons of Nd,Eu,O,F, (l), Eu,03 (2) and Nd203 (3) 798 J Muter Chem, 1996, 6(5),795-799 6877 68157 binding energy/eV Fig. 12 XP spectra for the F 1s electrons of PTFE (l), EuF, (2) Nd,Eu,03F6 (3) and KF (4) P 4.0E 3.0 E;2.0 2 5 I I 0-(3) -686685 7 cV Fig. 13 Binding energy shifts of the F 1s electron from the standard (686 eV) for all compounds of Nd,Ln,03F, (Ln=Y-Lu) (1) PTFE, (2) BaF2, (3) KF, (4)LnF3, (5)Nd2Ln203Ffj metal ion and oxide ion in Nd2Eu203F6 is weaker than that in Eu20, or Nd203 Fig 12 shows the F 1s spectra of Nd2Eu2O3F6, PTFE (as a compound with covalent bonds), EuF, and KF (of which the chemical bond is almost completely ionic) The binding energy of the F 1s electron in Nd,Eu203F6 is lower than that in EuF, The binding energy shifts of the F 1s electron from the standard energy of 689eV for all Nd2Ln2O3F6compounds are summarized in Fig 13 All energy shifts for Nd2Ln2O3F6 compounds are smaller than those in the rare-earth-metal fluorides This indicates that the bond between the fluoride ion and the rare-earth-metal ion in Nd2Ln2O3F6is stronger than in the rare-earth-metal fluorides, ie the ionic interaction of the oxide ion with the rare-earth- metal ion becomes weaker, and the mobility of the oxide ion increases This work was partially supported by a Grant in Aid for Scientific Research on Priority Areas (grant no 07239218) from the Ministry of Education, Science and Culture of Japan References 1 T H Etsell and S N Flengas, Chem Rev, 1970,70,339 2 A Kvist, in Physics ofElectrolytes, ed J Hladik, Academic Press, London, 1972, vol 1, p 319 3 K S Goto, Solid State Electrochemistry and Its Applications to Sensors and Electronic Devices, Elsevier, Amsterdam, 1988 4 5 A.S. Nowick and D. S. Park, in Superionic Conductors, ed. G. D. Mahan and W. L. Roth, Plenum Press, London, 1976,p. 395. H. L. Tuller and A. S. Nowick, J. Electrochem.SOC.,1975,122,255. 14 15 16 M. Takashima and G. Kano, J.Fluorine Chem., 1988,40,375. M. Takashima and G. Kano, Solid State Zonics, 1987,23,99. M. Takashima, G. Kano and H. Konishi, Nippon Kagaku Kaishi, 6 H.L. Tuller and A. S. Nowick, J. Electrochem. SOC., 1979,126,209. 1986,1892. 7 8 9 T. Takahashi, in Physics of Electrolytes, ed. J. Hladik, Academic Press, London, 1972, vol. 2, p. 989. J. Schoonman, K. E. D. Wapenaar, G. Oversluizen and G. J. Dirksen, J. Electrochem. SOC.,1979,126,709. P. Laborde, J. M. Reau, S. F. Matar and P. Hagenmuller, Mater. Res. Bull., 1985,20, 1501. 17 18 19 20 F. Beniere, in Physics ofElectrolytes, ed. J. Hladik, Academic Press, London, 1972, vol. 1, p. 299. C. Wagner, Adv. Electrochem. Eng., 1966,4,1. M. Takashima, G. Kano, T. Fukui and T. Ogura, Nippon Kagaku Kaishi, 1984,1083. M. Takashima, K. Ohwaki, H. Takahshi and S. Yonezawa, Solid 10 D. Avignant, I. Mansouri, J. C. Cousseins, J. Alizon, J. P. Battut, J. Dupuis and H. Robert, Mater. Res. Bull., 1982,17, 1103. State Commun., submitted. 11 12 J. G. Pepin and G. J. Mccarthy, J. Am. Ceram. SOC.,1981,64,551. M. Takashima, G. Kano, N. Tabota and K. Higashimoto, Denki Paper 6100135A; Received 5th January, 1996 13 Kagaku, 1987,55706. M. Takashima, G. Kano and M. Kawase, Denki Kagaku, 1985, 53, 119. J. Mater. Chem., 1996, 6(5),795-799 799

ISSN:0959-9428    

DOI:10.1039/JM9960600795

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Structure refinement, magnetic susceptibility, electrical conductivity and europium-151 Mossbauer spectroscopy of EuNiIn, Rainer Pottgen,"" Ralf Mullmann,b Bernd D. Moselb and Hellmut Eckertb "Max-Plunck-lnstitut fur Festkorperforschung, Heisenbergstrasse 1, 0-70569 Stuttgart, Germany blnstitutfur Physikalische Chemie der Wesgalischen Wilhelms-Universitat, Schlossplatz 417, 481 49 Munster, Germany The title compound was prepared by a reaction of the elemental components at 970 K in a tantalum tube. EuNiIn, adopts the orthorhombic YNiA1,-type structure. It was refined from single-crystal X-ray data: space group Cmcm, a =447.3 l(4) pm, b= 1695.88(15) pm, c= 722.32(6) pm, V=0.5479( 1)nm3, Z=4, wR2=0.0461,637 F2 values and 24 variable parameters. Magnetic susceptibility measurements show Curie-Weiss behaviour above 50 K.At 16( 1)K, a phase transition to the antiferromagnetic state is observed in the temperature dependence of the inverse magnetic susceptibility. The experimental magnetic moment pexp= 7.86(5) pB/Eu is close to that of the free Eu2+ ion of peff=7.94 pB.EuNiIn, is a good metallic conductor with a specific resistivity of 14 p Rcm at room temperature. '"Eu Mossbauer measurements can be fit by a single Eu site with an isomer shift 6 = -10.9 mm s-l against EuF, which is typical for divalent Eu. At TN=32(1)K magnetic order begins, which can be detected by Mossbauer spectroscopy when the fluctuation rate is smaller than the inverse half-life of the excited state of the '"Eu nuclide, which occurs in parts of the present sample only at temperatures significantly lower than TN.The YNiA1,-type structure' is formed by many intermetallics, e.g. LnNiAl, (Ln=Y, Ce, Pr, Nd, Gd-Tm, Lu),lP5 LnNiGa, (Ln=Y, Nd, Sm, Gd-Tm, Yb, Lu)~ and LnNiIn, (Ln =La-Nd, Eu, Yb).7 With copper as the transition-metal component, EuCuIn, and CaCuIn, have been reported.* Although a large number of such compounds has been synthesized, little is known about the physical properties of these intermetallics. CeNiAl, was reported to be a non-magnetic dense Kondo compo~nd.~~'~CeNiIn, is Curie-Weiss paramagnetic without magnetic order above 2 K." X-Ray absorption LIE* spectra suggest mixed valence (Eu"/Eu"') in EuCuIn, .12 We have now investigated the crystal structure of EuNiIn, by X-ray diffractometry on single crystals.In addition, we report on the magnetic susceptibility, electrical conductivity and '"Eu Mossbauer measurements of this indide. Experimental Starting materials for the preparation of EuNiIn, were ingots of europium (Johnson Matthey, >99.9%), nickel wire (Johnson Matthey, diameter 0.38 mm, >99.9%) and indium tear drops (Johnson Matthey, >99.9 Yo).The large europium ingots were mechanically cut into small pieces in a glove-box. They were not allowed to contact air prior to the reactions. The elemental components were mixed in the ideal atomic ratio and sealed in a tantalum tube under an argon pressure of cu. 800mbar. The argon was previously purified over molecular sieves, titanium sponge (900 K) and an oxisorb cata1y~t.I~The tantalum tube was subsequently sealed in a silica tube to prevent oxidation, and heated at 1270 K for three days followed by annealing for two more weeks at 970 K.The reaction resulted in a compact button which could be separated readily from the tantalum tube. For an ICP-AES (inductively coupled plasma atomic emis- sion ~pectrometry)'~ investigation, small pieces of the sample were dissolved in aqua regia and analysed in an ARL 3580 spectrometer. Guinier powder patterns of all samples were recorded with Cu-Ka, radiation using 5 N (Aldrich, purity >99.999 %) silicon [a =543.07( 1) pm] as an internal standard. The indexing of the diffraction lines was facilitated by intensity calculations'5 using the positional parameters of the refined structure.The lattice parameters (Table 1) were obtained by least-squares refinements of the Guinier powder data. Single crystals were first examined by Buerger precession photographs to establish both symmetry and suitability for intensity data collection. Intensity data were collected on a four-circle diffractometer (CAD4) with graphite monochrom- ated Ag-Ka radiation and a scintillation counter with pulse- height discrimination. The magnetic suceptibilities of polycrystalline pieces of EuNiIn, were determined with a SQUID magnetometer (Quantum Design, Inc.) between 4.2 and 300 K with magnetic flux densities up to 2 T. The specific resistivities were measured on a small block (1 x 1 x 1.6 mm3) with a conventional four-probe technique as described previously." The cooling and heating curves meas- ured between 4.2 and 300 K were essentially identical, also for different samples.For the Mossbauer measurements the 21.53 keV transition Table 1 Lattice parameters of the orthorhombic indides with YNiA1, structure (standard deviations in parentheses) compound alpm blpm clpm LaNiIn, CeNiIn, CeNiIn, PrNiIn, NdNiIn, EuNiIn, EuNiIn, YbNiIn, 448.4(4) 446.0( 2) 445.9( 1) 443.0( 2) 447.3 1 (4) 446.3(2) 439.8( 1) 444.3( 1) 1688.5( 4) 1678.8( 5) 1676.7( 2) 1668.1 (4) 1665.6( 6) 1695.88( 15) 1694.2( 5) 1660.6(5) 719.9( 2) 722.0( 2) 719.3( 1) 719.3(2) 717.8( 2) 722.32( 6) 720.2( 2) 727.8(2) v/nm3 ref. 0.5451 7 0.5406 7 0.5378(2) 11 0.5331 7 0.5296 7 0.5479( 1) this work 0.5447 7 0.5315 7 J.Muter. Chew., 1996, 6(5), 801-805 801 of '"Eu with an activity of 130 MBq (2% of the total activity of a 15'Srn EuF, source) was used The measurements were performed with a commercial bath cryostat The temperature of the absorber could be varied from 42 to 300 K and was measured with a metallic resistance thermometer with an accuracy better than +O 5 K The source was kept at room temperature The material for the Mossbauer absorber was taken from the same batch as those for the susceptibility and resistivity measurements The sample was poured into a PVC sample holder with a thickness of 10 mg Eu cmP2 Results and Discussion Powders and single crystals of EuNiIn, are light grey and stable in air over long periods of time No decomposition was observable after several months The single crystals exhibit a metallic lustre and have an irregular platelet-like shape The elemental analysis of the EuNiIn, sample with the ICP-AES technique gave the following atomic percentages Eu Ni In, 165(4) 167(4) 668(8), which are close to those calculated for the ideal composition (Eu Ni In, 166 166 66 6) No impurity elements were observed Lattice parameters The present powder data as well as the lattice parameters determined on the four-circle diffractometer [25 reflections, Ag-Kcr, 42" <28 <49", u =445 4( 1) pm, b =1693 6(7) pm, c = 719 2(2) pm, V=O 5425( 1) nm3] are in agreement with the data reported by Kalychak et d7(Table 1) The cell volumes of these isotypic compounds decrease more or less linearly from the lanthanum to the neodymium compound as expected from the lanthanoid contraction, however, the cell volume of EuNiIn, is similar to that of LaNiIn, (see Table 1) This clearly indicates the tendency of the europium atoms for the divalent state, in agreement with the magnetic measurements and Mossbauer data discussed below Structure refinement and crystal chemistry Single crystals of EuNiIn, were isolated from the crushed sample prepared in the tantalum tube Buerger precession photographs showed orthorhombic mmm Laue symmetry and the extinctions were compatible with space group Cmcm (no 63), in agreement with the previous investigations on YbN11n,~ and CeNiIn, l1 Some crystallographic data and experimental details for the data collection are listed in Table 2 The atomic positions of CeNiIn," were taken as starting values and the structure was sucessfully refined using SHELXL-93,I6 with anisotropic atomic displacement param- eters for all atoms A final difference Fourier synthesis revealed no significant residual peaks The results are summarized in Table 2 Atomic coordinates and interatomic distances are listed in Tables 3 and 4 Listings of the anisotropic displacement parameters and the observed and calculated structure factors are available The present structure refinement confirms the YNiA1,-type structure for EuNiIn, The refined atomic parameters agree very well with the ones recently obtained for isotypic CeNiIn, l1 The structure of EuNiIn, belongs to a large structural family which derives from the binary Re,B-type struct~re,'~ as shown in Fig 1 Re,B has two rhenium sites, crystallographically different, which may be occupied by two different atoms This is realized in the MgCuAl,-type'* structure of CaNiIn, l9 In the latter compound (see Fig I), the [Re6] trigonal prisms are formed by calcium and indium resulting in the prism composi- t Details may be obtained from Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, by quoting the registry number CSD-404251 802 J Muter Chem, 1996, 6(5), 801-805 Table 2 Crystal data and structure refinement for EuNiIn, empirical formula EuNiIn, formula mass/g mol 670 0 TIK 293(2) 4Pm 56 086 crystal system orthorhombic space group Cmcm (no 63) (unit-cell dimensions see Table 1) formula units per cell 2=4 calculated densitylg cm 8 121 crystal size/pm3 25 x 75 x 125 absorption correction from psi-scan data transmission ratio (max , min ) 1, 0569 absorption coefficient/mm 16 39 F(000) 1148 0 range for data collection/degrees 2-26 range in hkl O<h<6, -26<k<26, -11<1,<11 scan mode 016 total no reflections 2304 independent reflections 637 (R,,, =0 0483) reflections with I >241) 583 (R,=O0306) refinement method full-matnx least-squares on F2 data/restraints/parameters 63 710124 goodness-of-fit on F2 1175 final R indices [I >2a(Z)] R1= 0 0199, wR2 =0 0446 R indices (all data) R1 =O 0239, wR2=00461 extinction coefficient 0 001 1 ( 1 ) largest diff peak and hole/e nm 2543 and -2145 Table 3 Atomic coordinates and isotropic displacement parameters (pm') for EuNiIn,, Ue, is defined as one third of the trace of the orthogonalized U,, tensor atom Cmcm x Y Z ue, Eu 4c 0 012103(2) 1/4 102( 1) Ni 4c 0 077486(5) 114 104(2) W1) 8f 0 031145(2) 004827(5) 99(1) In(2) 4c 0 092527(3) 133( 1) 0IN31 4h 0 112 'I4 147(1) Table 4 Interatomic distances (pm), calculated with the powder lattice parameters, in the structure of EuNiIn, [all distances shorter than 545 pm (Eu Eu, Eu Ni, Eu In), 475 pm (Ni-In) and 430 pm (Ni-Ni, In-In) are listed, standard deviations are all 60 1 pm] 4 331 0 In(2) 1 255 1 1 332 0 4 314 1 2 343 6 4 329 4 4 353 2 1 332 0 2 354 3 2 369 6 2 369 6 2 402 0 In(3) 4 314 1 2 447 3 2 321 7 4 353 2 1 255 1 2 361 2 2 260 5 4 274 0 2 343 6 2 402 0 1 260 5 2 274 1 1 291 4 2 313 6 1 321 7 2 329 4 2 331 0 1 354 3 tion [Ca21n,] The prisms are centred on the nickel atoms In both structures the trigonal prisms form one-dimensional infinite columns, which extend along x If the compounds become richer in indium, up to the composition 1 1 4,ze EuNiIn, in the present study, the same [NiEu,In,] trigonal 45 r 35 Y CaNilnz EuNiIn, Fig.1. Projection of the crystal structures of Re,B, CaNiIn, and EuNiIn, (all space group Cmcm) onto the yz planes.20All atoms lie on mirror planes at x=O and x=+, indicated by thin and thick lines, respectively.The [BRe,], [NiCaJn,] and [NiEuJn,] trigonal prisms and the corrugated indium networks in EuNiIn, are outlined. prisms are formed. The excess indium atoms form corrugated indium networks which separate the prism columns, shown in the lower part of Fig. 1. This is also the case for the aluminides YNiA1, and YNiAl,.' The substitution of the rhenium atoms in the Re,B structure by rare-earth-metal and indium atoms has a large influence on the prism-centring atoms. While the boron atoms in Re,B are more or less in the prism centre, the nickel atoms in the [Ca,In,] and [EuJn,] prisms have moved towards the In, square.This difference is certainly correlated to the difference in size between the rare-earth-metal and indium atoms. The nickel and indium atoms in EuNiIn, form a three-dimensional [NiIn,] polyanionic network in which the europium atoms are embedded. A detailed discussion of the individual atomic coordinations and a view of the polyanion are given in ref. 11 for isotypic CeNiIn,. Magnetic and electrical properties The temperature dependence of the inverse magnetic suscepti- bility of EuNiIn, is shown in Fig. 2. EuNiIn, shows Curie- Weiss behaviour above 50 K. The magnetic moment obtained from the linear portion of the 1/x us. T plot above 50K of peXp=7.86(5) ~B/Eu compares very well with the theoretical effective moment peE= 7.94 pB for free Eu2+ calculated from peff=g[J(J+ 1)]li2pB.The extrapolation of the linear relation above 50 K to 1/x=O resulted in a Weiss constant of 0= -8 ( 1) K, indicating antiferromagnetic ordering. Below cu. 30 K, the temperature dependence of the inverse susceptibility shows a positive deviation from Curie-Weiss behaviour, as already observed for isotypic CeNiIn,." This deviation may be attributed to first antiferromagnetic interactions. At 16( 1 ) K a pronounced upturn is observed in the 1/x us. T plot, indicating a phase transition to the antiferromagnetic state. In 5 0 50 100 150 200 250 300 TIK Fig. 2. Temperature dependence of the inverse magnetic susceptibility of EuNiIn, measured at a magnetic flux density of 2 T order to analyse this behaviour in more detail, we also investi- gated the low-temperature part of the susceptibility in external magnetic fields of 1, 3 and 5 T (see Fig.3). These measurements show a slight decrease of the ordering temperature with increasing external field strength as expected for such an antiferromagnet. The magnetization us. external field strength dependence for EuNiIn, is shown in Fig. 4. The magnetization curve taken at 25 K is linear and shows no hysteresis. At the highest obtainable field strength of 5.5 T, the magnetic moment amounts to 1.55(5) pB/Eu. The magnetization curve at 5 K is 0.22 (a)-0.21 ---0.20 -. 00.19 -0 0 -0 0-0.18 -5 10 IS 20 25 30 0.19 - Y - !-0.18z - 3 - Ez 0.17 - Ex: - 0.16 - 0 0 - 0 0.15 1 I I I 1 1 5 10 15 20 25 30 t 0 0 0 0 0.15 00.16 1 0 5 10 15 20 25 30 TIK Fig.3. Low-temperature magnetic susceptibility of EuNiIn, measured at external magnetic flux densities of B=(u)l, (b)3 and (c)5T J. Muter. Chern., 1996, 6(5),801-805 803 'r 0 1 2 3 4 5 Be, Fig. 4. MagnetizaEuNiIn, at 25 K tion us exter nal m agnetic flux density, B,,,, for 14 10 5 5 Q6 2 0 50 100 150 200 260 300 TIK Fig. 5. Temperature dependence of the specific resistivity of EuNiIn, rather similar, yielding a slightly larger moment of 163(5) PB/EUThe positive deviation from the Curie-Weiss line can be explained with the structural peculiarities of EuNiIn, From Fig 1 it can be seen easily that the [Eu,In,] prism rows are well separated by the indium networks Thus, from the mag- netic point of view, the europium atoms form linear chains which extend along the x axis with Eu-Eu intrachain distances of 447 3 pm The shortest Eu-Eu interchain distances (546 8 pm) are much longer For comparison, the Eu-Eu distances in the Eu-Eu zigzag chains of ferromagnetic EuAuGe21 of 373 8 pm are much shorter It is therefore possible that the first antiferromagnetic coupling occurs at 32 K within the linear chains, while complete three-dimensional antiferromagnetic '"Eu Mossbauer spectroscopy Mossbauer spectra between 4 2 K and room temperature are shown in Fig 6 together with theoretical fits The fitting parameters and the results of some additional measurements are presented in Table 5 As can be seen in Fig 6, at T=28 2 K and higher temperatures a small amount of an Eu"' impurity (ca 1% in area) occurs at 0 9 mm s-' which is included as a simple Lorentzian component in all fits but is not reported in Table 5 Above the magnetic ordering temperature, TN=32 K, the spectra can be fit well by a single Eu" site subject to quadrupole interaction with an axially symmetrical electric field gradient Below TNthis part of the spectrum diminishes with decreasing temperature and is gradually replaced by a spectrum revealing magnetic hyperfine interactions with a local field of increasing intensity It was possible to arrive at excellent fits while constraining the isomer shifts 6, and 6, of both spectral components to be equal at any given temperature We therefore conclude that the signals of Eu" with 6= -107mm s-l at 4 2 K and 6 = -10 9 mm s ' at 300 K arise from a single crystallographic site, which is in accordance with the X-ray results Comparison of the quadruple splitting above and below the Neel temperature reveals that Be, lies perpendicular to the principal axis of the electrical field gradient, leading to the theoretically expected change in the quadrupole splitting by a factor of -1/2 below the Neel temperature From the non-vanishing 1/x of the magnetic measurements at low tem- perature (see Fig 2) it can be deduced that the magnetic field is an tiferromagne tic The temperature dependence of the spectra below TNcan be explained by magnetically interacting single domain volumes V, which are reorienting their magnetization with different frequencies owing to their different sizes This phenomenon has been analysed in the literature 23 24 The jump frequencies are a function of V/Tdepending on the distribution of volumes, which need not be identical to the particle sizes, and on temperature At temperatures close to TNthe magnetization of the large volumes reorients slowly on the Mossbauer timescale, I 93 lh -100 0 0psv) 94 4 100 0F"!!Yordering is present below 16 K The ordering procedure is described in more detail below, together with the results obtained from '"Eu Mossbauer measurements The specific resistivity of EuNiIn, decreases with decreasing temperature (Fig 5) as is typical for metallic conductors The room-temperature value of the specific resistivity amounts to 14 pi2 cm Thus, EuNiIn, is a good metallic conductor when compared to the specific resistivities of 6 84 and 8 37 pR cm for nickel and indium at room temperature, respectively 22 The specific resistivity at room temperature determined for CeNiIn, of 33 pR cm'l is comparable, however, polycrystalline CeNiA1, has a larger room-temperature value of ca 140 pR cm At low temperatures, the p us Tcurve has a small bend (see Fig 5) 4 0 -20 0 0 $20 0 t40 0The decrease of the resistivity is slightly steeper below the Neel v/mm s-l temperature This is attributed to a decrease of the spin- disorder resistivity resulting from the antiferromagnetic order Fig.6. '"Eu Mossbauer spectra of EuNiIn, relative to EuF, 804 J Mater Chem , 1996, 6(5),801-805 Table 5 Fitting parameters for EuNiIn," ~ ~ 300 -10.92(5) +8.6(8) 2.2(2) 78.0 -10.86(3) +8.7(4) 2.1(1) 35.0 -10.69(9) +9(2) 2.2(3) 33.0 -10.71(8) +9(1) 2.3(3) 32.0 -10.62(4) +9 2.3 -10.62(4) -3.8 2.3 6(5)28.2 -10.93(5) +9 2.3 -10.93(5) -3.8 2.3 10.5(4) 23.4 -10.57(9) +9 2.3 -10.57(9) -3.8 2.3 17.6(2) 18.5 -10.73(7) +9 2.3 -10.73(7) -3.8(9) 2.3 21.4(2) 13.8 ----10.76( 6) -3.8(9) 2.3 23.2(2) 9.1 ----10.76( 7) -3.8(9) 2.3(1) 24.8(2) 4.2 ----10.71(6) -3.8 2.3(1) 26.4(2) " Isomer shifts 6 are given with respect to EuF,.Numbers in parentheses represent the statistical errors of the last digits. Parameters without parentheses were kept fixed during the fitting process. and therefore the Eu" sites situated within them give rise to a static magnetic hyperfine splitting pattern. In contrast, the Eu" sites within the (rapidly reorienting) small volumes will still show the dynamically averaged spectrum observed for all sites above TN. When the fraction of Eu" sites in volumes with 5 6 7 R. M. Rykhal', 0. S. Zarechnyuk and T. I. Yanson, Dopov. Akad. Nauk Ukr. RSR, Ser. A, 1979,41,1057 V. A. Romaka, Yu. N. Grin and Ya. P. Yarmolyuk, Ukr. Fiz. Zh., 1983,28,1095. Ya. M. Kalychak, V. M. Baranyak, V. I. Zaremba, P. Yu. Zavalii, 0.V. Dmytrakh and V. A. Bruskov, Sov. Phys. Crystallogr., 1988, intermediate sizes, i.e.with frequencies that lie in the Mossbauer time window is insignificantly small, the spectra are well fit by a superposition of the two limiting cases. Since the frequencies depend on the V/Tratio, the fraction of the magnetically split spectral component changes from zero near TN to unity at 8 9 10 33, 602. L. V. Sysa and Ya. M. Kalychak, Crystallogr. Rep., 1993,38,278. T. Mizushima, Y. Isikawa, A. Maeda, K. Oyabe, K. Mori, K. Sat0 and K. Kamigaki, J. Phys. SOC. Jpn., 1991,60,753. T. Mizushima, Y. Isikawa, K. Oyabe, K. Mori and J. Sakurai, Physica B, 1993,186-188,457. sufficiently low temperature (seen at 16 K in the magnetic measurement ). 11 12 R. Pottgen, J. Muter. Chem., 1995,5, 769. L. V. Sysa, Ya. M. Kalychak, I.N. Stets' and Ya. V. Galadzhun, Crystallogr. Rep., 1994,39,743. 13 H. L. Krauss and H. Stach, Z. Anorg. Allg. Chem., 1969,366,34. We thank Prof. Dr. A. Simon for his interest and steady support of this work. We are also grateful to Dr. R. K. Kremer for helpful discussions, to W. Rothenbach for taking the 14 15 G. L. Moore, Inductively Coupled Plasma-Atomic Emission Spectrometry, Elsevier, Amsterdam, 1989. K. Yvon, W. Jeitschko and E. Parthe, J. Appl. Crystallogr., 1977, 10,73. Guinier powder patterns, to E. Brucher for the susceptibility measurement, to N. Weishaupt for the electrical conductivity measurement and to 0. Buresch for the ICP-AES analysis. The Stiftung Stipendienfonds des Verbandes der Chemischen Industrie supported our research with a Liebig grant to R.P. 16 17 18 G. M. Sheldrick, SHELXL-93, Program for Crystal Structure Refinement, University of Gottingen, Germany, 1993. B. Aronsson, M. Backman and S. Rundqvist, Acta Chem. Scund., 1960,14,1001. H. Perlitz and A. Westgren, Ark. Kemi, Mineral. Geol., 1943, 16B, 1. 19 V. I. Zaremba, 0. Ya. Zakharko, Ya. M. Kalychak and 0. I. Bodak, Dopov. Akad. Nauk Ukr. RSR, Ser. B., 1987,44. References 20 E. Keller, SCHAKAL 92, Kristallographisches Institut, Universitat Freiburg, 1993. R. M. Rykhal', 0.S. Zarechnyuk and Ya. P. Yarmolyuk, Sou. Phys. Crystallogr., 1972, 17,453. 21 22 R. Pottgen, J. Muter. Chem., 1995,5,505. Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Boca Raton, FL, 59th edn., 1978. 0.S. Zarechnyuk, T. I. Yanson and R. M. Rykhal', Izv. Akad. Nauk SSSR Met., 1983,4, 192. 23 D. B. Rancourt, S. R. Julian and J. M. Daniels, J. Magn. Magn. Muter., 1985,51, 83. R. M. Rykhal', 0.S. Zarechnyuk and A. V. Pyshchik, Dopov. Akad. 24 U. Pegelow, M. Winterer, B. D. Mosel, M. Schmalz and Nauk Ukr. RSR, Ser. A, 1973,35,568. R. Schollhorn, Z. Naturforsch., Teil A, 1994,49, 1200. R. M. Rykhal', 0. S. Zarechnyuk and Ya. I. Kuten, Dopov. Akad. Nauk Ukr. RSR, Ser. A, 1978,40,1136. Paper 5/06424D; Received 28th September, 1995 J. Mater. Chem., 1996, 6(5),801-805 805

ISSN:0959-9428    

DOI:10.1039/JM9960600801

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Magnetic properties of ternary chromium sulfides, V,Cr, -,S4 (0 <x <1.0) Anthony V. Powell* and Sascha Oestreicht Department of Chemistry, Heriot- Watt University, Riccarton, Edinburgh, UK EH14 4AS A series of phases V,Cr3 -,S4 (0.0d x d 1.0) have been prepared by high-temperature synthesis and characterised by thermogravimetry, energy dispersive X-ray microanalysis and powder X-ray diffraction in conjunction with Rietveld profile analysis. Data are consi:tent with the formation of single-phase products which all adopt the Cr3S4 structure (space group 12/m: a M 5.9, b =3.4, c M 11.2 A, 91.4"). Magnetic susceptibility data indicate that spin-glass behaviour persists in the range 0.4d x <1.0, whilst data for x =0.2 suggest that there is a slow freezing of spins prior to the onset of long-range magnetic ordering observed for x =0.0.A large number of binary transition-metal chalcogenides of stoichiometry M3X4 adopt the Cr3S4 structure (Fig. 1). This structure may be considered to be derived from that of CdI,. In the latter structure, cations (M) occupy octahedral sites between alternate pairs of layers in a hexagonally close-packed array of anions (X); the resulting stacking sequence may be represented schematically as MXXMXX. In the Cr3S4 struc- ture, additional cations are accommodated in half of the vacant octahedral sites in the X.-.X van der Waals gap in an ordered fashion resulting in an XMXM,.,XMX stacking sequence. The structure of Cr3S4 may therefore be viewed as an ordered defect structure intermediate between those of CdI, and NiAs.The presence of two crystallographically distinct cation sites suggests that a more appropriate formulation is (M)[M,]X,, where parentheses and square brackets denote sites in the vacancy layer and the fully occupied layer, respectively. When a second cation is present, two extreme cation arrange- ments corresponding to the normal (M')[ M,] X4 and inverse structures (M)[M'M]X4 have been ident5ed.l In addition, a number of non-stoichiometric phases have been and found to exhibit interesting variations in their physical properties as a function of composition. The precise structure adopted by non-stoichiometric phases will depend on the competing site-preferences of the constituent cations and in a small number of cases, studies of cation distributions have been carried out5-' We have recently commenced an investi- gation of structural and magnetic properties of such non-stoichiometric phases, and here present magnetic and structural data for the system V,Cr3 dxS4 (0.0dx <1.0).The end-member phases exhibit contrasting properties.VCr2S4 has been shown to be a semiconductor,* consistent with the presence of localised moments and formal oxidation states of V" and Cr"'. These properties are in agreement with predictions based on one- electron energy-level diagrams.* By contrast, Cr3S4 is an itiner- ant electron antiferromagnet. On the basis of powder neutron diffraction data,g the NCel temperature TNwas estimated to be 280 K.Subsequently, other workers have suggested" that the presence of impurity phases might result in an overestimate of the Nee1 temperature. More recent susceptibility and specific heat measurements" indicate TNto be in the range 200-220 K. The series V,Cr3 -,S4 therefore provides an opportunity to investigate the competition between itinerant and localised electron behaviour with changing chemical composition. Tazuke', has suggested, on the basis of magnetic susceptibility measurements, that phases in this system show a comparatively rare13314double transition from a paramagnetic to an antiferro- t Present Address: Max-Planck Institute for Colloid and Interface Science, Kanstr. 55, P-14513Teltow-Seehof, Germany. n Fig. 1 Ball-and-stick representation of the structure of Cr,S,.Large filled circles represent chromium ions in the fully occupied layer, small open circles represent chromium ions in the vacancy layer and large open circles represent sulfide ions. magnetic to a spin-glass state and has described these materials as antiferromagnetic re-entrant spin-glasses. However, doubts remain as to the phase purity of the samples used in these experiments: for example, large uncertainties in TNwere attri- buted to imperfections in the sample. We have endeavoured to produce well characterised single-phase samples of these materials and here present magnetic data which show differ- ences to those previously presented. In particular we find no evidence for the double transition.Experimental Appropriate quantities of vanadium, chromium and sulfur powders (all Matthey Catalogue Sales) were ground in an agate mortar. All reaction mixtures were prepared with a slight deficiency of sulfur, corresponding to compositions of AB2S3.93, as it has been shown15 that the phase range of the Cr3S4 structure does not extend to the fully stoichiometric composi- tion. Reaction mixtures were sealed into evacuated, silica ampoules and fired at a temperature of 850°C for 4 days with one intermediate regrinding; a further regrinding was followed by a final firing in a silica ampoule at 950°C for 3 days. Samples were cooled to 300°C prior to removal from the J. Muter. Chem., 1996, 6(5),807-813 807 furnace Reaction progress was momtored by powder X-ray Quantum Design MPMS2 SQUID susceptometer Samples diffraction using a Philips PA2000 diffractometer with nickel- were loaded into gelatin capsules at room temperature and filtered Cu-Ka radiation Powder X-ray diffraction data for data were collected over the temperature range 6-296 K both subsequent Rietveld refinement were collected in step-scan after cooling the sample in zero applied field (zfc) and after mode using a step sue of 28=0 02" and a counting time of 5 s cooling in the measunng field (fc) Measuring fields of 0 1, 1 step-Energy dispersive X-ray microanalysis to determine and 10 kG were used All magnetic data were corrected for V Cr ratios was performed using a JEOL 200FX electron diamagnetism of the gelatin capsule and for intrinsic core microscope fitted with a Tracor Northern analysis system diamagnetism VCr04 was used as an intensity standard Sulfur contents were determined thermogravimetncally by oxidation in a flow of Resultsdry oxygen on a Stanton Redcroft TG-750 thermobalance The mass loss on conversion to the corresponding oxides was The results of the analysis of chemical composition are shown related directly to the sulfur content Magnetic susceptibility in Table 1 Data are in good agreement with the stoichiometry measurements on powdered samples were made using a of the initial reaction mixtures, consistent with the formation Table 1 Results of thermogravimetry and energy dispersive X-ray microanalysis for V,Cr3 -,S4 (0 0 <x <1 0) nominal nominal expenmentally expenmentally expenmentally determined composition V Cr determined V Cr determined S (V +Cr) composition cr3s3 93 --1 31( 1) Cr3S3 94 vO zCr2 $3 93 0 07 0 07(1) 1 31( 1) vO zCr2 $3 93 vO 4cr2 gS3 93 0 15 0 15( 1) 1 30( 1) vO 4cr2 6% 91 vO 6Cr2 4s3 93 0 25 0 23( 1) 1 31( 1) vO 56cr2 44s3 93 0 36 0 39( 3) 1 32( 1) vO 84cr2 16s3 97vO BCr2 2s3 93 VCr2S3 93 0 50 0 47(2) 1 36( 1) vO 92cr1 97s3 93 I I I I I 1 1 I I I r 1 I I I I I I 0 r(CrJS4 %'o 2cr28% 0 * .-I Y) 0 I I I I I I I I I I 1 I 1 I I I 1 I 02 03 04 05 06 07 08 09 10 02 03 04 05 06 07 oe 09 10 1 1 I 1 I 1 1 1 I I I I 1 I 1 I I I 1 - 0 Ic.v) Va 4CrZ 6s4 0 VO 6cr14s4 5 a-so 4 9 0-O D $ e-c Ya," rn 0 - E c.4- 0 0 Q 0 0 .I I I I I 1 I I I I 1 I I I I I I I ri I I I I I I I 1 n . I I I I I 1 1 I I - VO8Cr22% 0 4 0 4 v,vr 0 0 ...8 . .*I --.-I--..I.*..111). 111.1 .I--., .*0 II u, Ia.n .I$ ,in i-mi ma.# mm om0-no-*-il..l........,-:-4 .I--:-:-I I I I I t I I I I I I I I I I I I 02 03 04 05 06 01 OR 09 10 02 03 04 05 06 07 oe 09 10 Fig. 2 Final observed (points), calculated (full line) and difference (lower full he) profiles for V,Cr3 .S4 (00 <x < 1 0) phases 808 J Muter Chem, 1996, 6(5),807-813 of single-phase products. For the phase with nominal composi- behaviour may be divided into one of three types, according tion VCr,S,, results indicate a slight deficiency of metal, to composition.In the compositional range 1.0<xd0.4, zfc suggesting that some attack of the silica ampoule has taken and fc data overlie each other in the temperature range place. This is consistent with energy dispersive X-ray micro- Tp d T d296 K. A maximum in susceptibility is observed in the analysis data which indicated, for all phases prepared, the zfc curve at Tg whilst the fc curve continues to rise. This indicates presence of trace amounts of silicon. The levels of the resulting magnetic frustration, in agreement with the previous description minor impurity phase are extremely low and below the limits of these materials as spin-glasses. Spin-glass behaviour persists of detection by powder X-ray diffraction.to x=0.4,the glass transition temperature, <,increasing linearly Initial examination of powder X-ray diffraction patterns with decreasing vanadium content. At x=O.2, zfc data show a indicated that all phases could be indexed on the basis of a broad ill-defined maximum at Tz 14 K. However, zfc and fc monoclinic unit cell with parameters similar to those for Cr,S4. data overlie each other only over the range 126d TG296 K However, in view of the existence of a number of phases with and start to diverge at 126 K, a temperature considerably above and that at which there is a susceptibility maximum. This may closely related structures and differing stoi~hiometriesl~ because of the significant influence that such impurities can correspond to a slow freezing of the spins resulting in magnetic have on magnetic properties, it was desirable to obtain further clusters of finite size, with associated uncompensated magnetic evidence of phase purity.Diffraction data were analysed by moments, prior to the onset of long-range antiferromagnetic the Rietveld16 method of profile analysis as incorporated in ordering which has been observed for Cr3S4. Data for x=O.O the GSAS suite of programs.” Starting models for all phases (Cr,S,) are in agreement with those reported previously and were derived from the previous X-ray diffraction study of indicate an apparently antiferromagnetic transition at Tz50 K. Cr,S, .I5 Recent work on VCr2S418 demonstrates that vanadium However, deviations from a modlfied Curie-Weiss law begin at is evenly distributed between sites in the vacancy and fully significantly higher temperatures (Tz220 K): in the region to occupied layers.However, since the difference in X-ray scat- which TNhas been assigned previously. tering power between vanadium and chromium is insufficient Attempts to fit magnetic susceptibility data in the high- to discriminate between them in the mixed phases, and as temperature region to a Curie-Weiss law resulted in relatively Rietveld refinement was carried out solely to establish phase poor agreement. However, introduction of a temperature-purity, in this work, vanadium was arbitrarily introduced at independent term, xo, in a modified Curie-Weiss law of the the metal site in the vacancy layer such that overall stoichi- form x =xo + C/(T-Q), led to a significant improvement in the ometry was maintained.fit in the high-temperature region as exemplified by Fig. 4 in The background was modelled using a cosine Fourier series which reciprocal observed data for VCr,S, are compared with with the coefficients introduced as refinable parameters. Initial calculated data derived from the fit to the modified Curie- refinement of background, zero-point, cell parameters and Weiss law. Derived magnetic parameters are given in Table 3. atomic positions proceeded smoothly. Isotropic temperature factors when introduced into the refinement became unstable Discussionfor phases with 0.4dxd 1.0. Thermal parameters for these phases were subsequently fixed at those refined values appro- All peaks in the powder diffraction patterns are fitted by the priate to Vo.2Cr2.8S4, prior to the introduction of peak shape structural model and there is no evidence to suggest the parameters as variables in the refinement.Final observed, presence of any significant impurity phases. Weighted profile calculated and difference profiles are shown in Fig. 2 and the R factors were of the order of 4-7%. A detailed discussion of resulting parameters are given in Table 2. the structures of these phases will not be given here, as results Magnetic susceptibility data for a measuring field of 1000 G of a recent structural study utilising a combination of powder are presented in Fig. 3. In all cases, field-cooled (fc) and zero- neutron and powder X-ray diffraction data will folloy in due field-cooled (zfc) susceptibilities overlie each other completely at course.19 Mean cation-anion separations of ca.2.4A are in higher temperatures. No appreciable field dependence was reasonable agreement with sums of respective ionic radii,20 observed over the range of applied field strengths investigated. whilst cation-cation separations relevant to the discussion of A broad maximum is observed at low temperatures (T,)for all magnetic properties which follows are summarised in Table 4. compositions. The insets to Fig. 3 show the differing behaviours The variation of cell parameters with composition is shown in of the fc and zfc susceptibilities below this transition.Magnetic Fig. 5. There is an overall decrease in each of the unit-cell edge Table 2 Parameters resulting from Rietveld refinement of powder X-ray diffraction data for V,Cr3-,S4 (0.0bx < 1.0)” x in V,Cr, -xS4 0.0 0.2 0.4 0.6 0.8 1.o 5.9538(4) 3.4211 (3) 11.2407( 7) 91.526(3) 5.9609( 4) 3.4164( 3) 11.2322(7) 91.428(3) 5.9762( 2) 3.4211 (1) 11.3145(5) 91.281 (3) 5.9 6 12(4) 3.4032( 2) 11.2172(7) 91.364( 2) 5.9488 (4) 3.3920( 3) 11.2026(8) 91.377(2) 5.9381 (5) 3.3827(3) 11.2086(8) 91.437(3) 0.85(9) 0.2586(3) 0.85(9) 0.33 72( 8) 0.3643 (4) 0.29(8) 0.3350(8) 0.8800(4) 0.29(8) -0.0238(5) 0.83(8) 0.2588( 3) 0.83(8) 0.3387(8) 0.3651(4) 0.52(8) 0.3361(8) 0.8807 (4) 0.52(8) -0.02 18( 5) 0.83(-) -0.0196( 6) 0.83(-) 0.2576(3) 0.3382( 9) 0.3684( 4) 0.3421(9) 0.8824(4) 0.52(-) 0.52(-) 0.83(-) -0.0222(5) 0.2579( 3) 0.3381(7) 0.3665( 3) 0.3388(7) 0.8809( 4) 0.52(-) 0.83(-) 0.52(-) 0.83(-) -0.0238( 5) 0.83(-) 0.2574( 3) 0.3357( 7) 0.3663 (4) 0.3396( 7) 0.8819(4) 0.52(-) 0.52(-) 0.83(-) -0.0262( 5) 0.2571(3) 0.3367(8) 0.3647( 4) 0.3369(9) 0.8830(4) 0.83(-) 0.52(-) 0.52(-) 4.4 4.7 5.5 5.1 5.7 6.8 3.3 3.4 4.0 3.7 4.2 5.1 a Space group: Z2/m, M on 2a (O,O,O), site occupancy factors: (1 -x)Cr, (x)V, Cr on 4i (x,O,z), S( 1) on 4i (x,O,z),S(2) on 4i (x,O,z).J. Muter. Chem., 1996, 6(5),807-813 809 I a a oxk O k 00 OaOn0 OOaOaO 6 J , . I . , . , . , . , . , o so 100 150 zoo 250 300 vcr,s4 -,S, (00 <x < 1 0) phases measured in a field of tion data show that solid-solution behaviour exists over the entire compositional range studied, it is evident from Fig 5 that changes in cell parameters with increasing vanadium content do not follow Vegard's law In particular, there are anomalies in all four plots at a composition of Vo4Cr2,S4 It is notable that this composition corresponds to that at which marked changes in the magnetic properties occur and may be indicative of changes in relative site preferences of the vanadium and chromium cations Powder neutron diffraction measure- ments have been used to investigate the site preferences of cations in the related systems Fe,V3-,S45 and CrxTi3-,Se4 In both systems, the partitioning of cations between sites in the fully occupied and vacancy layers was found to be incom- plete, although in the latter system, chromium was found to show a marked preference for sites in the vacancy layer Recent l'work,lg using a combination of X-ray and neutron scattering, has shown that a more complete randomisation of the two cations over the two types of site is found for VxCr3-,S4 Moreover, a marked change in the distribution of vanadium a cations between the two types of site occurs in the region in which anomalies in the plots of cell parameters ZIS composition are observed (0 2 <x < 0 4) Data in Table 3 reveal that there is a temperature-indepen- dent contribution of ca 3 x emu to the paramagnetic susceptibility which is virtually independent of composition In addition, the negative Weiss constants indicate that the predominant cation-cation interactions in this system are antiferromagnetic The Curie constants decrease with increas- ing vanadium content, as would be expected for replacement of the Cr3+ d4 ion with V2+ d3 However, the values determined here are considerably lower than would be expected from the spin-only value for localised moments, which for example Table 3 Parameters denved from magnetic susceptibility data for V,Cr, ,S4 (0 0 < x < 1 0) x in V,Cr, ,S, 04 06 08 10 3 O(1) 2 7(1) 2 8(1) 20(1)1 66(4) 1 32( 3) 1 14(4) 0 82(2) -55(3) -33(2) -65(4) -65(4) 44 40 36 32 X -0 NE 20 18 16 14 12 10 0 6 Fig.3 Zero-field-cooled (zfc) and field-cooled (fc) molar magnetic susceptibilities for V,Cr, lo00 G Insets show detail around the transition measured with a field of 100 G 500-400 -3 1-E h r;." 300--E Y 7 rY 200-100-~ l ~ l ~ l ~ l ' ~ l 'l 0 50 100 150 200 250 300 TIK Fig.4 Observed (m) and calculated (-) reciprocal susceptibility plot for VCr,S4 The calculated plot is denved from the fit to modified Cune-Weiss law lengths on moving from Cr,S4 to VCr2S4 1on:c radii given by Shannon'' [r(V2')=0 79 A, r(Cr2+)=0 80 A] suggest that replacement of Cr2+ by V2+ would lead to very little change in the size of the unit cell However, examination of the one- electron energy-level diagrams presented by Holt et al reveals that the presence of Cr2+ d4 requires the population of e8 energy levels which are effectively antibonding in nature Replacement of Cr2+ d4 with V2+ d3 ions would lead to the removal of electrons from these levels and could account for the observed contraction in the unit cell Whilst X-ray diffrac- xo/10 emu C/emu K-' T,/K 810 J Muter 00 02 2 O(2) 24(1) 3 44(2) 2 03(6) -324(4) -97(4) -14 Chem, 1996, 6(5), 807-813 Table 4 Selected cation-cation distances (A)for V,Cr,-,S, (0.0<x <1.0) x in V,Cr,-,S4 0.0 0.2 0.4 0.6 0.8 1.o M-M 3.421 1(3) x 2 3.4164( 3) x 2 3.4211(1) x 2 3.4032( 2) x 2 3.3920( 3) x 2 3.3827(3) x 2 Cr-Cr 3.4211( 3) x 2 3.4164(3) x 2 3.4211( 1)x 2 3.4032( 2) x 2 3.3920( 3) x 2 3.3827( 3) x 2 3.193(5) x 2 3.214( 5) x 2 3.243(6) x 2 3.206( 5) x 2 3.182(5) x 2 3.151(5) x 2 3.691 (5) x 2 3.672( 5) x 2 3.656( 6) x 2 3.672( 5) x 2 3.680( 5) x 2 3.697(6) x 2 Cr-Ma 2.914(3) x 2 2.913(3) x 2 2.920(4) x 2 2.899( 3) x 2 2.890( 3) x 2 2.889(4) x 2 Cr-Mb 4.494(2) x 4 4.489(2) x 4 4.498(2) x 4 4.471(2) x 4 4.556(2) x 4 4.449( 2) x 4 4.423(3) x 4 4.417( 3) x 4 4.439( 3) x 4 4.423(3)~4 4.424( 3) x 4 4.428( 3) x 4 4.329( 3) x 4 4.333( 3) x 4 4.364( 3) x 4 4.330(2) x 4 4.320( 3) x 4 4.310( 3) x 4 MS, and CrS, octahedra with a common face.MS, and CrS, octahedra with common vertices. M is defined in Table 2. O0 o0 O0-3A2 397--3A1 o0 O oo 0. -3M 0596 --339 5 -Q -33 oc' 0 0. 0 oo59s --337 SW --336 o0 i4 I . 1 . , . 1 . , . I !335 O i 0uIb OD 02 OA I6 oa ID 1132 -oo-91s 1130--913 iim--91.45 $ 0 0 o0 !?! 1126-b, -91.40 3 IIW- .Q -9135 1122-' -Y130 1120- 4 I ou - , 03 . 0.4 0.6 , , 08 . , ID !913 x in V, Cr,-,S Fig. 5 Compositional variation of lattice parameters: (a) a and b parameters (b)c and parameters predicts C=6.75 for Cr,S,, and differ from those obtained in earlier work" where Curie constants close to those predicted for spin-only moments were obtained from data in the range 295 < T <800 K. This discrepancy probably indicates that the high-temperature limit for collection of magnetic data in this work is insufficiently high for the moments to be non-inter- acting. In addition, electron delocalisation may contribute to the lowering of Curie constants.One of the chromium ions in Cr,S4 is formally present as Cr" d4. In a crystal field of regular octahedral symmetry, this would result in a t2,3eg1 configur- ation. Whereas the tag levels in Cr,S, are effectively localised, covalent mixing of the eg orbitals with anion s and p orbitals leads to a broadening of eg levels into a narrow band, thereby reducing the effective moment. All phases for which x>O.O have eg levels populated. Furthermore, for vanadium-rich phases, broadening oft,, states is possible, as found previously in binary vanadium sulfides.21,22 Previous work" found magnetic anomalies in the tempera- ture range 100d TG296 K which were ascribed to a double magnetic transition.Careful examination of our data in this region, including calculations of first derivatives of the x us. T O0 Fig. 6 Magnetic structure of Cr,S,. The crystallographic unit cell is outlined and anions are omitted for clarity. Light and dark circles denote cations with (-) or (+) spin, respectively. curves, provides no evidence for a transition to an antiferro- magnetic state above q.In addition, the data in this tempera- ture region were well fitted by a modified Curie-Weiss law and we therefore conclude that for our samples, the magnetic behaviour in this compositional range involves a change from a paramagnetic to a spin-glass state with decreasing tempera- ture. The existence of a spin-glass phase suggests some degree of electron localisation consistent with the known semiconduct- ing properties of VCr,S4 itself.However, the reduction in the values of the Curie constants below a value predicted on the basis of an ionic structure, together with the almost constant temperature-independent contribution to the susceptibility, suggests that localised and delocalised electrons are simul- taneously present. The magnetic structure of Cr3S4 (Fig. 6) determined by powder neutron diffraction measurements at 4.2 K9 consists of ferromagnetic sheets parallel to the (101) planes which in turn are coupled antiferromagnetically with respect to each other. This results in a doubling of the magnetic unit cell in the a and c directions. By considering the eB band to be infinitely narrow, application of the qualitative rules previously devel- oped by Go~denough~~ permits analysis of and Kanam~ri~~ the observed magnetic structure in terms of individual super- exchange interactions. In the fully occupied layer each Cr"' d3 ion has six nearest-neighbour Cr"' d3 ions (Fig.7). A 90" correlation superexchange via an anion p orbital is predicted J. Muter. Chem., 1996, 6(5), 807-813 811 (3.12) Crtn Cr, f Fig. 7 Illustration of interionic magnetic interactions in Cr& defining fully occupied layer; (b) interactions within the vacancy layer, and (c) cation-cation separations. to be ferromagnetic, whereas antiferromagnetic exchange results from direct cation-cation interactions. The latter decrease more rapidly with increasing cationic separation than do tee former.Hence, the two longer-range [d(Cr-Cr) =3.42, 3.69 A] interactions are dominated by correlation super-exchange resulting in &(2) >0 a,nd &b >0, whereas the relatively short-range [d(Cr-Cr) =3.19 A] interaction is dominated by direct cation-cation exchange resulting in JF(l) <0. The short separation betwoeen Cr"-and Cr"'-centred octahedra [d(Cr-Cr)=2.91 A] which share a common face results in direct cation-cation interactions dominating the JVFterm. As the d shell is less than half-full, these are predicted to be (JVFantiferr~magnetic~~ <O). In addition, each Cr" in the vacancy layer has six Cr'" neighbours, to which it is linked by a common anion, in each of the fully occupied layers above and below it (Crl-Cr12 in Fig.7). Whilst cation-anion-cation angles of ca. 130" lead to some uncertainty over the sign of correlation superexchange interactions, comparison with other systems23 suggests that they will carry the sign of the 180" interaction. The signs of correlation superexchange interactions involving Cr" are difficult to predict owing to the degenerate ground state of octahedral Cr" d4. One possible model may be derived in a similar manner to that applied to Mn"' oxides." The vacant d orbital can participate in four dsp2 hybrid orbitals which form a square-plane directed towards those (5.42) the exchange constants discussed in the text. (a) Interactions within the interlayer interactions.Numbers in parentheses indicate corresponding sulfurs which are common to Crllll-Crgl'l (Fig. 7). All 180" superexchange interactions between Cr" and the latter will therefore be antiferromagnetic (JVF(2) <0). Con-c0, JVF(3) versely, the half-occupied d, orbital directed towards sulfurs common to Cr$'-Crlp1 will give rise to a ferromagnetic 180" correlation superexchange interaction with the latter (&(I) >0). The remaining interactions are the 90" cation- anion-cation intralayer Cr'I-Cr" interactions for which d,-p,/p,-d, coupling results in ferromagnetic exchange (Jv,>O). Allowing for a finite width to the eg band does not alter this analysis significantly as only interlayer Cr"-Cr"' 180" interactions via a common anion and intralayer Cr"-Cr" 90" interactions are affected by the width of the eg band.Weakening of these interactions does not change the overall magnetic structure which is primarily determined by the signs of JF(l), JF(2), JFb and JVF* The spin-glass behaviour of VCr2S, is a consequence of mag- netic frustration which prevents the establishment of long-range order. Extension of the above treatment permits rationalisation of this behaviour. It has been shown18 that this compound is more correctly formulated as (Vo.51Cro.49)[ S4.Vo.49Cr1.51] Whilst ambiguity exists regarding the oxidation state of vanadium in the fully occupied layer, Hayashi et uL7 have indicated that for TiCr2Se,, chromium is present as Cr"' irrespective of the type of site which it occ-812 J.Muter. Chem., 1996, 6(5),807-813 upies. Similar behaviour in VCr,S, would result on (Vo.5111Cro.4~11)[ S, with all ions being isoelec- Vo.4~1Cr1.511'1] tronic (d3). Cation-cation separations show little variation in traversing the series from Cr3S, to VCr2S, and the relative magnitudes of competing exchange mechanisms within the fully occupied layer would not be expected to vary from those of Cr3S,. The signs of JF(l)and JF(2)are therefore identical to those for Cr3S4. Intralayer 90" cation-anion-cation inter-actions between d3 ions are ferromagnetic leading to JFb>O and JVb>0. Furthermore, interlayer interactions, of the direct cationsation type, result in JVF<0. Other interlayer inter- actions of the 180" correlation type involve d3 ions only and all are predcted to correspond to antiferromagnetic exchange.Given the presence of both ferromagnetic and antiferromag- netic coupling in the fully occupied layer, it is immediately apparent that cations in the vacancy layer cannot be simul- taneously antiferromagnetically aligned with all six neighbours in each of the two neighbouring fully occupied layers. The model therefore predicts the magnetic frustration implied by the spin-glass behaviour of VCr,S,. Structural studies" demon- strate that a similar statistical distribution of vanadium persists in the compositional range 0.4,<x,< 1.0. The increase in with decreasing x can be associated with the corresponding increase in the amount of Cr" d4, permitting the growth of magnetically ordered regions.At some critical composition, x,, it is to be expected that exchange pathways exist which permit the establishment of long-range magnetic order. From this work, it would appear that x, ~0.2.Further investigation of the properties of phases with compositions close to x=O.2 are required. In particular, low-temperature neutron diffraction measurements would establish whether Vo.,Cr,$, is magneti- cally ordered. Financial support from the EPSRC (grants GR/J36075 and GR/J34231) is gratefully acknowledged. We wish to thank Dr A.M. Chippindale, University of Oxford who performed the energy dispersive X-ray microanalysis measurements and Mr D.C. Colgan of Heriot-Watt University for thermogravimetric measurements.References 1 A. Wold and K. Dwight, in Solid State Chemistry: Synthesis, Structure and Properties of Selected Oxides and Sulphides, Chapman and Hall, New York, 1993, ch. 11. 2 Y. Ueda, K. Kosuge, M. Urabayashi, A. Hayashi, S. Kachi and S. Kawano, J. Solid State Chem., 1985,56,263. 3 H. Wada, M. Onoda, H. Nozaki and I. Kawada, J. Solid State Chem., 1986,63,369. 4 A. Hayashi, T. Kishi, Y. Ueda and K. Kosuge, Mater. Res. Bull., 1989,24,701. 5 J. M. Newsam, Y. Endoh and I. Kawada, J. Phys. Chem. Solids, 1987,48,607. 6 A. Hayashi, Y. Ueda, K. Kosuge, H. Murata, H. Asano, N. Watanabe and F. Izumi, J. Solid State Chem., 1987,71,237. 7 A. Hayashi, Y. Ueda, K. Kosuge, H. Murata, H. Asano, N. Watanabe and F.Izumi, J. Solid State Chem., 1987,67,346. 8 S. L. Holt, R. J. Bouchard and A. Wold, J. Phys. Chem. Solids, 1966,27,755. 9 E. F. Bertaut, G. Roult, R. Aleonard, R. Pauthenet, M. Chevreton and R. Jansen, J. Phys. (Paris), 1964,25,582. 10 Y. Tazuke, J. Phys. SOC.Jpn., 1981,50,413. 11 D. Babot, M. Chevreton, J. L. Buevoz, R. Langier, B. Lambert- Andron and M. Wintenberger, Solid State Commun., 1979,30,253. 12 Y. Tazuke, J. Phys. SOC.Jpn., 1986,55,2008. 13 G. V. Leconte, H. v. Lohneysen, W. Bauhofer and G. Guntherodt, Solid State Commun., 1984,52, 535. 14 J. Dumas and C. Schlenker, J. Phys. C, 1979,12,2381. 15 F. Jellinek, Acta Crystallogr., 1957,10,620. 16 H. M. Rietveld, J. Appl. Crystallogr., 1969,2,65. 17 A. C. Larson and R. B. Von Dreele, General Structure Analysis System, Los Alamos National Laboratory Report, LAUR 86-748, 1994. 18 A. V. Powell and D. C. Colgan, Proc. IVth European Powder Diflraction Conference, Chester, UK, 1995, in press. 19 A. V. Powell and D. C. Colgan, in preparation. 20 R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 751. 21 A. B. de Vries and C. Haas, J. Phys. Chem. Solids, 1973,34,651. 22 Y. Kitaoka and H. Yasuoka, J. Phys. SOC.Jpn., 1980,48,1949. 23 J. B. Goodenough, in Magnetism and the Chemical Bond, Wiley, New York, 1963. 24 J. Kanamori, J. Phys. Chem. Solids, 1959,10,87. 25 J. B. Goodenough, Phys. Rev., 1955,100,564. Paper 5/07103H; Received 27th October, 1995 J. Mater. Chem., 1996,6(5), 807-813 813

ISSN:0959-9428    

DOI:10.1039/JM9960600807

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Fabrication of La, -,Sr,CoO, -thin layers on porous supports by a polymeric sol-gel process Chunhua Chen,? Henny J. M. Bouwmeester," Henk Kruidhof, Johan E. ten Elshof and Anthony J. Burggraaf Laboratory of Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, P.O. Box 21 7, 7500 AE Enschede, the Netherlands A polymeric sol-gel process was developed to fabricate porous thin layers of the perovskite-type Lao.3Sro.7Co03 for membrane applications. A spin-coating technique was used for deposition of the layer on porous a-and y-Al,03 supports. Both supported and non-supported membranes were characterized by means of thermal analysis, X-ray powder diffraction, particle size analysis, scanning electron microscopy and permporometry. The main phase formed upon heating in air above 400 "C is cubic perovskite, although traces of SrCO, and of an unknown phase can be observed up to 800 "C.Above this temperature, strong chemical interaction occurs between the deposited perovskite layer and the support material.The effects of the sintering temperature and the type of the La-precursor on the pore-size distribution were investigated. The perovskite-type oxides La, -,Sr,Co03 -(LSCO) have attracted much attention in recent years because of their potential uses as electrode materials in solid oxide fuel cells (SOFCs),'v2 gas sensors3 and oxygen separation membrane^.^ For these applications, the use of thin layers of LSCO is beneficial because it can decrease the electrode resistance, favour the miniaturization of a gas sensor and optimize the oxygen permeability. Furthermore, if the layer is porous, it can also increase the triple-phase boundary in SOFC electrodes and the active surface area in gas-sensing elements. For oxygen separation membranes the thin barrier layer should be dense, i.e.free from connected-through porosity, and crack free. The layer needs to be supported by a porous substrate if its thickness is less than ca. 150 pm in order to obtain sufficient mechanical strength. The successful application of the electro- chemical vapour deposition (ECVD) technique for the prep- aration of dense thin layers of yttria-stabilized zirconia (YSZ) on porous alumina supports has been reported.'^^ However, it is not easy to find a good substrate material for LSCO layers owing to its high reactivity with many substrate materials at high temperatures and thermal mismatch.It is therefore con- sidered advantageous to coat a substrate with a thin porous layer of LSCO, as an intermediate layer, prior to deposition of a dense top layer with similar or adapted composition. In the present paper, the preparation and characterization of such porous (intermediate) LSCO layers are described. The depos- ition of a dense layer on top of these will be a subject for future research. Sol-gel methods can be divided into colloidal and polymeric types. Although both approaches have been applied success- fully in the preparation of ceramic porous membranes, the polymeric approach is more often used for microporous and multi-component systems.798 The polymeric route is supposed to produce an inorganic network (gel) by gelation which results in an appropriately viscous sol preventing the compo- nents from penetration into the pores of the support.Nevertheless, there are a few reports on the deposition of thin oxide layers on porous supports by this polymeric sol-gel process.' In many cases crack-free films can be obtained when the thickness is less than 0.5 pm." In this study, a polymeric sol-gel process has been developed for the fabrication of our perovskite Lao.3Sro,7Co0, layers. Two types of porous alum- t Present address: Laboratory of Applied Inorganic Chemistry, Delft University of Technology, Julianalaan 136, 2628 BL Delft, the Netherlands.ina supports were used, both with well defined sharp pore-size distributions. As suitable metal alkoxide precursors of cobalt, strontium and lanthanum were not available, we first synthe- sized methoxyethoxides of these three elements with hydrolys- abilities suitable for easy manipulation. Details will be reported elsewhere." A spin-coating technique was adopted to prepare the thin LSCO layers. Experimenta1 The synthesis procedure is schematically shown in Fig. 1. Methoxyethoxide solutions of cobalt, strontium and lantha- num were synthesized separately according to the method developed in our laboratory and described elsewhere." Two kinds of lanthanum precursors were used in this study for comparison.The first is a distilled methoxyethanol (MOE) solution of hydrated lanthanum nitrate, as shown in Fig. 1. The second precursor is lanthanum methoxyethoxide synthe- sized from the reaction between lanthanum nitrate and stron- dissolved distilled, dissolved, in MOE repeat Fig. 1 Sol-gel La, -.Sr,CoO, mixed solution . I partial hydrolysis yjspin coating Ifilm drying and firing perovs kite synthesis flow chart for the preparation of -6 perovskite films J. Muter. Chem., 1996, 6(5), 815-819 815 tium methoxyethoxide Solutions containing La, Sr and Co were mixed in a 0 3 0 7 1 molar ratio to a final concentration of ca 0 2 mol dm-, Co The mixed solution was heated at reflux in a three-necked flask under pure nitrogen at 125°C for 3-5 h and subsequently cooled to room temperature to serve as a stock precursor solution If well sealed, the solution has a shelf-life of several months A few millilitres of the stock precursor solution was trans- ferred into a small glass bottle MOE and distilled water were added, under stirring, in order to partially hydrolyse the precursor solution into a sol Usually, the volume ratios of the precursor solution methoxyethanol water were kept at 1 1 03 The viscosity of the sol increased with time due to gelation About 1-3 h after the aforementioned addition, the support was coated with the sol A few droplets of the sol were dripped onto the middle of a porous alumina substrate (diameter 12mmx2mm) which was held in a spin coater (model PMlOlD, Headway Research Inc, Garland, TX) placed in a class 100 clean hood (Down-flow unit, The Netherlands) Following this procedure, however, the layer thus obtained was usually thicker at the centre, and thinner towards the edge Therefore, the whole surface was flooded with the sol prior to spinning, in order to produce a uniform layer All results reported here refer to films prepared by the latter procedure The spinning was performed for 40 s with a speed of 4000 rpm The porous substrates used in the experiments were made of a-Al,O, with a pore size of ca 160nm, and lanthanum- doped y-Al,03 with a pore size of ca 17 nm l2 ', Both types of substrates were heat-treated at temperature > 1100 "C before spin coating in order to obtain a thermally stable pore size The supported gel layers were dried in air at room tempera- ture for several hours and successively heated to either 500, 600, 700 or 800°C for 5 h with a heating/cooling rate of 30°C h-l Sometimes the aforementioned steps were repeated to increase the layer thickness The surface and cross-section morphologies of 800 "C sintered samples were examined with a scanning electron microscope (SEM, Hitachi S800) Elemental analysis was performed on some selected areas by energy dispersive analysis by X-rays (EDAX) Direct analysis by standard X-ray diffraction (XRD) was not possible due to the fact that the layers were too thin to obtain reasonable intensities To enable XRD analysis, some samples were pre- pared with very thick layers, both on a-A1203 and y-A1203 supports, by using a highly concentrated sol [derived from La(NO,),] in the spinning process After sintering at 800°C for 5 h, some powder was scratched from the surface of the sintered layer for a step-scanning XRD analysis (Philips PW 1710, Ni-filtered Cu-Ka radiation) with a 28 step of 0 02" and a counting time of 5 s per interval In addition, a non-supported membrane was prepared by drying a partially hydrolysed sol in a glass dish under ambient conditions into a xerogel The resulting gel was crushed into a fine powder This powder was characterized by a TG-DSC analyser (Stanton Redcroft PL-STA 625) in the temperature range 25-1300°C at a heating/cooling rate of 10"Cmin-l in flowing air A step scanning XRD analysis with a 20 step of 0 04" (5 s per interval) was also applied to the powder samples heat treated at 600, 700 and 800 "C A laser diffraction particle- size distribution analyser (HORIBA, LA-500) was used to analyse the non-heat-treated xerogel powder after ultrasonic dispersion in distilled water The pore-size distribution of deposited layers was examined by means of permporometry In this method, a mixture of cyclohexane vapour, oxygen and nitrogen is used to flush one side of a supported membrane which is sealed with O-rings in a diffusion cell Initially, the gas mixture is saturated with cyclohexane so that all the pores in the membrane become filled with liquid cyclohexane owing to capillary condensation 816 J Mater Chem, 1996, 6(5),815-819 No gas can permeate through the membrane at this stage By decreasing stepwise the relative pressure of the cyclohexane in the mixture, pores with radii equal to or larger than the corresponding Kelvin radii are opened and oxygen diffuses through the membrane to the other side The relation between the accumulated oxygen permeability and the relative pressure of cyclohexane provides information about the pore-size distri- bution of the membrane under consideration For further details on the principles and experimental set-up of this technique, see ref 12 Results and Discussion Non-supported membrane Fig 2 shows the TG-DSC curves of the La, 3Sro ,Coo,-, gel powder A senes of mass losses and several exothermic peaks appear below 400"C, which can be attnbuted to the evapor- ation and burn-out of organic components (such as residual solvent MOE, acetate groups and unhydrolysed alkoxy groups) and dehydration of hydroxides existing in the gel The large exothermic peak around 400"C corresponds to the formation of the perovskite phase, as was confirmed by XRD Based on the XRD results, discussed below, the small mass loss and the very small endothermic peak between 600 and 750°C can be attnbuted to the decomposition of residual SrCO, No chemi-cal reaction nor any phase transition is observed above 800 "C The DSC peaks between 1000 and 1200 "C are due to baseline instabilities The initial state of the gel powder is assumed to be either highly amorphous or poorly crystalline Fig 3 shows the XRD patterns of gel powders calcined at 600,700 and 800 "C It can be seen that the cubic perovskite-type La, $r0 ,COO,-, phase is the main phase14 The peak widths in the pattern of the powder calcined at 600°C indicate a low level of crystallinity 105 95 -85 s Y 5 75E 65 55 45 0 200 400 600 800 100012001400 TI"C Fig.2 TG-DSC curves of the La, 3Sro ,COO, gel powder 30 35 40 45 50 55 60 2 eldegrees Fig.3 XRD patterns of the non-supported Lao,Sr,,CoO, mem-branes calcined at (a) 600, (b) 700 and (c) 800°C Lines belonging to the cubic perovskite phase are indexed The asterisks indicate SrC0, Unidentified lines are marked by open circles or small particle size.From the change of width of the corresponding diffraction peaks with increasing temperature, it can be concluded that either the crystallinity improves or the particle size increases. The XRD pattern of the powder calcined at 600 "C contains several peaks which belong to SrCO,. Upon increasing the calcination temperature to 700-800 "C, these peaks almost disappear. Apparently, most of the SrCO, that is present in the sample is decomposed below 700 "C. The latter temperature is much lower than the onset temperature for SrCO, decompo- sition reported in ref. 15. This is probably due to the high reactivity of the sol-gel derived powders. This result is also in agreement with the DSC-TG curves and thus the small endothermic peak between 600 and 750°C can be attributed to the decomposition of SrCO,.A few unidentified diffraction peaks were found in the XRD patterns of powders calcined at 700 and 800°C. A metastable tetragonal phase has been observed previously in the La, -,Sr,CoO, system.14 Some of the unidentified peaks, for instance the peak at 28 cu. 44", may be attributed to this metastable phase. Supported membrane The morphology of the supported membranes was studied by SEM. The SEM pictures shown in Fig. 4 were taken after sintering the samples at 800°C. In the cross-section of a y-Al,O,-supported sample, shown in Fig. 4(u), three regions can be distinguished. At the bottom, the porous a-Al,O, phase can be seen. The intermediate layer with a thickness of cu.2.5 pm consists of porous y-Al,O,. The top layer is the perovskite phase, having a thickness of ca. 300nm, and was Fig. 4 SEM images of the supported La,, ,SrO 7C003 single-layer coated (a)-(e) and triple-layer coated (f),(8)membranes sintered at 800 "C for 5 h. (a)-(f) supported on porous y-Al,O,, (8)supported on porous a-Al,O,. (a)cross-section, La(NO,), as precursor (10000x); (b)La(NO,), as precursor (100O00x); (c) La(MOE), as precursor (100000x); (d) La(NO,), as precursor (10000 x); (e) La(MOE), as precursor (loo00 x),(f)La(NO,), as precursor (100000 x); (g)La(NO,), as precursor (100000 x). J. Muter. Chern., 1996, 6(5), 815-819 817 I h yAIpO, Supported v) a-AI,O, supported 30 35 40 45 50 55 60 2Bldegrees Fig.5 XRD patterns of powders scratched from the supported La, 3Sr0$003 membranes after sintenng at 800°C for 5 h Reflections belonging to cubic perovskite are indexed Open circles indicate A1,0,, while asterisks indicate CoAI,O, I 1025I: 0 5 10 15 20 25 30---. pore radudnrn Fig. 6 Pore-sue distnbution of the supported La, 3Sr0 7C003 mem-branes prepared with La( NO3), as precursor matenal Sintenng temperatures (a)500, (b) 600 and (c) 700 "C found to exhibit a mirror-like appearance The layers obtained are porous, which can be clearly seen from Fig 4(b)-(e) The grain size is estimated as ca 30-50 nm EDAX indicated that the composition of the surface is approximately the same as that in the sol This implies that selective penetration of components into the pores, giving rise to a non-stoichiometric composition, did not occur within the EDAX experimental error 818 J Muter Chem, 1996,6(5), 815-819 The layer prepared with La(NO,), as the lanthanum precur- sor was found to be somewhat inhomogeneous in its strontium distribution by EDAX The bright area in Fig 4(d) is rich in strontium This is probably due to the fact that during the reflux of the mixed precursor solution, La(N03), reacted with Sr(OCH,CH,OCH,), resulting in La(OCH2CH20CH,), and Sr(N03), Since the La Co ratio used in this study was 3 7, this implies that when all La(NO,), was converted to La(OCH,CH,OCH,), ,the strontium precursor consisted of a mixture of Sr(OCH,CH,OCH,), and Sr(NO,), Although an inorganic salt of one component can be used as a precursor in a multi-component polymeric sol-gel process,16 a certain extent of inhomogeneity can occur when salt mixtures are used This may be due to the precipitation of one the salts during drying and gelation as a result of the difference in the solubility of the salts The layer that was prepared with Sr(OCH,CH,OCH,), as the strontium precursor is found to be homogeneous, but more porous, which can be seen by comparing Fig 4(d) and 4(e) The multiple-coated membranes, shown in Fig 4(f) and (g),exhibit denser morphologies than the single-coated ones This is especially clear for the a-Al,O,- supported layer in Fig 4(g) Gas-tightness measurements indi- cated that the obtained layer was still porous A fully dense supported layer can be obtained only when sintering occurs by a viscous mechanism Even though the existence of an i11- defined liquid phase is assumed to promote densification of bulk ceramics of La, ,S~,COO,-~,~~the present results suggest that densification is retarded by the interaction with the support material Fig 5 shows the XRD patterns of powders scratched from a-Al,O,- and y-Al,O,-supported La, 3Sro ,COO, -membranes sintered at 800°C The results are essentially similar to those obtained for the unsupported membranes, the main phase formed being cubic perovskite Diffraction peaks from Al,O, and a diffraction peak from CoA120, can also be seen in the patterns The latter compound may result from the interaction between the La, 3Sro layers and the A1,0, supports during sintenng The spinel CoA120, was also found to be formed in an investigation of CoO-A1,03 catalysts '* Although fully densified La, -,Sr,CoO, layers could possibly result at sintering temperatures >800°C, in view of the high sinter- ability of the perovskite phase, alumina is not suitable as support material because of its strong chemical interaction with La, 3Sro 7C00,-s A search for other, more inert, porous supports is deemed necessary to develop dense structured layers Fig 6 shows estimates of the pore-size distribution of some y-Al,O,-supported perovskite membranes sintered at different temperatures and prepared with different lanthanum precur- sors The results can be used only in a qualitative manner owing to the badly developed accumulated oxygen per- meability plots It can be seen from Fig 6(u)-(c) that the average active pore size of a perovskite membrane tends to increase with increasing sintenng temperature Increasing the sintering temperature also broadens the pore-size distnbution It should be noted that the estimated average pore radius of the membrane sintered at 700°C is cu 12 nm, which is larger than the average pore radius of La-doped y-Al,O, (8 5 nm) This may be caused by the large error in the estimated pore size In addition, the interaction between the deposited layer and the alumina support, as confirmed by XRD, may play a role The membranes sintered at 400 and 800°C could not be charactenzed by the permporometry technique, probably because their pore sizes are beyond the range of this technique (1-50 nm) Conclusions A polymeric sol-gel process was developed to fabricate thin La, 3Sro &oO, layers on porous alumina supports Partially hydrolysed mixed precursor solutions in methoxyethanol were spin-coated on porous ct-and y-A1203 supports to form gel layers The perovskite phase was formed slightly above 400 "C Most of the residual strontium carbonate existing in the gel layers could be decomposed by heating to 600-700°C When the preparation conditions were carefully controlled, the formed layers were smooth and crack free with an esti- mated thickness of ca 300nm after a single coating step The layer was found to be more homogeneous when La(OCH2CH20CH,), was used as the lanthanum precursor instead of La(NO,), The pore size of layers tended to increase with increasing sintering temperature Sintering had to be performed at tem- peratures below 800°C in order to prevent chemical inter- actions between the deposited layer and the alumina support, resulting in the formation of spinel-type CoA1,0, The alumina-supported Lao 3Sro ,COO, layers developed in this study may be used either as intermediate layers onto which a dense layer of the same material can be applied (eg by vapour deposition) or as porous membranes for high- temperature liquid or gas separation This work was performed in the framework of a joint pro- gramme between the Academia Sinica and the Dutch Academy of Science (KNAW) and was supported financially in part by the KNAW We are indebted to Professor Meng Guangyao (Department of Materials Science and Engineering, University of Science and Technology of China) for his support in stimulating this project P Fransen (MESA, University of Twente), H W Brinkman and B de Boer are sincerely acknowledged for their technical assistance References 1 0 Yamamoto, Y Takeda, R Kanno and M Noda, Solid State Ionzcs, 1987,22,241 2 A Mackor, T P M Koster, J G Kraaijkamp, J Gerretsen and J P G M van Eijk, in Proc 2nd Int Symp on Solid Oxide Fuel Cells, Athens, Greece, ed F Grosz, P Zegers, S C Singhal and 0 Yamamoto, Commission of the European Communities, Luxumbourg, 1991, p 463 3 Y Yamamura, Y Ninomiya and S Sekido, in Proc Int Meeting on Chemical Sensors, Tokyo, 1983, p 187 4 Y Teraoka, T Nobunaga, K Okamoto, N Miura and N Yamazoe, Solid State Ionics, 1991,48,207 5 Y S Lin and A J Burggraaf, AIChE J, 1992,38,445 6 G Z Cao, H W Brinkman, J Meijennk, K J de Vries and A J Burggraaf, J Am Ceram Soc ,1993,76,2201 7 C J Brinker, T L Ward, R Sehgal, N K Raman, S L Hietala, D M Smith, D-W Hua and T J Headley, J Muter Sci, 1993,77, 165, W F Maier, I-C Tilgner, M Wiedorn, H-C KO, A Ziehfreund and R Sell, Adv Muter, 1993,5730 8 C D E Lakeman and D A Payne, J Am Ceram Soc, 1992,75, 3091, G Moore, S Kramer and G Kordas, Muter Lett, 1989, 7, 415, K Okuwada, M Imai and K Kakuno, Jpn J Appl Phys, 1989,28, L1271 9 T W Kueper, S J Visco and L C De Jonghe, Solid State Ionics, 1992,52,251 10 C J Brinker and G W Scherer, Sol-Gel Science The Physics and Chemistry of Sol-Gel Processing, Academic Press, Boston, 1990 11 C H Chen, H Kruidhof, H J M Bouwmeester and A J Burggraaf, to be published 12 G Z Cao, J Meijennk, H W Brinkman and A J Burggraaf, J Membr Sci ,1993,83,221 13 Y S Lin and A J Burggraaf, J Am Ceram SOC,1991,74,219 14 J Kirchnerova and D B Hibbert, J Muter Sci , 1993,28,5800 15 M J Scholten, J Schoonman, J C van Miltenburg and H A J Oonk, Solid State Ionics, 1993,61, 83 16 P Griesmar, G Papin, C Sanchez and J Livage, J Muter Scz Lett, 1990, 9, 1288, S-F Ho, L C Klein and R Caracciolo, J Non-Cryst Solids, 1990, 120, 267, H Murakami, S Yaegashi, J Nishino, Y Shiohara and S Tanaka, Jpn J Appl Phvs, 1990, 29, L445 17 H Kruidhof and H J M Bouwmeester, unpublished results 18 P Arnoldy and J A Moulijn, J Catal, 1985,93,38 Paper 5/06083D, Received 14th September, 1995 J Muter Chem, 1996, 6(5),815-819 819

ISSN:0959-9428    

DOI:10.1039/JM9960600815

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Additive-assisted pressureless sintering of carbothermal /r-sialon: an X-ray and solid-state MAS NMR study Kenneth J. D. MacKenzie” and Richard H. Meinhold New Zealand Institute for Industrial Research and Development, P.O. Box 31-310, Lower Hutt, New Zealand /?’-Sialon pellets (Si6-zAl,0,N8 -z; z =2.45) prepared from carbothermally synthesised sialon powder were sintered in N2 at 1400-1700°C with additions of 15 mol% each of A1203, MgO and Y203, and 10 mol% each of various combinations of these oxides. The most effective densifying agent is Y203, both alone and in combination, followed by MgO and A1203. XRD and multinuclear MAS NMR (27Al, 29Si, ”Mg and 89Y) studies revealed traces of X-phase sialon, Sic and an X-ray amorphous glass- like phase in the starting material, and were used to monitor phase changes during sintering.Y203 by itself and with other additives forms Y3A15012 (YAG) with /?’-sialon; at higher temperatures, an aluminium-rich polytypoid sialon and yttrium- containing glass are formed, to the detriment of the strength of the sintered body. MgO initially forms MgA120,, lowering the sialon z-value. At higher temperatures, this further reacts to form a glass containing magnesium in both tetrahedral and octahedral sites. A1203reacts with p’-sialon, increasing its z-value. The NMR spectra confirm the XRD phase analyses and provide information about phases not detectable by XRD, and about the possible constitution of the glasses formed. The substitution of aluminium and oxygen into silicon nitride results in sialon ceramics having the advantages of a wider composition range, greater ease of fabrication, improved ther- mal shock resistance and oxidation resistance.One of the most useful sialons is p’-sialon, which is isostructural with /?-Si3N4 and has the general formula Si6-zAlzOzN8-z, where z can vary from 0 (corresponding to Si3N4) to ca. 4.3. Under sintering conditions, some sialon compositions produce a liquid phase which renders them more sinterable than silicon nitride, but for other sialons, including p’-sialon, the addition of additives such as MgO or Y203 is desirable; these promote efficient pressureless sintering by forming intergranular liquid phases. The experiments of Lumby et al., who noted the beneficial effect on the sintering of p’-sialon compositions of small additions (1%) of MgO,’ were extended to higher MgO concentrations (up to 7 mass%) by Lewis et who reported that in hot-pressed samples most of the Mg goes into a liquid silicate phase, with only a small solubility of Mg in p’-~ialon.~ In pressureless-sintered p’-sialons containing MgO, spinel (MgA1,04) was reported, with a slightly expanded lattice parameter taken2 to indicate some substitution of Si for Al, balanced by the substitution of some N for 0.2The presence of Mg also facilitated the formation of 15R-sialon polytypoid containing significant Mg subs tit ution.Sialon compositions containing much greater MgO concentrations form p”-sialon, MgXAl4-xSi204+xN,-x,4 characterised by hexagonal cell parameters larger than those of p’-sialon.’ Lewis et aL6 reported the formation of a Y-Al-rich glass during pressureless sintering of low-z (ca.0.5) sialon composi- tions containing 6 mass% Y203; this glass crystallised when further heat-treated to yttrium aluminium garnet (YAG) Y3A15012, with a slightly enlarged lattice parameter which was taken as evidence for some Si substitution in this phase.6 The maximum degree of (Si +N) replacement for (A1 +0)in YAG was subsequently found to be small (0.4).7 The solubility of Y in p’-sialon is also reported to be negligible.6 Inspection of the phase diagrams for sialon compositions containing Y203 ’ confirms the importance of YAG as a secondary phase in sialons sintered with Y203, and suggests the possible formation of other crystalline phases, YSi0,N and Y2SiA10SN, both of which have structures based on a-w~llastonite.~ All the previous p’-sialon sintering studies have been made on sialons formed in situ from the appropriate compositions plus the sintering additive, but the focus of this work is on the post-synthesis behaviour of p’-sialon powders prepared by carbothermal reduction of clay minerals.The present studies were therefore undertaken to investigate the interactions of some commonly used sintering additives with carbothermal /?’-sialon for future comparison with sialons formed and sin- tered in situ. In a previous study of additive-assisted sintering of Si3N,,8 solid-state NMR with magic angle spinning (MAS NMR) was used to complement X-ray powder diffrac- tion (XRD) by providing information about some of the non- crystalline phases present.In this work, the same combination of techniques was brought to bear on the more complex sialon systems sintered with MgO, Y203, A1203 and various combi- nations of these. Experimenta1 The p’-sialon was produced in this laboratory by carbothermal reduction of New Zealand halloysite.’ XRD investigations showed it to be predominantly crystalline /?’-phase, z =2.45, determined from a careful measurement of the cell parameters. A very small trace of X-phase sialon was the only impurity detectable by XRD. The particle size distribution of the pre- prepared p’-sialon, determined by laser interferometry, was 100% <6 pm, 50% <2 pm, 10% <1 pm.The additives, A1203, Y203 and MgO, were used separately in concentrations of 15 mol%, or in binary and ternary combi- nations at individual concentrations of 10 mol%. These addi- tive levels, which are considerably greater than additions made in normal fabrication practice, were dictated by the needs of the NMR study. Eu203 (73pmol g-’) was added to the samples containing Y203 to increase the 89Y relaxation rate; a previous study” has shown that Eu20 in this concentration is very effective for this purpose, without significantly broadening the ”Y or 29Si spectra of yttrium silicon oxynitride phases. Mixtures were batched and ball-milled for 18 h in ethanol using Si3N4 milling media, then the solvent was removed by rotary vacuum evaporator, and the powder brushed through a 600 pm sieve.Pellets of mass 1g and diameter 13 mm were pressed without binder at 100 kPa and fired for 2.0 h at 1400-1700°C in a powder bed containing BN and p’-sialon (2:1m/m)contained in an alumina crucible with a loose-fitting alumina lid. All firings were performed in a graphite resistance furnace (Thermal Technology Inc.) evacuated for 0.5 h, then back-filled with oxygen-free nitrogen, which flowed into the J. Muter. Chern., 1996, 6(5), 821-831 821 firing chamber continuously throughout the sintering process. Temperature measurements were made with a boron graphite- graphite coaxial thermocouple, periodically checked by optical pyrometry.A blank series of pellets without additives was also fired under the same conditions. Before and after sintering, the pellets were weighed and their dimensions measured. The pellets were then broken by being loaded across their diameter in an Instron Model 1122 univer- sal tester at a crosshead speed of 5 mm min-', from which a value for the tensile strength T was deduced [eqn. (l)]:" T=2P/ndt (1) where P is the breaking load, and d and t the pellet diameter and thickness respectively. The samples were then ground to pass through a 152 p sieve and examined by XRD using a Philips PW 1700 computer-controlled goniometer with graph- ite monochromator and Co-Ka radiation. Careful lattice par- ameter measurements, made using Si as the external angular calibrant, were used to deduce the sialon z-value from the relationships of Ekstrom et Solid-state MAS NMR spectra were obtained at 11.7 T using a Varian Unity 500 spectrometer and 5 mm Doty probes with ZrO, rotors spun at 10-12 kHz, under the following instrumental conditions: 29Si: 6 ps 42 pulse, recycle delay of 30 or lOOOs, shifts referenced to tetramethylsilane (Me$).27AI: 1ps n/10 pulse for solution, recycle delay of 5 s, shifts referenced to 1 mol dm-3 aqueous A1(N03)3 solution. 25Mg: 3 ps n/10 pulse for solution, recycle delay of 0.1 s, 90 ps ringdown delay prior to acquisition, shifts referenced to 1mol dm-3 aqueous MgS04 solution. *'Y: 20 ps n/2 pulse, recycle delay of 3 s, 170 ps ringdown delay between excitation and sampling, shifts referenced to aqueous YC13 solution.No interference was found in any of the spectra from probe background signals. Results and Discussion Physical properties of the sintered samples To monitor the progress of the sintering at different tempera- tures and additive additions, the bulk densities and radial shrinkages of the pellets were recorded. As was the case in the previous Si3N4 sintering experiments,* the primary focus of this work was the intergranular chemistry rather than the preparation of highly dense sialons; the green densities of the pellets pressed without binder were thus all rather low (1.40-1.65 g CM-~).Fig. 1 shows the bulk densities of the fired pellets as a function of sintering temperature. Comparing the single oxide additives (Fig.lA), Y203 pro-duces the greatest densification at all temperatures, followed by MgO, then A1203. Little densification was observed in undoped pellets, even at 1700 "C. All the additive combinations containing Y203 were effective densifying agents (Fig. 1B); the particularly beneficial effect of the ternary additive combination even at 1500°C may be due in part to the high total additive concentration (10 mol% of each component). The combination of MgO and A120, is very little different from the effect of A1203 alone, because the MgO is bound up in the refractory MgAl2O4 phase (see later). As expected, this densification behaviour is closely reflected by the radial shrinkage measure- ments (Fig.2), but the tensile strengths, derived from room- temperature radial crushing measurements (Fig. 3) show some differences, especially for the additive combinations contain- ing Y,03. Although the actual strength values obtained by radial crushing should be treated with caution, since they are based on single measurements made by a crushing method, the results are internally consistent, and are considered to reflect 822 J. Muter. Chem., 1996,6(5), 821-831 3.0 2.5 / / / 2.0 / / 1 --d'y 7.5 ' --/ E' I I I P 14 1500 1600 1700.-a w:8 7% 5 3.0 2.5 ,'MgO +2.0 p' 40,1 / . //---? It 14 1500 1600 1700 sintering temperature/"C Fig. 1. Densification of carbothermal p'-sialon pellets as a function of sintering temperature.A, Undoped and with single additives; B, with combinations of additives. 20 I* t I I 1400 1508 1600 1700 sintering temperature/"C Fig.2. Shrinkage of carbothermal p'-sialon pellets as a function of sintering temperature. A, Undaped and with single additives; B, with combinations of additives. Fig. 3. Strength of carbothermal p'-sialon pellets as a function of sintering temperature. A, Undoped and with single additives; B, with combinations of additives. real changes within the sample microstructures. The reduction in strength of the 1700 "C samples containing additive combi- nations with Y203 (Fig. 3B) may be related to differences in the glass composition of these samples and its effect on their response to the radial crushing force applied; some of these samples failed in a distinctly more brittle fashion than others.X-Ray diffraction The phases detected by XRD in the sintered samples are summarised in Table 1, which also lists the mean z-values of the p'-sialon, determined from accurate measurements of both the a and c lattice parameters by the relationships given by Ekstrom et al." The tabulated z-values represent the average for both the grain surface and the interior, which may remain relatively unchanged by the reactions originating at the grain surfaces. Table 1 also lists the phases detected in these samples by solid-state NMR studies, for comparison. A selection of typical XRD traces is shown in Fig. 4. When sintered without additives, a small amount of cor- undum (a-alumina) appears at 1500 "C [Fig.4(b)], probably resulting from the conversion to p'-sialon of the trace of X-phase sialon impurity in the original sialon [Fig. 4(a)], by reaction [eqn. (211 with the small amount of Sic detected in the sample by NMR (see later). 2Si6A1 0 4Si,A1303N, 21N3+18N2+ 12SiC-t (X-phase) (P', z =2) +3A12O3+3CO +9CO2 (2) As the sintering temperature increases, some Si is lost as SiO, leading to an increase in the sialon z-value and, at 1700"C, to the appearance of a trace of 15R polytypoid sialon SiA140,N4 [Fig. 4(c)], with a higher A1:Si ratir, than P'-sialon. The changes in the p'-sialon z-value with temperature for all undoped and doped sintered samples are plotted in Fig.5. In samples containing A1203alone [Fig. 4(d)], incorporation of the additional A1 into the p' structure occurs at ca. 1600"C, indicated by the disappearance of the corundum reflections and the increase of the sialon z-value to ca. 3 (Fig. 5A). The small amount of glassy phase formed at 1400"C, evidenced by a broad hump in the XRD baseline, disappears from the trace at higher temperatures. The addition of MgO results in the formation of spinel, MgA1204, at 1400-1500 "C [Fig. 4(e)], the necessary A1 being supplied by the p'-phase, reflected in a small decrease in its z- value (Table 1). Measurements 0.f the spinel cell constants yielded values of a =7.9!5-7.995 A, satisfactorily close to the reported value of 8.093 A fo; pure spinel (PDF no.21-1152), and unlike the value of 8.2A found by Lewis et al.' in Mg- doped sialon systems and cited as evidence for the substitution of Si in their spinels. The spinel is replaced by a glassy phase at > 1600 "C, the concomitant increase in the p'-sialon z-value (Fig. 5A) suggesting that the A1 component of the spinel is taken into the sialon. Most of the Mg may reside in the glassy phase, although the possibility of limited uptake of Mg in the sialon is not inconsistent with its increased z-value. The Mg- containing p"-sialon with cell parameters quoted by Jack' is stable only up to ca. 11OO"C, and was not expected in the present samples. The samples sintered with Y203 below 1700°C all contain YAG, as expected from the phase diagram.' Incomplete reaction at 1400°C is indicated by very small traces of Y203 and N-melilite (Y2Si303N4), which disappear at 1500"C [Fig.4(f)]. The A1 component of the YAG is furnished by the p'-sialon, evidenced by the continuous decrease of the z-value with temperature (Fig. 5A). The cubic lattice parameter of the YAG, measured from the XRD traces of the present samples, was in the range 12.00-12.02 A, more similar to the tabulated value of 12.0089 for th: pure phase (PDF no. 33-40) than to the value of a= 12.11A reported by Lewis et aL6 for an Si- substituted YAG identified in sialon systems sintered with Y203. By 1700"C, the YAG has been replaced by Al-rich 15R sialon, with a significant amount of glass indicated by the pronounced baseline hump [Fig.4(g)]. The wollastonite-type structure YSi02N was not observed in the present samples, and neither was Y2SiA10SN, which is stable only to about 1150 "C. Binary additions of MgO and A1203, each at 10mol% concentration, give rise to spinel formation at 1400 and 1500 "C [Fig. 4(h), Table 11, with some of the additional alumina also entering the sialon structure at 1500"C, resulting in a slightly increased z-value. By 1600 "C, the spinel has decomposed, with a significant increase of the alumina content of the B'-sialon (Fig. 5B) and, at 1700"C, the appearance of the alumina-rich 15R sialon. The latter phase was not found in samples contain- ing A1203 or MgO alone; its appearance in the presence of these oxides together could result from the combined effect of extra A1 and the removal of Si to form an Mg-containing glass.The combination of A1203 and Y203 gives rise to YAG at temperatures < 1700 "C, with none of the additional A1 appar-ently being available for substitution into the p'-sialon, since its z-value remains low (Fig. 5B). By 1700"C, the YAG has decomposed, its A1 being taken up into a 15R phase and the Y being accommodated in a glassy phase, as in samples containing Y203 alone. Samples containing both MgO and Y203 also form YAG, possibly via the Mg-containing intermediate MgY,Si,O,, , of which a very small trace may tentatively be identified at 1400 "C [Fig. 4(i)]. The A1 required for the formation of YAG, and for the 15R sialon formed subsequently, is derived from the p-sialon, evidenced by the progressive decreases in its z-value during the reaction (Fig.5B). As with both MgO and Y203 used separately, considerable glass appears at higher temperatures. J. Muter. Chew., 1996, 6(5), 821-831 823 Table 1 Phases in sintered p’-sialon detected by XRD and NMR studies phases“ -p’-sialonadditive T/“C XRD NMR z-value none unheated 1400 1500 1600 1700 A1203 1400 1500 1600 1700 p’, cor, glass p’, cor, glass(tr) p’ p’ 1 400 1500 1600 1700 p’, SP p: SP p’ p’, glass y2°3 1400 1500 1600 1700 MgO + A1203 1 400 1500 1600 1700 Y203+ A1203 1 400 p’, Y, cor(tr) 1500 1600 1700 p’, y p’, y, pp’, P, glass Y203+ MgO 1400 1500 1600 1700 Y203+ MgO + Al,03 1400 1500 1600 1700 p’, y, sp(tr)p’, Y, P(tr), glass (tr) p’, P, glass p’, P, glass ~~ p’, Sic,glass 2 45 p’, Sic, glass 2 45 p’, Sic, cor, glass 2 39 p’, glass 2 86 p’, P, glass 2 74 p’, Sic, cor, glass 2 42 p’, Sic, cor, glass 2 28 p’, cor, glass 3 12 p’, glass 2 97 p‘, sp, glass (Mg-Si-0-0) 2 37 p’, sp, glass (Mg-Si-O-C7) 2 41 p’, glass 2 94 p’, glass 2 79 p’, Y, SiC(tr) 2 29 p’, y 2 27 p’, y, p 2 09 p’, P, glass (AI-Si-0-0) 2 02 p’, cor, sp, glass 2 42 p‘, cor, spyglass 2 49 p’, glass 3 11 p’, P, glass(tr) 2 88 p’, Y, cor, SiC(tr) 2 27 p’, y 2 59 p’, Y, P, glass (Y-Si-C-07) 2 40 p’, P, glass(tr) 2 32 p’, y 2 42 p’, y 2 40 p’, P, glass(tr) 2 11 p’, P, glass(tr) 2 06 p’, y, SP 2 49 p’, y 2 62 p’, P, glass(tr) 2 36 p’, P, glass(tr) 2 30 p’=p’-sialon (PDF no 24-1492), X=X-phase sialon (PDF no 36-832), cor=corundum, a-A1203 (PDF no 10-173), P= 15R sialon (PDF no 42-160), sp=spinel, MgA1204 (PDF no 21-1152), Y =Y3Al5OI2 (PDF no 33-40), N=N-melihte, Y2Si303N, (PDF no 28-1457) tr= trace The combination of the three additives Y203, MgO and Al,03 in equal molar percentages produces YAG and a trace of spinel at 1400°C By 1500”C, the spinel has given way to 15R polytypoid sialon and some glass, these two phases becoming dominant after the disappearance of the YAG at > 1500“C (Table 1) This high-temperature behaviour is a common feature of all samples containing Y203, but the composition of the glass presumably varies with the presence of the other additives, since Mg can readily enter into glassy phases under these conditions MAS NMR Studies Samples sintered without additives.Representative 27Al and 29S1 MAS NMR spectra of these samples are shown in Fig 6 The 27Al NMR spectrum of the starting material [Fig 6(a)] is typical of a high-z /?’-sialon, containing a major broad AlO, resonance at 6 66 and a weaker, sharper resonance at 6 103 which has been ascnbed13 to AlN, units, but in the light of recent studies14 may be more correctly descnbed as A10N3 units The presence of the shoulder at 6 ca 3 has been reported in p’-sialon spectra by Dupree et a1,13 Sjoberg et all5 and Smith,16 who ascribe it to a low-level octahedral A1 impurity either in the p’-sialon itself, or in an impunty phase undetect- able by XRD. The present sample contains a small amount of X-phase sialon impurity [Fig 4(a)], of which the 27Al NMR spectrum is expectedi7 to consist of a major A106 resonance 824 J Muter Chem, 1996, 6(5),821-831 at 6 2 8 and a minor AlO, resonance at 6 59, the present NMR spectrum is thus entirely consistent with the XRD indication of a p’-sialon of z>2, containing a small amount of X-phase sialon The 27Al NMR spectrum of this material sintered at 1400 “C without additives [Fig 6(b)] is identical to the unsintered sialon, but at 1500“Cchanges occur which are again consistent with the XRD results, a strong sharp octahedral resonance at 6 12 superimposed on the p’-sialon resonances [Fig 6(c)] reflects the presence of corundum in this sample, apparently formed at the expense of the X-phase The 27Al NMR spectrum of the sample sintered at 1600°C [Fig 6(d)] is of a high-z p’-sialon with no resolvable octahedral component, but at 1700“C [Fig 6(e)], a small octahedral component may be reappeanng as a shoulder at 6 ca 10 on the p’-sialon spectrum This shoulder could reflect the presence of the small amount of 15R polytypoid sialon, the spectrum of which consists16 of an octahedral resonance at 6 ca 10 and a second resonance at 6 ca 93, obscured under the broad envelope of the p’-sialon spectrum The 29S1 NMR spectra of all the samples, both with and without sintenng additives, contain the strong, sharp peak of p’-sialon at 6 -47 to -48, the position of this resonance varies similarly with sintering temperature irrespective of the additive (Fig 7) Although Dupree et al l8 suggest that the position of this resonance is independent of the sialon z-value, Y ? dB(f) SA L1, I 1 I I I I I I 10 20 3040 50 60 7080 10 20 3040 50 60 70 80 2Mdegrees (Co-KCX) Fig.4. Representative XRD powder patterns of carbothermal p'-sialon sintered under various conditions: (a) unheated starting material; (b) 1500 "C without additive; (c) 1700 "C without additive; (d) 1500 "C,Al,O,; (e)1400"C, MgO; (f)1500"C, Yz03;(g) 1700 "C, Y,O,; (h) 1400 "C, MgO +A1,0,; (i) 1400 "C, Yz03+MgO; (j) 1600 "C, Y,O, +Al,O,. B =p'-sialon (PDF 24-1492); X =X-phase sialon (PDF 36-832); P = 15R sialon (PDF 42-160); C=a-Al,O, (PDF 10-173); S=MgA1,04 (PDF 21-1152); Y =Y,A1,0,, (PDF 33-40); M=MgY,Si,O,, (PDF 20-1410).Major unmarked peaks correspond to p'-sialon. J. Mater. Chem., 1996, 6(5),821-831 825 47 0 (.o 480 / 28 / 20 ' I 1400 1500 1600 1700 sintering temperaturePC Fig. 5. z Value of carbothermal j? -sialon as a function of sintering temperature A, Undoped and with single additives, B, with combi- nations of additives 200 100 0 -100 40 0 40 80 -120 -160 6 Fig. 6. Typical 11 7 T MAS NMR spectra of carbothermal j? -sialon sintered at various temperatures without additives "Al NMR, ref [Al(HzO),]3' (a)unsintered,(b) 1400, (c) 1500, (d)1600, (e) 1700 "C 29S1 NMR, ref Me,& (A unsintered, (g) 1400, (h) 1500, (1) 1600, (1) 1700°C Asterisks denote spinning side bands 49 0 I I I I I I 1400 1500 1800 1700 sintering temperaturePC Fig.7. 29S1 NMR resonance position of carbothermal /?-sialon with various additives as a function of sintering temperature+, MgO+A1203, 0, MgO, A, A1,03+MgO+YzO3,A,Yz03+Mg0, M, Y2O3+AIz0,, 0,Y203, 0,A1203,0,blank the more recent data of Sjoberg et a1 l5 suggest that the small downfield shift with increasing sintering temperature up to 1600°C is consistent with the observed trend to higher z-values with increasing temperature in all samples except those containing Y203 The upfield shift in all samples at 1700°C probably reflects the increased contribution from 15R sialon, which has its major resonance at 6 -48 1 l9 Another major feature seen in most of the 29S1 NMR spectra acquired with a delay time of 30 s is a broad peak centred at 6 ca -95 (Fig 6) This feature is obscured by noise in spectra acquired with a longer delay time (1000 s) but fewer transients A few measurements made at different delay times suggest its relaxation rate is faster than the j'-sialon peak The shape, position and faster relaxation of the broad feature suggest it is due to a glassy silicate phase Attempts to deconvolute this feature suggest the presence of at least two components, at 6 ca -75 to -85, and at 6 -102 to -112 The area of this resonance, measured for spectra acquired with a 30s delay time, gives a comparative estimate of the amount of glassy phase present, and changes with temperature and with the sintering additive (Fig 8) In the present samples, the glass content reaches a maximum at 1600 "C Although appreciable glass is present in the original sialon, the presence of Y203 removes this at the lower temperatures, and suppresses glass formation at higher temperatures MgO, both alone and in combination with A1,0,, maintains a high glass content over the whole temperature range The 29S1 NMR spectrum of the unheated sialon [Fig 6(f)] shows, in addition to the sialon and glass resonances, a small peak at 6 -15, corresponding to an unreacted Sic residue from the carbothermal synthesis, this is too small or insufficiently crystalline to be detected by XRD, but persists in the NMR spectrum of the sample sintered at 1500°C [Fig 6(h)] At 1700°C, the Sic peak has gone, and the area of the glass peak has been significantly reduced [Fig 6(j)] The ability of NMR spectroscopy to detect the otherwise unsuspected presence of Sic will prove useful in optimising the carbothermal synthesis process, since this undesirable impunty hinders the sintering process p-Sialon+ A1,OY Representative 27Al and 29S1 MAS NMR spectra of these samples are shown in Fig 9 The 27Al NMR spectra of samples heated at 1400 and 1500°C [Fig 9(a), (b)] are typically of p-sialon with a superimposed corundum resonance at 6 12 At 16OO0C,the sharp octahedral peak has decreased in intensity [Fig 9(c)], indicating the continuing 826 J Muter Chem, 1996,6(5), 821-831 Fig.8. Glass content of carbothermal r-sialon with various additives at various temperatures, estimated from 29Si MAS NMR spectra 4 4 -48.1 12 /-".;t;, 47.5& -47.2 -47.8 -95 6 Fig. 9. Typical 11.7 T MAS NMR spectra of carbothermal p'-sialon sintered with A1203 at various temperatures. 27Al NMR, ref. [Al(H,0)6]3+: (u) 1400; (b) 1500; (c) 1600; (d) 1700°C. 29SiNMR, ref. Me& (e) 1400; (f)1500; (g) 1600; (h) 1700°C. Asterisks denote spinning side bands. presence of corundum, even though the XRD results suggest that unreacted corundum is no longer present in particles large enough to produce the characteristic XRD pattern. By 1700 "C, the additional alumina has been fully incorporated into the p'-sialon, the 27Al NMR spectrum [Fig.9(d)] being typical of a high-z sample. The 29Si NMR spectra [Fig. 9(e)-(h)] show identical behaviour to those of the undoped samples. /r-Sialon+MgO. Typical 27Al, 29Si and "Mg MAS NMR spectra of these samples are shown in Fig. 10. The 27Al NMR spectra of p'-sialon sintered with MgO at 1400-1500 "C [Fig. lo@), (b)] contain p' resonances and a superimposed octahedral peak at 6 8-9, arising from spinel (MgA1204) in which the major resonance is at 6 7-11.20 The weaker tetra- hedral spinel resonance at 6 68-7220 is located under the broad tetrahedral envelope of the p'-sialon. On sintering at higher temperatures, the spinel resonance disappears [Fig. lo@)]. The 29Si NMR spectra of these samples [Fig. 10(e)-(h)] are similar to those of the undoped samples, except that the presence of the MgO has essentially removed the Sic even at 14OO0C,possibly by the formation of a glassy silicon oxycar- 6 Fig.10. Typical 11.7 T MAS NMR spectra of carbothermal p-sialon sintered with MgO at various temperatures. 27Al NMR, ref. [Al(H20)6]3+: (a) 1400; (b) 1500; (C) 1600; (d) 1700°C. 29si NMR, ref. Me,%: (e) 1400; (f> 1500; (g) 1600; (h) 1700°C. 25Mg NMR, ref. MgS0,: (i) 1400; (j)1500; (k) 1600; (I) 1700°C. Asterisks denote spinning side bands. J. Muter. Chem., 1996, 6(5),821-831 827 bide phase, which is known to be unusually stable and to form readily in the Mg-Al-Si-0-C system2' (see below) The 25Mg NMR spectra, shown in Fig lO(z)-(l), are broadly similar to those found for Si3N4 sintered with Mg0,8 and contain two general features the resonance at 6 ca -40 to -50 is characteristic of Mg in octahedral sites,22 while the feature at 6 33-40 is more characteristic of tetrahedral Mg The similarity between the spectra of Mg in MgO-sintered Si3N4 and those found for nitrogen-containing compounds such as MgAlSiN, has been taken to indicate the presence of N in the glassy Mg-containing phase,' and similar reasoning seems likely here At increasingly higher reaction temperatures, the proportion of tetrahedral Mg resonance appears to increase [Fig 10(k), (l)], with a concomitant increase in the proportion of A1 in the p'-sialon (Table l), suggesting that the proportion of A1 in the glass (predominantly in tetrahedral sites) is decreasing, with these sites being increasingly occupied by Mg 8-Sialon +Y,03.Representative 27Al, 29S1 and "Y MAS NMR spectra of these samples are shown in Fig 11 The 27Al NMR spectrum of the sample sintered at lower tempera- tures [Fig ll(a)] contains a narrow tetrahedral lineshape in which can be distinguished the p'-sialon resonance at 6 59 and the tetrahedral component of the YAG spectrum at 6 69, the position of this resonance is in reasonable agreement with the reported shift of 6 74,13 since it is present here as a shoulder on the tetrahedral envelope The sharp octahedral peak at 6 -1 in this spectrum is attributable to the octahedral component of YAG, (reported shift 6 0 8) l3 The well resolved feature at 6 80 1 does not arise from the A1 component of the N-melilite phase present at 1400 "C, which should have reson- ances at 6 110 1, 60 5 and 19 7 (our own unpublished results) The origm of the peak at 6 80 1 is at present unknown At 1500-1600 "C,the components of the tetrahedral envelope are no longer resolved, but the spectra retain the characteristics of p'-sialon and YAG [Fig ll(b), (c)] By 1700"C, the YAG spectrum has disappeared [Fig 11 (41 with a broadening of the spectral envelope due to the presence of a range of A1 t -1A environments, including the tetrahedral and octahedral compo- nents of 15R sialon and probably also the glassy phase evident from XRD The 29S1 NMR spectra of the 1400 "C sample [Fig 1l(e)] shows little evidence of the glassy phase or Sic, at 6 -95 and -18 respectively, but all these spectra show pronounced spinning side bands resulting from the presence of the paramag- netic Eu203 added to increase the "Y relaxation rate At 1700°C a shoulder at 6 -36 which was evident at 1600°C becomes sharper [Fig ll(h)], this is in the position reported for 15R sialon, which is also detected by XRD in these samples sintered at 1600 and 1700°C (Table 1) The 1700°C sample also shows the development of a shoulder at 6 ca -24, in the reDon attributed to the SiC2O2 groups in silicon oxycarbide glasses 23 Such glasses are also reported to show 29S1 resonances at 6 ca -75 and -112, corresponding to SiC03 and SiO, groups re~pectively,~~ this could provide an explanation for the fine structure observed in the 1600 and 1700°C spectra, and also for the two peaks at 6 ca -75 and -110 which can be fitted to the glassy phase spectra in most of the other glass- containing samples As was found in a previous sintenng study of Si3N4,8 all the "Y spectra are broad and fairly featureless [Fig ll(z)-([)],the centre-of-gravity (c o g ) positions were therefore measured from the mid-point of the integrated intensities of the complete resonances The spectra of samples fired at 1400-1600°C consist of a sharp peak at 6 218-222, consistent with the reported position for YAG (6 22224) This is superimposed on a broad underlying envelope with a c o g ranging from 6 203 to 229 in samples sintered at 1400-1600 "C The "Y resonances arising from the trace of N-melilite (Y2Si303N,) and unreacted Y203 noted by XRD in the 1400°C sample should appear on either side of YAG, at 6 160" and 6 314, 272 524 respectively, 1 e they are included under the broad envelope With increasing sintering temperature, the YAG content decreases and the glass content increases, at 1700"C, the Y may occur predomi- nantly in the glass, in which, however, the broad distribution of Y sites is not significantly different from the lower tempera- ture samples, evidenced by its c o g of 6 214 [Fig 11(1)] 483 --A 47 6 47 0 487 Fig.11. Typical 11 7 T MAS NMR spectra of carbothermal -sialon sintered with Y203 at various temperatures 27AlNMR, ref [A1(H20)6]3t (a) 1400, (b) 1500, (c) 1600, (d) 1700°C 29S1 NMR, ref Me& (e) 1400, (f)1500, (g) 1600, (h) 1700°C 89Y NMR, ref YCIJ 0) 1400, (1) 1500, (k) 1600, (1) 1700 "C Asterisks denote spinning side bands 828 J Muter Chem, 1996, 6(5),821-831 p-Sialon+A1,03 +MgO. Below 1500 "C, the 27Al NMR spectra of these samples are similar to those of samples containing MgO alone (Fig.lo), and correspond to a mixture of p'-sialon, corundum and spinel. The octahedral peak of the latter (expected at 6 7-1l2') overlaps with the corundum resonance to give a combined peak at 6 12. As with MgO alone, these octahedral features are lost at 1600"C, the spec- trum being that of /3'-sialon only [Fig. lO(c)]. At 1700"C, the spectral features of a mixture of and 15R sialon are found, the latter evidenced by the broad octahedral shoulder at 6 ca.9. The 29Si NMR spectra are very similar to those of samples containing MgO alone (Fig. lo), indicating the low-tempera- ture removal of Sic. A shoulder at 6 -36.2 which becomes prominent in the 1700°C sample reflects the development of 15R sialon in this sample, consistent with the XRD results (Table 1). This feature was also noted in samples containing y2°3 [Fig* I1(g)l.Some typical "Mg NMR spectra of p'-sialon sintered with MgO +A1203 and other combinations of additives are shown in Fig. 12. The spectra all contain peaks in both the octahedral and the tetrahedral regions, the tetrahedral peak in the sample containing MgO +A1203 [Fig.12(u)] having a shift much closer to that reported for MgA1204 spinel (6 5222)than the analogous sample containing MgO alone [Fig. lO(i)], even though both samples contain significant amounts of spinel, according to XRD results (Table 1). The presence of the 6 Big. 12. Typical 11.7 T 25MgMAS NMR spectra of carbothermal p-sialon sintered with various combinations of additives (a)MgO +A1203 at 14OO0C, and (b) at 1700°C; (c) MgO+Y,O, at 1700°C; (d) MgO +Al,03 +Y20, at 1400"C, and (e)at 1700 "C. octahedral Mg peak at 6 -42 indicates that even at this temperature, appreciable partitioning of the Mg into the glassy phase has occurred. With increasing sintering temperature, the tetrahedral and octahedral peaks assume shifts of 6 38 and -35 respectively; the similarity of these peak positions with those found in other sintered p'-sialon samples containing MgO as one of the sintering additives suggests these "Mg shifts are characteristic of the glassy phase, and seem to be essentially independent of the presence of other additives.fl-Sialon +Y203+A1203. At 1400 and 1500"C, the 27Al NMR spectra contain partly resolved tetrahedral reson- ances from /l'-sialon and YAG, at 6 ca. 59 and 69 respectively, together with the octahedral YAG resonance at 6 ca. 1. The additional octahedral peak at 6 14.1 in the 1400°C sample results from the unreacted corundum in this sample, which has fully reacted by 1500°C. At higher temperatures, the spectra are similar to those of samples containing Y203 alone, with considerable spectral broadening, reflecting the presence of 15R sialon and glass.Although the octahedral YAG reson- ance is still resolvable at 1600"C as in Fig. 11(c), by 1700"C it has been replaced by the broad 15R sialon resonance at 6 ca. 0-4 as in Fig. ll(d). At higher temperatures, the small 8'-sialon resonance at 6 ca. 106 is no longer resolved in the sample containing Y203 +A1203,but comes under the broad envelope which spans the complete range of Al-0-N composi-tions (6 75-10614,16). The 29Si NMR spectra are dominated by the effect of the yttrium, which removes the Sic and suppresses glass formation at lower temperatures and facilitates the formation of 15R sialon and glassy silicon oxycarbide at higher temperatures, as in the samples containing Y203 alone.The 89YNMR spectra of all these samples differ from those containing Y203 alone, in not containing a sharp, dominant YAG peak at 6 222, even in samples sintered at 1400-16OO0C, which contain significant amounts of YAG. The YAG reson- ance is largely masked by the broad spectral envelope with a c.0.g. of 6 275-309, significantly downfield of the spectra of samples containing Y203 alone, and tending towards the spectral region of the pure oxide (Fig. 13).This broad envelope is also present in the sample sintered at 1700"C, but the c.0.g. is shifted to 6 246 by the appearance in this spectrum of a broad upfield shoulder at 6 ca. 110, in the spectral region reported for two of the polymorphs of Y2Si207.24 These results suggest that with increasing temperature, more of the Y is becoming associated with a silica-rich glassy phase, which may also contain N. The additional A1 forms progressively more 15R sialon, but the present results provide no information about the association of Y with this phase.fl-Sialon+Y,O,+MgO. The 27Al NMR spectrum of the 1400 "C sample is again similar to that of the sample containing Y,03 alone [Fig. 11(u)], having a sharp octahedral YAG resonance at 6 0.7, superimposed on the broader /?'-sialon resonances, of which the tetrahedral component and that of the YAG are not well resolved. Also discernable in this spectrum in a shoulder at 6 ca. 81 similar to that found at 1400°C with Y203 alone, and which in the present sample becomes better resolved at 1500"C,before becoming absorbed into the broad tetrahedral envelope which accompanies the formation of glass and 15R sialon at 1600-1700°C as in Fig.11(d). The 29Si NMR spectra are also identical with those of samples containing Y203 alone, particularly with respect to the development of features at 6 -25 to -37. The development of 15R sialon and glassy silicon oxycarbide therefore appears to be a function of the presence of yttrium rather than magnesium. As is the case with samples sintered with Y203+A1203, the 89Y NMR spectra of all the samples containing MgO are J. Matev. Chem., 1996,6(5), 821-831 829 330-broad, with c o g values of 6 271-329 (Fig 13) The samples sintered at 1400 and 1500°C contain XRD evidence of YAG, 320-Y,OI + &OI but the *'Y NMR peak of this phase, at 6 222, occurs under +w the broad spectral envelope The absence of an upfield shoulder 310 -at the higher temperatures may indicate that, by companson with the samples containing Y203 +Al,03, the glassy phase 300-formed in these samples at 1700 "C is less silica-rich The 25Mg NMR spectra of these samples are similar to those of the other Mg-containing samples, showing a well290-defined tetrahedral and octahedral band [Fig 12(c)] At all A E sintering temperatures, the position of the tetrahedral peak gm-(6 33) is upfield of that in the spinel-containing samples Y s [Fig 12(a)], and is in the regon expected for Mg in the glassy E 270-phase As the sintering temperature increases, the intensity of the tetrahedral resonance decreases, suggesting a progressive $280-change in the composition of the glassy phase formed under '> 250-these conditions 240 -p-Sialon + Y203+ A1203+ MgO.In the sample heated at 1400"Cthe 27Al NMR spectral features include the octahedral signatures of YAG and spinel at 6 0 9 and 13 6 respectively,230-together with an unresolved tetrahedral envelope containing p'-sialon, YAG and spinel resonances [Fig 12(u)] A slight 220 -downfield shift of the octahedral spinel peak suggests the presence of a small amount of unreacted corundum, undetected 210 -by XRD studies The spinel/corundum disappears at 1500 "C, but the spectrum is otherwise unchanged At 1600-1700 "C, the spectra are broadened into the tetrahedral and octahedral 1350 1450 1550 1650envelopes consistent with a mixture of p' and 15R sialons and stntering temperaturePC glass, as seen with Y203 alone [Fig 11 (d)] Fig.13. 89Yresonance position in sintered carbothermal p'-sialons as The 29s1NMR spectra are Of containing a function of sintering temperature yttrium, showing evidence of the formation of 15R sialon and a glassy phase, possibly silicon oxycarbide, at 1600-1700 "C The *'Y NMR spectra of all these samples are very similar to those of the samples containing Y203 in combination with Corundum+ Glass Spinel + I I Corundum + Spinel + Glass I J YAG Glass1 1 Sialon + Glass INCREASING SINTERING TEMPERATURE 14oooc 17OO0C Fig.14. Schematic representation of the interactions between p'-sialon and the vanous sintenng additives, simplified by omitting the F-sialon present throughout the reaction 830 J Muter Chem, 1996, 6(5), 821-831 either Al,O, or MgO, being broad, with c.0.g. values ranging from 6 276 to 315 (Fig. 13). Again, at the two lowest temperatures, the samples show XRD evidence of YAG, the NMR peak of which remains unresolved under the broad spectral envelope representing Y in a range of non-crystalline environments. The 25Mg NMR spectra show at the lowest temperature a tetrahedral resonance which reflects the presence of some spinel [Fig. 12(d)].At higher sintering temperatures, the spec- tra take on the tetrahedral and octahedral positions typical of the Mg-containing glass formed in all these samples [Fig.12(e)]. Conclusions The NMR results generally confirm the XRD phase identifi- cation in the sintered samples, but also indicate the presence of Sic and an X-ray amorphous residual glassy phase in the starting material. Under the present conditions, the chemistry of intergranular phase formation during sintering is determined at the lower temperatures by reactions with the aluminous component of the sialon (with consequent changes to its z-value), and at the higher temperatures by glass formation, which occurs to a greater or lesser degree with all the present additives. Samples containing MgO or Al,O, form the glassy phase via magnesium aluminate (spinel) or corundum respectively; an equimolar mixture of these two additives behaves similarly to the individual components. Samples containing Y,03 as one of the additives all react via an yttrium aluminate (YAG), with other lower-temperature phases (spinel, corundum) depending on the other additives present.The presence of Y,O, also promotes at higher tempera- tures the formation of an Al-rich polytypoid sialon similar to 15R sialon, in addition to the higher-temperature glassy phase. The reaction pathways pertaining to the various additives are shown schematically in Fig. 14. Details of the evolution and disappearance of the glassy phase in the presence of various combinations of additives, as revealed by NMR studies, indicate that at lower temperatures, glass formation is significantly suppressed in all systems con- taining yttrium.At higher temperatures, systems containing magnesium (but not yttrium) contain comparatively larger glass contents, as judged from the NMR spectra, which also indicate the occurrence of Mg in both tetrahedral and octahedral sites. The residual Sic initially present may be implicated in the glass chemistry, by the formation of glassy silicon oxycarbide. References 1 R. J. Lumby, B. North and J. A. Taylor, in Ceramics for High-Performance Applications 11, ed. J. J. Burke, E. N. Lenoe and R. N. Katz, Brook Hill, Chestnut Hill, MA, 1978, p. 893. 2 M. H. Lewis, A. R. Bhatti, R. J. Lumby and B. North, J. Mater. Sci., 1980, 15,438. 3 D.P. Thompson, Mater. Sci. Forum, 1989,47,21. 4 G. Leng-Ward, M. H. Lewis and S. Wild, J. Mater. Sci., 1984, 19, 1726. 5 K. H. Jack, in Alloying, ed. J. L. Walter and M. R. Jackson, ASM, Ohio, 1988, ch. 13. 6 M. H. Lewis, A. R. Bhatti, R. J. Lumby and B. North, J. Muter. Sci., 1980,15, 103. 7 D. P. Thompson, in Tailoring Multiphase and Composite Ceramics, ed. R. E. Tressler, G. L. Messing, C. G. Pantano and R. E. Newnham, Plenum Press, New York, 1986, p. 79. 8 K. J. D. MacKenzie and R. H. Meinhold, J. Mater. Chem., 1994, 4, 1595. 9 K. J. D. MacKenzie, R. H. Meinhold, G. V. White, C. M. Sheppard and B. L. Sherriff, J. Mater. Sci.,1994,29,2611. 10 R. H. Meinhold and K. J. D. MacKenzie, Solid State Nucl. Magn. Reson., 1995,5, 151. 11 M. M. Frocht, Photoelasticity, vol. 2, Wiley, New York, 1948, p. 121. 12 T. Ekstrom, P. 0.Ka11, M. Nygren and P. 0. Olsson, J. Mater. Sci., 1989,24, 1853. 13 R. Dupree, M. H. Lewis and M. E. Smith, J. Appl. Crystallogr., 1988,21, 109. 14 J. J. Fitzgerald, S. D. Kohl, G. Piedra, S. F. Dec and G. E. Maciel, Chem. Mater., 1994,6, 1915. 15 J. Sjoberg, R. K. Harris and D. C. Apperley, J. Mater. Chem., 1992, 2,433. 16 M. E. Smith, J. Phys. Chem., 1992,96,1444. 17 M. E. Smith, Solid State Nucl. Magn. Reson., 1994,3, 111. 18 R. Dupree, M. H. Lewis, G. Leng-Ward and D. S. Williams, J. Mater. Sci. Lett., 1985,4,393. 19 J. Klinowski, J. M. Thomas, D. P. Thompson, P. Korgul, K. L. Jack, C. A. Fyfe and G. C. Gobbi, Polyhedron, 1984,3,1267. 20 R. L. Millard, R. C. Peterson and B. K. Hunter, Am. Mineral., 1992, 77, 44. 21 J. Homeny, G. G. Nelson and S. H. Risbud, J. Am. Ceram. Soc., 1988,71, 386. 22 K. J. D. MacKenzie and R. H. Meinhold, Am. Mineral., 1994, 79,250. 23 H. Zhang and C. G.Pantano, J. Am. Ceram. Soc., 1990,73,958. 24 R. Dupree and M. E. Smith, Chem. Phys. Lett., 1988,148,41. Paper 5/07244A; Received 2nd November, 1995 J. Mater. Chem., 1996, 6(5),821-831 831

ISSN:0959-9428    

DOI:10.1039/JM9960600821

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Thermal reactions of alkali-leached aluminosilicates studied by XRD and solid-state 27Al, 29Si and 23Na MAS NMR Kenneth J. D. MacKenzie,*a Richard H. Meinhold," Akshoy K. Chakravortyb and M. H. Dafadarb "New Zealand Institute for Industrial Research and Development, P.O. Box 31 -31 0, Lower Hutt, New Zealand bCentral Glass and Ceramic Research Institute, Calcutta 32, India The interactions of dehydroxylated Zettlitz kaolinite with NaOH solution have been studied using X-ray powder diffraction and solid-state MAS NMR to characterise the crystalline and amorphous products, respectively, and to provide further information about the reaction sequence of kaolinite. Under the present conditions, leaching for very short times (ca. 10 min) preferentially removes uncombined amorphous silica (but not the quartz impurity), and also introduces Na into the remaining amorphous aluminosilicate phase.Leaching for longer times continues to remove Si from the amorphous phase, with a concomitant increase in Na incorporation and an increase in the proportion of available Al. On reheating to 1140 and 1300 "C, the resulting crystalline phases reflect the increase in A1 and Na content with increased leaching time, forming a-alumina and, in samples leached for >60 min, a nepheline solid solution at the expense of mullite, residual quartz and cubic spinel-type phase. When aluminosilicate minerals such as kaolinite are heated, they pass through an amorphous or semi-amorphous state en route to crystalline high-temperature phases such as mullite (A16Si2013) and cristobalite (SO,).' The constitution of the amorphous phases is difficult to determine; they may include poorly crystalline mullite, amorphous silica, amorphous alumi- nosilicates and a cubic spinel phase in amounts which vary depending on the crystallinity of the starting mineral, the impurities present and the thermal treatment.' The composi- tion of some of these phases, especially the spinel, has also been the subject of considerable speculation for many years., Although solid-state nuclear magnetic resonance with magic- angle spinning (MAS NMR) is a promising technique for studying amorphous phases, its usefulness in studying the present system is limited by the fact that the aluminium in octahedral and tetrahedral sites has similar 27Al NMR chemical shifts in all the phases, and the 29Si NMR spectra are broad and relatively featureless, spanning a range of Si environment^.^ To facilitate the study of the aluminosilicate phases in heated kaolinite, Chakravorty and Ghosh have developed methods for selective dissolution using NaOH solutions under carefully controlled condition^.^ During leaching experiments of heated kaolinite, it has been noted that dissolution appears to occur in two steps.' In the first step, representing the first ca.30-40 min of reaction, dissolution is very rapid, and is thought to involve only the removal of uncombined SiO,, representing 35 mass% after 40 rnin reaction. Samples leached to this point did not form cristobalite on heating to higher temperatures.At longer leaching times, when the dissolution rate becomes much slower, it is thought that the amorphous aluminosilicate phase reacts next, followed by the poorly crystalline mullite component. The cubic spinel phase is considered to be the most stable to NaOH treatment. Samples leached for >40 rnin formed corundum (a-A1203) on heating to higher temperatures.' These observations suggest that the interpretation of the 27Al and 29Si MAS NMR spectra of heated kaolinite could be facilitated by the progressive removal of the various phases using NaOH dissolution techniques. This paper reports the results of such a study, which also addressed the question of Na incorporation during leaching, using 23Na MAS NMR. Experimental The kaolinite used in this study was a well characterised material from Zettlitz, calcined at 980°C for 4 h to remove all structural (hydroxy) water.The calcined kaolinite was treated with 5% NaOH solution on a boiling water bath for 10-90 min. The leached material was then recovered and washed thoroughly to free it of adsorbed alkali. Combined TG-DSC experiments were performed on these samples (Polymer Laboratories PL-STA thermal analyser) in air, heating rate 10 "C min-' to 1350 "C. Two sub-samples of this material were then heated in air as follows: (i) to 1140°C at 5 "C min-' with no dwell time at 1140°C; (ii) to 1300°C at 5 "C min-', 30 min dwell time at 1300°C. All samples were examined by X-ray powder diffraction (Philips PW 1700 computer-controlled diffractometer with graphite monochromator and Co-Ka radi- ation).Their room-temperature 27Al, 29Si and MAS NMR spectra were acquired at 11.7 T (Varian Unity 500 spectrometer with 5mm Doty MAS probe spinning at 10-12 kHz), with parameters as follows: 27Al: 1 ps (.n/10 solu-tion) pulse width, 1s delay, shifts referenced to 1 mol dmP3 A1(N03),; 29Si: 6 ps (n/2)pulse width, 10 s delay, referenced to tetramethylsilane (Me4Si); 23Na: 1 ps (n/10 solution) pulse width, 1 s delay, referenced to 1 mol dme3 NaC1. The samples heated to 1140 "C were analysed for Al, Si and Na by atomic absorption spectroscopy (AAS). Where appro- priate, analyses for quartz (SiO,) and corundum (a-Al,O,) were made by quantitative X-ray powder diffraction (QXRD) with an internal standard of 20 mass% Cr,O, (JCPDS Intensity Standard 674), calibrated with quartz and corundum standard phases diluted in finely ground (<44 mesh) glass.Results and Discussion Leached samples without further heating Representative XRD traces of both unheated and heated materials are shown in Fig. 1. The unleached unheated material (Fig. 1A) contains a significant proportion of amorphous mate- rial, evidenced by the broad baseline hump centred at 28 =25", with a small amount of a crystalline quartz impurity (estimated as 1.6% by QXRD) and poorly crystalline mullite. Broad reflections are also present, corresponding to a cubic spinel phase with a cell parameter similar to y-A1203 (PDF no.10-425). Leaching for 10-30 rnin progressively reduces the amount of amorphous material, but never entirely removes it; the other poorly crystalline phases become more clearly visible with the J. Muter. Chem., 1996, 6(5),833-841 833 Q A I M M F 40 mln A M+A IA AI A+ A 1 1 I I I I I 1 I I I L 1 1 IJ 10 20 30 40 50 80 70 m~10 20 30 40 50 60 70 80 2#/degrees (Co-Ka) Fig. 1 Representative XRD traces of Zettlitz kaolinite, heated at 980 "C for 4 h, then leached in NaOH for the indicated times A, Leached but not reheated, B-D, reheated at 1140"C, E-G, reheated at 1300°C M =mullite (PDF no 15-776), Q=quartz (PDF no 33-1161), S=spinel-type y-alumina (PDF no 10-425) C =cristobalite, A =corundum, N =nepheline-type solid solution reduction of the amorphous component Leaching for phases identified by XRD in leached samples which were 40-90 min has little further effect on the remaining amorphous reheated to 1140 and 1300 "C The DSC traces of these samples material, but may produce a slight deterioration in the crystal- are shown in Fig 2, on which are also marked the mass losses linity of the other phases The 1 4-1 5% quartz content remains of each sample at 1350 "C unaffected by any of these leaching treatments The XRD XRD of the samples after thermal analysis shows that the results are summarised in Table 1, which also includes the unleached sample, which gave rise to an exotherm at 1232°C Table 1 Phases detected by XRD in NaOH-leached Zettlitz kaolinite, listed in approximate order of decreasing significance leaching time/min as-leached reheated to 1140°C reheated to 1300"C 0 M, Q, S(tr), am M, C, am 10 M, am M, am 20 M, S(tr), am M, A, am(tr) 30 M, S, am A, M, am(tr) 40 M, S, A(tr), am A, am(tr) 60 A, N, S, M, am A, N 90 A, N, S, M, am A, N Key M =mullite, S =cubic spinel-type phase, Q =quartz, C =cristobalite, A =corundum, N =nepheline-type solid solution, am =amorphous phase, tr =trace amount 834 J Muter Chem , 1996,6(5), 833-841 2.1% mass loss Unlwched leeched 20 mln k.ch.d#)mln Fig. 2 DSC traces of Zettlitz kaolinite, heated at 980 "C for 4h, then leached in NaOH for the indicated times.Heating rate 10°C min-' in air. with an enthalpy of -25.28 cal g- (Fig.2A),t contains reason- ably crystalline mullite, cristobalite, quartz and some residual amorphous material. Leaching for 10 rnin reduces the size and sharpness of the exotherm to an enthalpy of -12.42 cal g-l, and also lowers its peak temperature to 1167 "C (Fig. 2B). The product after heating this sample to 1350°C in the DSC is mullite and a small amount of residual amorphous material. In the DSC trace of the sample leached for 20min, the high- temperature exotherm is broadened, and possibly consists of two merged peaks with a mid-point temperature of ca. 1200 "C and a combined enthalpy of -23.12 cal g-' (Fig. 2C). The product after thermal analysis is well crystallised mullite and a small amount of residual amorphous material.Leaching for 30 rnin essentially removes the high-temperature exotherm (Fig. 2D), but a small endotherm at ca. 500 "C suggests the removal of some hydroxy water. The product from this sample consists of mullite, corundum and some amorphous residue. Leaching for 40 rnin produces little difference from the sample leached for 30min, but the heated product now contains significantly more corundum than mullite. After 60 rnin leach- ing, the first signs of an exothermic plateau are found at ca. 1270 "C (Fig. 2F), which becomes better defined after 90 rnin leaching (Fig. 2G). The crystalline products in both these samples are corundum and an aluminium-rich alkali-metal silicate similar to a nepheline solid solution (PDF no. 23-475), but in view of the present reaction conditions, our material must be the Na analogue.Typical 27Al and 23Na MAS NMR spectra of both unheated t 1 cal=4.184 J. and heated samples are shown in Fig. 3, and a selection of typical 29Si NMR spectra are shown in Fig. 4. The 27Al spectra (Fig. 3A,B) all contain an octahedral and a tetrahedral reson- ance at 6 ca. 6 and 58, respectively. The apparent growth of the tetrahedral resonance with respect to the octahedral as leaching progresses is misleading; the octahedral peaks have a high-field tail, probably arising from distortion of these sites. When the spectral intensity of the tail is taken into account by curve-fitting a third peak in this region, the proportion of octahedral spectral intensity ranges from 60 to 63%, i.e.within error it is essentially constant in all these spectra, but is considerably greater than expected for pure mullite (43 YO)?' The widths of the curves fitted to the octahedral region remain essentially constant with leaching time, but the fitted tetra- hedral width decreases smoothly up to ca. 60min leaching time, suggesting that the phases developing during leaching are better ordered than the amorphous phases originally present, at least with respect to the tetrahedral sites. As expected, the 29Si NMR spectra of the unheated samples (Fig. 4A-G) show considerable changes as leaching progresses. The resonance in the unleached sample (Fig. 4A) is at 6 ca. -110, in the region of uncombined SO2, but it may also partially mask a small signal at 6 ca.-88. On leaching for 10 min, the total Si intensity decreases and the 6 -88 peak becomes proportionally more intense (Fig. 4B), a trend which continues with further leaching (Fig. 4C). The resonance at 6 -88 arises from Si substituted by Al, and is in the spectral region of mullite,6 as well as other aluminosilicate phases. In order to satisfactorily simulate the 29Si NMR spectra several curves had to be fitted, the positions of which varied somewhat from sample to sample. In general, the fitted peaks occurred at 6 ca. -114, -108, -101 and -88, but not all the spectra required all these peaks for a satisfactory fit. Some typical curve-fitted 29Si NMR spectra are shown in Fig. 5. The relative contributions of these fitted peaks to the total spectral intensity changes with leaching time, as shown in Fig.6, in which the site distributions are shown as a percentage of the unleached Si remaining. The integrated intensities of the 29Si NMR spectra fall off with leaching time up to about 40 rnin leaching time (Fig. 7). From the chemical shifts and changes on leaching of the fitted 29Si NMR peaks (Fig. 6), the following assignments are suggested. (i) The species at 6 ca. -114 decreases to zero after 20min leaching, and is probably due to amorphous and uncombined SO2. (ii) The species at 6 -108 to -110 is removed more gradually, and eventually levels out at ca. 4-5% of the total Si intensity after 60 rnin leaching. Since the chemical shift of this resonance is in the region for quartz and cristoba- lite,' it must contain the quartz phase which is not removed by leaching, and probably represents the 4-5% residual 29Si intensity.The remainder of this resonance, which is removed slowly by leaching, could contain another silica-rich phase, since the fitted peak tends to be broad. The greater resistance of this species to alkali attack in comparison with the 6 -114 resonance may indicate the presence in this phase of the impurities originally occurring in the kaolinite (0.55% Na,O, 0.84% K20, 0.29% CaO, 0.27% MgO and 0.65% Fe203). (iii) The species at 6 ca. -101 has a chemical shift suggesting a degree of A1 substitution, and is similar to the 29Si chemical shift of metaka~linite.~ This species is relatively insignificant in the unleached sample, but it becomes more prominent during the initial stages of leaching, and disappears after 30 rnin reaction. Since no crystalline phase demonstrating this behaviour is detectable by XRD, it is assumed to be amorphous also, possibly a product of reaction with the sodium hydroxide.The disappearance of this species after longer reaction times may reflect the formation of other more stable alumina-rich alkali-metal silicates (see below). (iv) The resonance at 6 ca. -88, which becomes progressively more significant with leach- ing time, represents the most heavily Al-substituted species, J. Muter. Chem., 1996, 6(5),833-841 835 J 10 rnln /w46!50\ M 10 mln 200 100 0 -100 -200 Fig.3 Typical 11 7 T MAS NMR spectra of Zettlitz kaolinite, heated at 980 "C for 4h, then leached in NaOH for the indicated times A-G, 27Al NMR, [ref Al(H20)63+], H-M, 23Na NMR (ref NaCl solution) A, B, H, I, Not reheated C, D, J, reheated at 1140 "C, E-G, K-M, reheated at 1300"C Asterisks denote spinning side bands and will include the mullite-like parts of the structure, together with other aluminosilicates, the nepheline solid solution which crystallises from the amorphous phases in samples leached for 60-90 min is expected to have a 29S1 chemical shift similar to nepheline (6 -85 to -88)' and should therefore appear under this envelope The 23Na NMR signal from the unleached material was so weak as to be virtually indistinguishable from noise, but, by contrast, all the leached samples, both unheated and heated, show strong signals resulting from the incorporation of signifi- cant concentrations of Na (Fig 3H,I), apparent even after 10min leaching The amount of Na present, estimated from NMR peak area measurements, increases almost linearly with leaching time, up to 40 min, after which it becomes essentially constant A very good linear correlation (R=0 99) exists between the decreasing total Si intensity seen by NMR and the increasing Na NMR peak intensity as leaching progresses, 836 J Muter Chem, 1996,6(5), 833-841 suggesting that the NaOH is not just dissolving silica to form soluble sodium silicate species, but insoluble sodium silicate or aluminosilicate species must also be forming concurrently The resonance position and width at half height are essentially identical in all the unheated leached samples, independent of leaching time (Fig 3H,I), and are typical of less shielded, more isotropic Na environments such as those found in hydrother- mally treated sodium aluminosilicate glasses lo The present resonance positions (6 cu -2 to -3) are even closer to the hydrated Na' ion reference (6 0) than in hydrothermally altered glass previously reported (6 -7 7)," but are similar to the 23Na shifts found in halloysites hydrothermally treated with NaOH at 600-800°C containing incipient zeolite A (6 -1 9 to -4 5) and in commercial crystalline sodium zeolite A (6 -1 6) (our own unpublished results) These 23Na NMR shifts are all downfield of the values for Na in the interlayer sites of smectites, 6 -9 8 for Na-montmorillonite, 6 -9 0 for A H unleached -110.3 unleached -106.3 -84.0AA 10 min B I\ 10 mln C 20 min L/ \ K -1 10.8 D -88.9 unleached 30 min I \ E I\ JLM A-Fig.4 Typical 11.7 T 29Si MAS NMR spectra (ref. Me,Si) of Zettlitz kaolinite, heated at 900 "C for 4 h, then leached in NaOH for the indicated times. A-G, not reheated; H-J, reheated at 1140 "C; K-N, reheated at 1300"C. Na-bentonite and 6 -5.3 for Na-hectorite, our own unpub- lished results), consistent with the Na in the present leached samples being more highly hydrated than would be possible if the Na were in regular smectite-like interlayer sites.Removal of this hydration water by heating results in the expected upfield shift and broadening of the 23Na resonance (Fig. 3J-M) to an extent determined by the leaching time, and thus by the Na :Si ratio. The increased broadening and upfield shift may be predominantly due to an increased electric-field gradient (efg) at the Na, but chemical shift effects may also be present. At leaching times <60 min, the resonance position (6 -18.3 to -22, Fig. 3J-M) is typical of unhydrated sodium aluminosil- icate glass and rhyolite glass at 11.7 T (6 -18.4).11 At longer leaching times (i.e. lower Si content) the resonances move downfield and become narrower, especially in samples heated to 5300°C. The resonance positions in these samples (6 cu.-13) are similar to those reported in nepheline-kalsilite solid solutions, arising from the preferential occupation by Na of the crystallographically smaller of the two available alkali- metal sites in nepheline.12 Thus, the present 23Na NMR results are consistent with the incorporation of Na into the aluminosil- icate structure at the earliest stages of leaching, initially in a highly hydrated amorphous phase which on heating forms an anhydrous amorphous sodium aluminosilicate or an aluminous crystalline nepheline solid solution, according to the amount of Si available. A good linear relationship (R=0.94) also exists between the 23Na NMR intensity of the leached samples and the percentage of Si in the 29Si NMR peak at 6 -88.The 6 -88 resonance probably contains contributions from several phases, including mullite and various possible alkali-metal aluminosilicates which have 29Si peaks in this region (e.g. nepheline (NaAlSiO,) 6 -84, -88; jadeite [NaAl(Si03)2] 6 -91.8; albite (NaA1Si30,) 6 -92.8 to -104.7, natrolite (Na2A12Si,01,~H20) -87.7, -95.4).8 Thus, the NaOH may 6 J. Muter. Chem., 1996, 6(5),833-841 837 A Leeched 10 min A -20 40 -100 -1 40 -1 80 s Fig.5 Curve-fitted 29S1 MAS NMR spectra of Zettlitz kaolinite, leached as indicated Observed spectrum is upper trace of each group, fitted envelope in middle and component peaks at bottom of group be reacting to form insoluble sodium aluminosilicates which are providing a major contribution to the 6 -88 NMR resonance.Leached samples heated to 1140 and 1300 "C A selection of typical XRD traces of samples heated to 1140 and 1300°C are shown in Fig. 1B-G. The chemical analyses of the samples heated to 1140°C are shown in Table 2. On heating to 1140 "C, the XRD trace of the sample leached for 10 min (Fig. 1B) shows only reasonably well crystallised mull- ite, in contrast with the unleached material which under the same heat treatment contains mullite, unreacted quartz, a trace of cubic spinel and significant amorphous material. Heating to 1300°C for 30 min produces mullite of better crystallinity in both the 10 min leached and unleached samples; in the latter the quartz was completely converted to cristobalite, there was no trace of cubic spinel, and the amount of amorphous phase had been considerably reduced, as judged from the curvature of the diffraction baseline.The 20 min leached sample heated at 1140 "C contained mullite and a small trace of cubic spinel. Heating this sample 838 J. Muter. Chem., 1996,6(5), 833-841 20 leaching time/min Fig. 6 Distribution of Si amongst the vanous sites in leached Zettlitz kaolinite, estimated from the fitted 29S1 NMR spectra, as a function of leaching time 0U-A 0 20 40 60 80 leaching time/min Fig. 7 Integrated intensity of total 29S1 NMR signal in leached Zettlitz kaolinite, as a function of leaching time Table 2 Chemical analyses of unleached and leached Zettlitz kaolinite, heated to 1140"C leaching time/min Si02(%) A1203 (Yo) Na20 (YO) 0 54.5 45.1 0.28 10 45.2 51.4 2.98 20 36.3 58.5 4.11 30 32.8 61.9 4.3 1 40 29.2 64.5 5.10 60 25.5 65.2 7.27 90 25.9 64.4 7.05 at 1300°C improved the mullite crystallinity and led to the formation of a small amount of corundum (a-A1203), at the expense of both the spinel and the amorphous phase; appar- ently even at this early stage in the leaching, the amorphous phase is becoming aluminium-rich.The sample leached for 30min and heated at 1140°C (Fig. 1C) contains mullite and cubic spinel which appears more distinctly crystalline than with 20 min leaching. Heating this sample at 1300°C produces a small amount of mullite and a considerable amount of corundum, again with the disappear- ance of spinel and a reduction in the amount of amorphous phase. Spinel is still evident in the sample leached for 40min and heated at 1140 "C, together with mullite and small amounts of corundum and amorphous phase.Heating at 1300 "C produces only crystalline corundum, with some residual amorphous phase still evident (Fig. 1F). When heated to 1140"C, the samples leached for 60 and 90min contain, in addition to corundum, the nepheline solid solution (PDF no. 23-475) identified in the thermal analysis samples (see above), together with a small amount of mullite, the cubic spinel phase and a significant amount of amorphous material (Fig. 1D). Heating to 1300°C completes the conver- sion of the mullite, spinel and amorphous material to crystalline corundum and nepheline solid solution (Fig.1G). The XRD results for the reheated samples are summarised in Table 1. The 27Al NMR spectra of samples leached for up to 30min and heated at 1140 "C (Fig. 3D) are all broadly consistent with previously published spectra of m~llite,~?~ containing an octa- hedral resonance at 6 -1.9 with a marked upfield tail, and a tetrahedral resonance at 6 47-53, which, however, shows no evidence of partial resolution into two components which is characteristic of highly crystalline mullite.6 The proportion of tetrahedral signal, estimated by curve-fitting the spectra, pro- gressively increases from 51% in the unleached sample to 60% in the sample leached for 30min; this is within the range previously reported for mullite derived from heated clay.7 Similar spectra were recorded for the mullite-containing samples heated at 1300 "C (Fig.3E,F). The onset of corundum formation is marked by the appear- ance of the characteristic octahedral resonance at 6 ca. 10-11 in samples leached for 40-90 min and heated at 1140 "C, and in samples leached for 20-90 min heated at 1300 "C (Fig. 3F,G). The proportion of tetrahedral resonance decreases in these samples as the corundum resonance grows with leaching time and heating temperature. The position of the single tetrahedral resonance in the samples containing nepheline solid solution (Fig. 3G) is in the range expected for fully polymerised frame- work aluminosilicates," but differs from the spectra reported for natural nepheline, which contain two resolvable tetrahedral resonances corresponding to two distinct A1 sites.12 The 27Al NMR spectra are thus generally consistent with the XRD data.The 29Si NMR spectra of the samples heated at 1140°C (Fig. 4H-J) are simpler than the corresponding unheated spec- tra (Fig. 4A-G). The unleached sample consists of two clearly resolved components, that at 6 -84 corresponding reasonably to the major mullite resonance at 6 -86.8,6 and that at 6 -106.3 corresponding to uncombined silica and quartz (6 -107.1).8 The change in this peak position to 6 -109 on heating to 1300 "C (Fig. 4K) reflects the conversion of the quartz to cristobalite (6 -108.5ppm).' In the samples leached for 10min the presence of mullite is still indicated by the shoulder at 6 ca.-85 to -86 (Fig. 4I,L), but the major component of the resonance intensity (probably associated with the residual amorphous phase) has moved downfield to 6 -97. The ,'Si NMR spectra of samples leached for times >10 min and reheated at 1140 and 1300 "C, all contain a single broad, featureless peak, the centre of which progressively moves downfield with increasing leaching time. This single peak envelope must contain contributions from the mullite and alkali-metal aluminosilicate phases, both amorphous and crystalline; all these spectra were therefore fitted with between one and four curves, some of which are shown in Fig. 8. Satisfactory fits for most spectra were obtained with three peaks (Fig. 8A,B), which in all the samples containing XRD indications of mullite and amorphous phase (but no significant cubic spinel) occur at 6 ca.-84, -95 and -102 (Fig. 8A). Samples containing mullite, cubic spinel and amorphous phase are best fitted by three similar peaks (Fig. 8B), and show no indication of an additional peak at 6 ca. -80, the calculated position of an Si-containing pin el;^ this question is discussed further below. The heated sample containing only corundum and amorphous phase would be expected to contain only 29Si resonances from the amorphous phase, and is best fitted by two peaks, at 6 -88.5 and -96.1 (Fig. 8D), suggesting that the peaks at 6 ca. -90 and -100 in the spectra of less-reacted samples are associated with the amorphous phase.By this reasoning, the peak at 6 -84 to -85 in the mullite-containing samples must arise from mullite. Although this peak is slightly downfield of the resonance reported for well crystallised mullite (6 -86.8),6 it is within the error expected in the fitting of broad spectra. The sample containing only crystalline nepheline solid solution and corundum is best fitted by two peaks, at 6 -86.5 and -92.4 (Fig. 8C), in reasonable agreement with the reported peak positions for nepheline (6 ca. -84 and -89);12 the incorporation of additional A1 into this phase does not appear to change its ,'Si NMR spectrum greatly. The similarity between the fitted 29Si NMR spectra of nepheline solid solution (Fig.8C) and the amorphous phase (Fig. 8D) suggests that the latter is the source from which the nepheline solid solution crystallises, and must therefore approach a similar composition in the later stages of leaching. During the course of the 29Si NMR curve-fitting, consider- able effort was made to locate and fit peaks below 6 -85, in the region expected for Si contained in the cubic spinel phase. Although the results for the reheated samples containing XRD indications of spinel were not convincing, inspection of the 29Si NMR spectra of some of the unheated leached samples (Fig. 4) suggests the presence of extra spectral intensity in this region, to which a small peak could be fitted in some cases. Fig. 8E shows an example of such a fit, in which the downfield shoulder has been fitted by a peak at 6 -76.2, representing a highly Al-substituted Si site such as could arise from an Si- containing y-alumina-like spinel.Assuming this peak assign- ment, and taking into account the significant amount of spinel indicated by XRD in this sample, the small number of such silicons, representing 0.9% of the total Si intensity, suggests a rather low degree of Si substitution in this spinel. Another possible explanation of this result is that early-stage leaching may remove Si from the spinel originally formed at 980"C, producing the observed cubic phase with a composition more like pure y-Al,03. Constitution of the amorphous phases deduced from AAS and QXRD analyses The analysis of the unleached material (Table 2) indicated an excess of ca. 20% SiO, above that required to ultimately form J.Muter. Chew., 1996, 6(5),833-841 839 A 10 mln, 1140 OC E DnB 40 mln, 1300 OC I I I I 1 A 60 80 -100 -120 -140 -160 6 1 1S.II1 1 0 -100 -200 0 -100 -200 Fig. 8 Typical curve-fitted 29S1NMR spectra of Zettlitz kaolinite, leached, then reheated as indicated E, leached in NaOH for 10 min, curve-fitted to five peaks 3 2 mullite, the low Na content suggests that this SiO, is relatively pure, but the analysis figures provide no indication of possible substitution of the Si into the spinel phase in the unleached sample On leaching for 10 min, Si02 is removed with a correspond- ing incorporation of Na20 in the sample Based on the 3 2 mullite composition which is ultimately achieved, this sample will contain 25% S102 which is surplus to its requirements for mullite formation, which, together with the analysed Na con- tent, calculated to an Na Si 0 ratio of 1 4 3 9 1, i e if the sodium is assumed to be associated with the amorphous silica component, its composition will be close to NaSi40, Applying similar reasoning to the sample leached for 20 min, but correcting the mullite composition for the 11 3% corundum estimated by QXRD in the sample heated at 1300"C, the composition of the amorphous phase (17 8% S102 to 4 1% Na,O) calculated to an Na Si 0 ratio of 1 22 5, ze the amorphous phase approximated to NaSi20, The same calculation for the sample leached for 30min leads to the same composition for the amorphous phase, although further SiO, has been removed from the sample, this is compensated for by the ultimate formation at 1300°C of a greater amount of corundum (26 6%), maintaining the Na Si 0 ratio at 1 227 5, corresponding to an amorphous phase composition of approximately NaSi,O, In contrast, after leaching for 40min, the only detected crystalline phase, after the thermal reactions are completed at 1300 "C, is corundum, estimated by QXRD to be 66 7%, in satisfactory agreement with the value for total A1203 (64 5%) The analysed Si02 and Na20 contents, which must eventually end up solely in the amorphous phase, correspond to an Na Si 0 ratio of 1 3 6 4,i e a composition of approximately NaSi306 After leaching for 60-90min, the compositions of the 840 J Mater Chem , 1996, 6(5),833-841 samples are virtually unchanged, but QXRD indicates the formation of 39 3% corundum, the balance being a nepheline- type solid solution with a calculated composition of NaA122S1,,0,, If the total Na content of this sample is assumed to be associated with a normal nepheline, NaAlSiO,, the additional material forming the solid solution contains both A1 and Si, with a composition All ,SiO 903 ,, z e the additional phase corresponds to O27moles of 3 2 mullite Thus, the nepheline-like phase has a composition which can be written 0 27(A1,S1,0,3) NaAlSiO, The structure of such a compound, if indeed it can be formed, is presently unknown Conclusions When Zettlitz kaolinite is heated at 980°C then leached with NaOH for short times (ca lOmin), the first species to be removed is uncombined amorphous SiO,, characterised by a 29S1 NMR peak at 6 ca -114 Crystalline SiO, (quartz) is largely resistant to alkaline attack A significant amount of Na and associated hydration water also comes into combination, probably with an amorphous aluminosilicate phase which is also relatively stable to NaOH attack, and contributes signifi- cantly to a 29S1 NMR peak at 6 ca -88 The removal of ca 30% of the total Si in this stage (based on NMR measurements) is sufficient to produce an overall mullite stoichiometry, since heating this material to 1300°C converts all the spinel and quartz and essentially all of the amorphous phase to mullite Leaching heated kaolinite for 20-30 min removes 50-60% of the total Si, principally from the amorphous phase which continues to retain Na, and contains Si sites not unlike nepheline, although even at this stage, it may be Al-rich with respect to nepheline composition The system now contains insufficient Si to form mullite with all the available Al, since on heating to 1140°C the crystalline silica and most of the spinel is taken into the amorphous phase, which forms a mixture of corundum and mullite on heating to 1300°C.Leaching for 40min removes a further 10% Si from the amorphous aluminosilicate phase, bringing about the forma- tion of some corundum even at 1140 "C.Heating to 1300 "C produces crystalline corundum at the expense of the mullite, spinel and amorphous phase still present at 1140°C. The remaining Si is located in a small amount of residual amorph- ous phase which also contains Na and tetrahedral Al, according to 27Al NMR data. After 60-90 min leaching, no further Si is removed, but the system now contains sufficient Na and A1 to form a solid solution of nepheline with alumina when heated at 1140 "C. Heating to 1300°C converts all the remaining mullite, spinel and amorphous phase into the nepheline solid solution, the excess A1 appearing as corundum. The leaching sequence and subsequent thermal reactions can therefore be understood in terms of the progressive removal of Si from the system, principally from the amorphous silica and aluminosilicate phases, the latter becoming progressively substituted by Na.We are indebted to Mr. D. Gedye for the thermal analyses and to Dr. R. Goguel for the AAS analyses. References 1 N. H. Brett, K. J. D. MacKenzie and J. H. Sharp, Quart. Rev. Chem. Soc., 1970,24,185. 2 (a)G. W. Brindley and M. Nakahira, J. Am. Ceram. Soc., 1959,42, 319; (b) A. K. Chakravorty and D. K. Ghosh, J. Am. Ceram. Soc., 1991,74,1401. 3 K. J. D. MacKenzie, I. W. M. Brown, R. H. Meinhold and M. E. Bowden, J.Am. Ceram. Soc., 1985,68,293. 4 A. K. Chakravorty and D. K. Ghosh, J. Am. Ceram. Soc., 1978, 61, 170. 5 A. K. Chakravorty, J. Muter. Sci.,1992,27,2075. 6 L. H. Merwin, A. Sebald, H. Roger and H. Schneider, Phys. Chem. Mineral., 1991, 18,47. 7 K. J. D. MacKenzie, R. H. Meinhold, G. V. White, C. M. Sheppard and B. L. Sherriff,J. Muter. Sci., 1994,29,2611. 8 B. L. Sherriff, H. D. Grundy and J. S. Hartman, Eur. J. Mineral., 1991,3,751. 9 J. F. Stebbins, J. B. Murdoch, I. S. E. Carmichael and A. Pines, Phys. Chem. Mineral., 1986,23, 371. 10 W-H. A. Yang and R. J. Kirkpatrick, Geochim. Cosmochim. Acta, 1989,53, 805. 11 W-H. A. Yang and R. J. Kirkpatrick, Am. Mineral., 1990,75, 1009. 12 G. L. Hovis, D. R. Spearing, J. F. Stebbins, J. Roux and A. Clare, Am. Mineral., 1992,77, 19. Paper 5/07325A; Received 7th November, 1995 J. Muter. Chem., 1996, 6(5),833-841 841

ISSN:0959-9428    

DOI:10.1039/JM9960600833

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

A 23NaNMR study of hydrous layered silicates Graham G. Almond,' Robin K. Harris,*' Kevin R. Franklinb and Peter Grahamb 'Department of Chemistry, University of Durham, Science Laboratories, South Road, Durham, UK DHl 3LE budever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK L63 3JW Sodium-23 single-pulse NMR spectra have been obtained for the layered sodium polysilicate hydrates, makatite, kanemite, octosilicate, magadiite and kenyaite. The kanemite spectra were severely distorted by second-order effects from strong quadrupolar interactions. However, the regular behaviour of the linewidth with frequency allowed the isotropic chemical shift and quadrupole interaction constant for the single peak to be deduced. The spectra of octosilicate, magadiite and kenyaite contain either one or two peaks.Powder X-ray diffraction indicates that the spectra with two lines correspond to samples contaminated by sodium chloride, which is a result of relevance to the conclusions of two previous authors. A 'H,23Na heteronuclear shift correlation spectrum for magadiite shows that strongly hydrogen-bonded protons are responsible for the single magadiite 23Na cross-polarisation signal. The presence of these hydrogen-bonded protons, and the detection of a single sodium site, indicate that there are significant differences between the interlayer conformations of kanemite, octosilicate, magadiite and kenyaite and the known structure of makatite, which has no hydrogen-bonded protons and two sodium sites. The 23Na isotropic chemical shifts of kanemite, octosilicate, magadiite and kenyaite indicate that the relevant sodium species have coordination numbers of five or six; both arrangements are known to occur with makatite.The estimated quadrupole interaction constants infer that the value is five for kanemite and six for octosilicate, magadiite and kenyaite. Makatite, kanemite, octosilicate, magadiite and kenyaite are related silicates with typical empirical formulae, Na20-(4-22)si02 .( 5-10)H20.1 They can be synthesised readily,2 while all but octosilicate have been found in nat~re.~ They are clearly hydrous, and it has long been understood that they are layered materials, containing sodium ions which are susceptible to facile ion-exchange.Such properties suggest potential appli- cations in catalyst and detergent systems. The structure of makatite (Na20* 4sio2 * 5H@) has been fully resolved by single-crystal X-ray diffra~tion,~ and it contains single silicate layers with two distinct interlayer sites for hydrated sodium ions. A similar arrangement might be expected in the other silicates, with thicker silicate layers in octosilicate, magadiite and kenyaite, as in the hypothetical model structures proposed by several author^.'.'.^ Many 29Si NMR spectra of these layered sodium polysilicate hydrates have been reported. These can provide information concerning the multiplicity of distinct silicon sites and also their connectivity, Q3 or Q4.7Our studies have detected water and strongly hydrogen-bonded silanol protons using 'H MAS NMR spectroscopy;' this has recently been confirmed by Apperley et al.for kanemite.g However, the literature on 23Na NMR spectroscopy for these potentially useful silicates is incomplete and sometimes misleading. Two previous workers have published 23Na NMR spectra for octosilicate, magadiite or kenyaite. Rojo et al. detected two different sodium sites in natural magadiite." The higher- frequency signal corresponded to sodium ions which were more readily exchanged with protons on titration with dilute acid, and the signals were assigned on the basis of their position in the interlayer space. Harris and Nesbitt found two signals in 23Na single-pulse (SP) spectra of some magadiite and kenyaite samples.','' The higher-frequency signal was not present in 23Na cross-polarisation (CP) spectra and was assigned to isolated intralayer sodium ions.However, it seems unlikely that an isolated sodium ion would be more susceptible to ion-exchange, and the work reported here will provide significant new information. Additionally, the mechanism for 23Na CP in magadiite will be elucidated from a two-dimen- sional 23Na,'H heteronuclear correlation spectrum. Theory The nucleus 23Na has spin 3/2. Thus it is quadrupolar, leading to phenomena which would not be observed in the 29Si or 'H NMR spectra of these silicates. There are three possible transitions for nuclear magnetic resonance, with frequencies that depend on the strength of the quadrupolar interaction. Weak interactions lead to first-order effects, where the central (%--%) transition is unaffected, while the outer transitions can give a broad pattern in a powder sample.Stronger interactions lead to second-order effects: the central transition can be distorted and shifted from the isotropic position, 6,,, (to an apparent shift, a*), while the outer transitions can be too broad for detection. The distortion of the central transition, 6,,,-6*, is given by eqn. (1),l2 where vL and x correspond to the Larmor frequency and the quadrupole interaction constant (e2qQ/h),respectively. It is obvious that the magnitude of any distortion decreases as the magnetic field is increased. Spectra of spin-4 nuclei tend to contain discrete signals which occur at informative chemical shifts and can be counted and quantified.Because of strong quadrupolar interactions, this is not always feasible for nuclei with spin >i.Magic-angle spinning does not completely average second-order effects because their angular dependence is not simply described by a (3 cos2 8-1) term, and a range of special techniques is available for line-narrowing: working with as high a magnetic field as possible; dynamic-angle spinning (DAS),13 where a spinning rotor can flip between two angles; variable-angle spinning (VAS),14 using an angle other than 54.7"; and double rotation (DOR)." For DOR, the sample is contained in a special rotor which allows rotation around two axes. The combination of orientations can provide efficient line-nar- rowing.Alternatively, a series of MAS spectra at different spectrometer frequencies can provide isotropic 23Na chemical shifts, through use of eqn. (1); this will also provide an estimate for the quadrupole interaction constant. Koller et al.16 studied the interpretation of 6,, and x, showing that they can be J. Muter. Chem., 1996, 6(5),843-847 843 related to the conformation of oxygen ions around the relevant sodium ion A high-frequency chemical shift corresponds to a high coordination number, while the value of the quadrupole interaction constant varies with the symmetry of the coordi- nation sphere Experimental The silicate samples used in this work were obtained from a variety of sources, and powder X-ray diffraction was used to check their integrity An important result from this screening process is discussed in the Results and Discussion section The powder patterns were acquired with a Phillips PW1050 powder diffractometer The results were analysed using Sietronics trace processing software There is some attenuation of lines at low values of 28, which was caused by the geometry of the sample holder The sodium chloride sample was obtained from BDH Sodium-23 MAS NMR spectra were acquired with Chemagnetics CMX200, Bruker CXP200, Varian VXR300 and Varian Unity Plus 300 spectrometers using a variety of Bruker, Doty and Chemagnetics probes In general, pulse durations were optimised for maximum signal intensity for each sample Magic-angle spinning spectra were referenced through replace- ment by a static sample of aqueous sodium chloride solution (1 mol dmP3) Though this is not necessarily the conventionally accepted reference, it has been used consistently to provide chemical shifts with a reproducibility of +O 5 ppm The two- dimensional 1H,23Na heteronuclear correlation CP spectrum was acquired with the pulse sequence of Vega17 and Fyfe et al l8 Hartmann-Hahn matching conditions were set using octosilicate All measurements were carried out at ambient probe temperature Results and Discussion Makatite and kanemite 23Na SP spectra Only one sample of makatite was available, and a spectrum was obtained at one frequency (79 346 MHz) This showed a broad unresolved signal around 6 -3 (see Fig 1) DOR spectra were obtained, courtesy of the Chemagnetics company These contained many spinning side-bands, but were consistent with the existence of two sodium sites in the system However, the resulting uncertainties in the chemical shifts render them too unreliable, in our view, to quote Single-pulse (SP) 23Na spectra of kanemite have been acquired with two spectrometer frequencies The 79 346 MHz and 52 938 MHz spectra are displayed in Fig 1, while Table 1 lists apparent chemical shifts and a measure of the total width of the absorption The spectra are distorted, the apparent chemical shift varies considerably with spectrometer frequency, and a sharper spectrum is seen at the higher magnetic field This indicates that these are spectra exhibiting second-order quadrupolar effects, showing the central transitions only The line-widths of these spectra behave regularly with spectrometer frequency, z e A+ cc l/v, Therefore, the distorted signals are likely to come from a single sodium species, rather than from the overlap of more than one line This is confirmed by a 23Na DOR spectrum on this sample (not shown), which consists of a single spinning side-band manifold Eqn (1) can be used to calculate an approximate value for the isotropic chemical shift, 6,,,, and the quadrupole interaction constant, x This can be attempted with reasonable precision as there is a large discrep- ancy between the two apparent chemical shift values With the reasonable simplification of ( 1+q2/3)3z 1, the following values are obtained 6,,,=5 ppm, x=2 MHz Octosilicate, magadiite and kenyaite 23Na SP spectra Harris and Nesbitt showed that some magadiite and kenyaite samples had two signals in their 23Na SP spectra, while others 844 J Muter Chem , 1996, 6(5),843-847 1 1 I I I I I I 1 200 0 -200 Fig.1 Single-pulse 23Na NMR spectra of (a) makatite (79 346 MHz, 200 transients, 2 s recycle time, spinning rate 5 2 kHz), (b) kanemite (79 346 MHz, 500 transients, 2 s recycle time, spinning rate 5 1 kHz), (c) kanemite (52 938 MHz), 1024 transients, 1 s recycle time, spinning rate 4 1 kHz) Table1 Apparent chemical shifts and widths for 23Na SP spectra of kanemite at two magnetic fields spectrometer apparent chemical half-width, frequency/MHz shift, dNa* A1,2/kHz 79 346 -12 1 22 52 938 -33 2 29 only had one ' In this work, octosilicate has been discovered to exhibit the same variety Fig 2 and 3 show 52 938 MHz 23Na SP spectra for samples with one peak and two peaks, respectively, while Table 2 lists apparent chemical shift and peak-width data All the spectra contain signals around BNa* -4 The extra signal, around dNa* 7, is only observed in a few samples and has quite different NMR properties to the other signal an extensive spinning side-band manifold, substan- tially longer Z3Na spin-lattice relaxation times (ca 10 s rather than ca 10 ms), and no signal on cross-polarisation Using a higher magnetic field shows another difference (The spectra are too similar to Fig 2 and 3 for display to be worthwhile, but apparent chemical shifts and peak-widths are also listed in Table 2) While the low-frequency peaks shift (by ca 2 ppm) and sharpen, in accordance with second-order quadrupolar effects, the rarer high-frequency peak changes little (A6 <0 5 ppm, the reproducibility of the referencing), in accord- ance with first-order effects Approximate isotropic chemical shift and quadrupole interaction constant values for the low frequency peaks can again be predicted from eqn (1) Apparent chemical shifts, dNa* -2 and -4 at the two spectrometer frequencies give the following values 6,,, x0 ppm and xz06 MHz, for all these silicates These are very approximate as the second-order quadrupolar distortion was small A 200 0 -200 I""I""I""1""I""I""I""I""I""I"'l 50 0 -50 Na Fig.2 Single-pulse 52.938 MHz 'jNa NMR spectra of layered sodium polysilicate hydrates exhibiting a single signal: (a) octosilicate (16 transients; 2 s recycle time; spinning rate 2.1 kHz); (b)magadiite (100 transients; 1 s recycle time; spinning rate 2.0 kHz); and (c) kenyaite (260 transients, 1 s recycle time; spinning rate 2.0 kHz) Identification of the extra signal using powder X-ray diffraction Powder X-ray diffraction has been used to identify the species responsible for the second signal seen in some samples of octosilicate, magadiite and kenyaite. Fig. 4 illustrates this with respect to two octosilicate samples.Two powder patterns are shown. Both are similar to the one reported by Borbkly et al.,19 so the samples are clearly octosilicate. However, the trace for sample 2 contains several extra lines. These are emphasised by the difference pattern: positive lines come from an impurity, while the negative signals correspond to octosilicate. The final pattern is sodium chloride, which is clearly responsible for the extra lines in the trace for sample 2. We conclude that sodium chloride is responsible for the extra signal seen in some octosilicate, magadiite or kenyaite 23Na SP spectra.l.'*,ll Sodium chloride has a chemical shift of 6 ca. 7. It also contains no protons, so no 1H,23Na CP signal would be expected and long spin-lattice relaxation times are under-standable. The spinning side-bands are somewhat surprising for a compound with a cubic structure, but they are also seen with pure sodium chloride.Heteronuclear 1H,23Na correlation spectroscopy for magadiite Cross-polarisation is an inherently selective technique, as it is only efficient between neighbouring species of a suitable rigid- ity. In fact, the contaminated samples of octosilicate, magadiite and kenyaite are a classic case of this as no CP signal is seen for sodium chloride, because the sodium ions have no neigh- bouring protons. The two-dimensional heteronuclear shift cor- Fig. 3 Single-pulse 52.938 MHz 'jNa NMR spectra of layered sodium polysilicate hydrates exhibiting two signals: (a) octosilicate (12 transi-ents; 60 s recycle time; spinning rate 4.1 kHz); (b) magadiite (156 transients; 100 s recycle time; spinning rate 3.9 kHz); and {c) kenyaite (244 transients; 10 s recycle time; spinning rate 3.9 kHz) relation cross-polarisation method of Vega and Fyfe et al.allows interactions between sodium ions and protons to be selectively In this experiment, the equivalent of a 'H MAS spectrum is acquired in the tl period of a two-dimensional CP experiment. For 1H,23Na CP, double Fourier- transformation gives a spectrum with 'H and 23Na chemical shift axes. Cross-peaks identify the source of proton magnetis- ation for each 23Na site. Such experiments for 23Na and 29Si CP in octosilicate and 29Si CP in magadiite have been reported Fig. 5 shows a 1H,23Na heteronuclear correlation spectrum for maga- diite.For this sample and 29Si CP, careful sample-drying was needed to obtain a selective spectrum. Otherwise proton spin- diffusion or chemical exchange was too extensive over the timescale of the experiment (8 msj for the undried material, and an ambiguous result was obtained. In this case, a shorter optimum contact time was feasible (2 msj, and we believe the effect of spin-diffusion or chemical exchange can be ignored. A 'H MAS spectrum of magadiite contains two peaks at BH 4 and 15, corresponding to water and strongly hydrogen-bonded protons;' the 23Na CP spectrum contains a single signal at BNa* -4. The two-dimensional spectrum contains a singlecross-peak at the apparent chemical shift of the single 23Na CP signal.In the proton dimension, this occurs at the chemical shift of the hydrogen-bonding protons rather than that of the water. Therefore, the cross-polarisation mechanism can be Table 2 Apparent chemical shifts and widths for 'jNa SP spectra of two types of sample for each of octosilicate, magadiite and kenyaite spectrometer frequency =52.938 MHz spectrometer frequency =79.346 MHz silicate &a* A l,Z/HZ &a* 4/2/Hz dNa* AI/Z/HZ bNa* A l,'lHZ octosilicate -4.0 330 -2.1 210 7.0 150 -4.8 430 7.1 125 -2.4 200 magadiite -4.6 350 -2.6 225 7.1 210 -3.1 320 7.3 140 -1.7 200 ken yaite -4.3 400 -1.9 200 7.5 250 -4.0 400 7.1 200 -2.3 225 J. Muter. Chem., 1996, 6(5),843-847 845 01020304050607080 2Bldegrees Fig.4 Powder XRD patterns of (a) octosilicate (one 23Na SP signal); (b) octosilicate (two 23Na SP signals); (c) a difference pattern; and (d) sodium chloride -10 1 Fig. 5 Two-dimensional 1H,23Na heteronuclear correlation CP spectra for magadiite. The two-dimensional data set was built up of 95 FIDs of 200 transients with a recycle time of 3 s, a contact time of 2 ms and a spinning rate of 4 kHz, while the spectral widths were 20.0 and 33.3 kHz in the sodium and proton dimensions, respectively. deduced: 23Na CP in magadiite occurs through a transfer of magnetisation from the hydrogen-bonding protons to the single sodium site. Conclusions The aim of this work was to obtain the number of distinct sodium species in four silicate samples using 23Na NMR 846 J.Muter. Chew., 1996,6(5), 843-847 spectroscopy. Two problems have been surmounted to achieve this. First, conventional 23Na SP spectra of kanemite were heavily distorted by second-order effects from strong quadru- polar interactions. However, this distortion was regular enough for the conclusion that kanemite has a single type of sodium species with an isotropic chemical shift of 6 ca. 5. Secondly, powder X-ray diffraction has been used to identify the second 23Na SP signal that has been observed in some octosilicate, magadiite and kenyaite samples as arising from sodium chlor- ide contamination. The other signal is ubiquitous and can be related to a single distinct type of sodium ion, with an isotropic chemical shift of 6 ca.0. The interlayer space of the related silicate, makatite, is known to contain two types of sodium ion. Our 23Na DOR spectra of this silicate (not shown) contain more than one signal. Only one sodium site is indicated by the 23Na NMR spectra of kanemite, octosilicate, magadiite and kenyaite. This observation can be compared with 'H NMR information for these silicates. In this work, hydrogen-bonded protons have been shown to provide the source of 23Na cross-polarisation in magadiite. Such protons have not been detected for makatite but are found for the other layered sodium polysilicate hydrates. Therefore, it is clear that the known structure of makatite does not provide a perfect model for the interlayer space in kanemite, octosilicate, magadiite and kenyaite.Nevertheless, the calculated isotropic chemical shifts and quadrupole interaction constants of kanemite, octosilicate, magadiite and kenyaite can be used to propose some structural conclusions, with respect to the coordination of oxygen atoms around each sodium ion. In makatite,4 the conformation of the two sodium sites are octahedral and trigonal bipyramidal, i.e. with coordination numbers of six and five, respectively (see Fig. 6). In an empirical study of several sodium-containing compounds (including several silicates), Koller et al. reported Z3Na chemical shift ranges of 6 -22-8 and -15+20 for coordination numbers five and six, respectively.16 The meas- ured isotropic chemical shifts of kanemite, octosilicate, magadi- ite and kenyaite (6 5 and 0), are in the overlap between these ranges.Therefore the relevant sodium species will have coordi- nation numbers of five or six. Kanemite has a significantly larger quadrupole interaction constant than octosilicate, maga- diite and kenyaite. Since the symmetry of the coordination sphere around the sodium ions will affect the magnitude of x, a less symmetrical environment can be predicted for kanemite. A five-coordinate system will be relatively asymmetric, so a coordination number of five can be tentatively proposed for the sodium ions in kanemite. Similarly, an octahedral system will be relatively symmetric, so a coordination number of six Fig.6 A schematic diagram of the makatite structure showing Si04 tetrahedra, water oxygen atoms (0)and sodium ions (0).Oxygen atoms lie at the corners of the Si04 tetrahedra, so sodium ions are present with coordination numbers of five and six.According to the notation of Annehed et al., this is a projection along the a crystallo-graphical axk4 can be tentatively proposed for the sodium ions in octosilicate, magadiite and kenyaite. We are grateful to the UK SERC for a research studentship for one of us (G.G.A.) under the CASE scheme (in collaboration with Unilever Research), for research grant 597557, and for access to the UK national solid-state NMR service based at Durham. We thank Dr. D. C. Apperley for useful discussions and practical assistance with the VXR300 and Unity Plus 300 spectrometers, Dr.C. Ridenour and Chemagnetics for provid- ing the DOR spectra, and Dr. C. W. Lehmann for help with powder X-ray diffraction. References 1 G. J. Nesbitt, PhD Thesis, University of Durham, 1986. 2 W. Schwieger, K.-H. Bergk, D. Heidemann, G. Lagaly and K. Beneke, 2.Kristallogr., 1991, 197, 1; K. Beneke and G. Lagaly, Am. Mineral., 1977, 62, 763; R. K. Iler, J. Coll. Sci., 1964, 19, 648; R. A. Fletcher and D. M. Bibby, Clays Clay Miner., 1987,35, 318; K. Beneke and G. Lagaly, Am. Mineral., 1983,68,818. 3 R. L. Hay, Contrib. Mineral. Petrol., 1968, 17, 225; Z. Johan and G. F. Maglione, Bull. SOC.Fr. Mineral. Cristallogr., 1972, 95, 371; H. P. Eugster, Science, 1967, 157, 1177. 4 H. Annehed, L.Falth and F. J. Lincoln, 2. Kristallogr., 1982, 159, 203. 5 W. Schwieger, D. Heidemann and K-H. Bergk, Rev. Chim. Miner., 1985,22,639. 6 A. Brandt, W. Schwieger and K-H. Bergk, Rev. Chim. Miner., 1987, 24, 564. 7 F. Liebau, Structural Chemistry of Silicates, Springer-Verlag, Berlin, 1985. 8 G. G. Almond, R. K. Harris and P. Graham, J. Chem. SOC., Chem. Commun., 1994,851. 9 D. C. Apperley, M. J. Hudson, M. T. J. Keene and J. A. Knowles, J. Mater. Chem., 1995,5, 577. 10 J. M. Rojo, E. Ruiz-Hitzky and J. Sanz, Inorg. Chem., 1988, 27, 2785. 11 R. K. Harris and G.J. Nesbitt, J. Magn. Reson., 1988,78,245. 12 E. Lippmaa, A. Samoson and M. Magi, J. Am. Chem. SOC., 1986, 108,1730. 13 K. T. Mueller, B. Q. Sun, G. C. Chingas, J. W. Zwanziger,T. Terad and A. Pines, J. Magn. Reson., 1990,86,470. 14 S. Ganapathy, S. Schramm and E. Oldfield, J. Chem. Phys., 1982, 77,4360. 15 A. Samoson and E. Lippmaa, J. Magn. Reson., 1989,84,410. 16 A. Koller, G. Engelhardt, A. P. M. Kentgens and J. Sauer, J. Phys. Chem., 1994,98, 1544. 17 A. J. Vega, J. Am. Chem. SOC., 1988,110,1049. 18 C. A. Fyfe, Y. Zhang and P. Aroca, J. Am. Chem. SOC., 1992, 114,3252. 19 G. Borbely, H. K. Beyer and H. G. Karge, Clays Clay Miner., 1991, 39,490. 20 G. G. Almond, R. K. Harris and K. R. Franklin, Solid State NMR, in press. Paper 5/07614E; Received 22nd November, 1995 J. Mater. Chem., 1996, 6(5),843-847 847

ISSN:0959-9428    

DOI:10.1039/JM9960600843

出版商:RSC    

年代:1996

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Organometallic cation-exchanged phyllosilicates: variable-temperature 57Fe Mossbauer spectroscopic and related studies of the adsorption of dimethylaminomethylferroceneon clays and pillared clays Christopher Breen," John S. Brooks, Susan Forder and Julian C. E. Hamer Materials Research Institute, Shefield Hallam University, Shefield, UK S1 1 WB Variable-temperature "Fe Mossbauer spectroscopy, thermogravimetry (TG), powder X-ray diffraction (PXRD) and temperature- programmed solid insertion probe mass spectrometry (TP-SIP-MS) have been used to study the interaction of dimethylaminomethylferrocene (DMAMF) with Westone-L (WL), a low iron montmorillonite. The hydrochloride salt of DMAMF, (ferrocenylmethyl)dimethylammoniumchloride (FMDMACl), was prepared and studied both prior and subsequent to exchange on the interlamellar sites of WL.X-Ray diffraction confirmed that the FMDMA cations were incorporated between the + clay lamellae and the observed spacing of 15.1 A was thermally stable up to 200 "C in air. TP-SIP-MS indicated that a small proportion of the incorporated metallocene was volatilised at temperatures below 400 "C, but that the majority decomposed via loss of cyclopentadienyl ligands leaving the metal centre between the sheets. A similar thermal degradation path was observed for DMAMF on aluminium pillared clay (A1-PILC). 57Fe Mossbauer spectroscopy revealed that the FMDMA cation occupied a + similar environment in the chloride salt, FMDMA+-WL and DMAMF-A1-PILC insofar as the isomer shift, 6, and quadrupole splitting, A, of the incorporated metallocene were essentially the same in all complexes and virtually identical to that observed for FMDMACl(6 =0.34 mm s-l, d =2.32 mm s-l at 300 K).The values for the Debye temperature 8, and recoil-free fractionf determined from variable-temperature 57Fe Mossbauer spectroscopy, were typically 140 K and 0.12, respectively, for FMDMACl and FMDMA+-WL, thus confirming the similar environment occupied by the cation in the chloride salt and in WL. In contrast, the corresponding values for DMAMF-A1-PILC were 118 K and 0.06, respectively, indicating that the the metallocene enjoyed much greater freedom in the galleries of the Al-PILC which exceed the dimensions of the metallocene compared to FMDMA+-WL where the organoiron cation itself determines the layer separation.The incorporation of metallocenes into zeolites and zeotypes continues to attract interest. One particularly attractive goal is the production of catalytically active molecular fragments or small metallic clusters within a host matrix which, in addition to its thermal stability, can impart size and shape selectivity on the product distribution. In pursuit of this goal, considerable emphasis is placed on proving that the molecule is in the channel network and not bound to the surface, the characterisation of the incorporated species, and an expla-nation of how it interacts with its new environment before undertaking a detailed investigation of how it degrades upon thermal treatment.Moller et al.' have presented a detailed investigation of the pyrolysis of ferrocene in zeolites presenting EXAFS and sup-porting mass spectral data which indicate the presence of half-sandwich fragments bound to the oxygens of the zeolite lattice. Indeed, attempts to incorporate neutral metallocenes within the confines of the zeolite framework can prove problematic since protons arising from residual water molecules readily oxidise ferrocene to ferrocenium.',2 Recent studies,., have resulted in the successful inclusion of ferrocene into the channel network of A1P04-5 and AlPO,-8 from which it cannot be sublimed. s7Fe Mossbauer spectroscopy3 indicates that the metallocene is (i) rapidly reorientating within the channel network and (ii) largely unchanged following incorporation.EXAFS analysis,, which provided independent evidence for the presence of unaltered ferrocene at temperatures up to 200 "C, indicated that the thermally degraded composite did not show any evidence of the clustering of iron atoms through either Fe-Fe or Fe-0-Fe interactions. Cobaltocene, however, was oxidised to cobalticenium upon incorporation within the channels of VPI-5.5 Once produced the cobalticenium remained thermally stable up to 130"C, even though VPI-5 converted to AlPO,-8 over this temperature interval. An increase in layer separation, which exhibits enhanced thermal stability, usually confirms incorporation of metallo- cenes into layered componds but the nature of the included species, its interaction with the host and its thermal degradation path are still important.Ferrocenylalkylammonium cations have been adsorbed into a number of layered hosts including a-Sn( HPO,),-H,O (a-SnP), a-Zr( HPO,),.H,O (a-ZrP), MOO, and VOPO,. Dimethylaminomethylferrocene (DMAMF) was readily intercalated into a-SnP from aqueous solution forming a bilayer of protonated amines.6 In MOO,, 15N CP MAS NMR provides evidence for two distinct environments for 15N between the 1aye1-s.~ A minor resonance was tentatively assigned to a small amount of oxidised guest species whilst the major resonance was intermediate between that for (FCCH,CH,~'NH~)+C~-and FcCH,CH,~'NH,, where Fc =Fe(q-C,H,)(q-C,H,). NMR evidence was also utilised to show that the amino group was interacting with the host layer via P-O..-H-N hydrogen bonds in Z~(HPO,),(FCCH,CH,~~NH~)~.,(H20), (x=0.1-0.5).7 The increase in layer expansion upon incorporation of FCCH,CH,'~NH, in both a-ZrP and MOO, suggests that the guests formed bilayers between the layers of each host.Large increases in d spacing, i.e. layer expansion, were also observed when ferrocenylalkylammonium iodides were incorporated between the layers of VOPO,. The length of the alkyl bridging unit apparently influences both the layer spacing and the extent to which the ferrocene moiety is oxidised upon intercalation.' DMAMF was the molecule of choice in this investigation for several reasons. Firstly, it is easily transformed into the hydrochloride salt thus allowing straightforward replacement of the Na' cations resident on the exchange sites of a low- +iron Texas bentonite.Secondly, incorporation into H -exchanged WL (H+-WL) should be possible via the in situ formation of the conjugate base, ferrocenylmethyldimethylam-monium (FMDMA+ ), in the interlamellar region. This second J. Muter. Chem., 1996, 6(5), 849-859 849 approach also has the added attraction of neutralising the protons which may contribute to the oxidation of the ferrocene unit Thirdly, it was our intention to incorporate DMAMF into the interlamellar gallery of an aluminium pillared inter- layer clay (Al-PILC) by using the protons, formed during calcination of the pillar, to produce FMDMA+ ions All these approaches have proven successful and the products have been characterised using a range of instrumental techniques includ- ing X-ray fluorescence spectrometry (XRF), powder X-ray diffraction (PXRD), thermogravimetry (TG), temperature-pro- grammed solid insertion probe mass spectrometry (TP-SIP- MS), and Fourier transform IR (FTIR) spectroscopy Moreover, variable-temperature Mossbauer spectroscopy has been extensively employed to determine how strongly DMAMF is held within the Al-PILC, where the gallery height exceeds the dimensions of the metallocene, compared to the situation in FMDMA+-WL where the dimensions of the metallocene itself determines the interlayer separation Experimental Materials The clay used in all the experiments was Westone-L (WL) a Texas bentonite, supplied by ECC International, which has a cation exchange capacity (cec) of 81 mequiv (100 g)-' and a low iron content of 05% m/m Fe203 This clay and the procedures used to convert it into the (nominal) < 2 ym particle size, Na-exchanged form, subsequently referred to as Na+-WL, have been described in detail elsewhere The H+-exchanged form of Westone-L, H+-WL, was obtained by treating Na+-WL with aqueous 1 mol dmP3 sulfuric acid for 2 h at 25 "C, and washing until the residual conductivity of the supernatant was <30 pS The product was dried at room temperature Elemental analysis using XRF spectrometry indicated that this treatment, as anticipated based on related results,1° had little effect on the layer composition This was confirmed when 27Al and 29S1 MAS NMR spectra of WL were unchanged following the acid treatment The aluminum pillared clay was prepared using the method described by Schoonheydt et a1 l1 Na+-WL was suspended in 100 cm3 of water and stirred for 6 h An aqueous solution of NaOH (17 cm, 0 4 mol dme3) was added dropwise at 1 cm3 min-l to an aqueous solution of AlCl, 6H,O (17 cm3, 0 2 mol dmP3, 10 cec) with vigorous stirring The resulting solution was heated at reflux for 3 h and then added dropwise at 8 cm3 min-' to the Na+-WL suspension This was stirred for 12 h, and then washed with deionised water until the conductivity of the supernatant fell below 30 pS The clay was air-dried and then calcined at 500 "C for 1h This yielded aluminym pillared WL (Al-PILC) with an interlayer spacing of 18 8 A (Ferrocenylmethyl)dimethylammonium chloride (FMD-MAC1) was prepared using N,N-dimethylaminomethylferro-cene (DMAMF) supplied by Aldrich Chemicals DMAMF (1g, 4 12 mmol) was added dropwise with stirring to 50 cm3 of 1mol dm-3 HCl This was evaporated (in vacuo) to give a green solid Recrystallisation from CHCl,-Et,O gave long golden brown crystals in 87% yield (Analysis Found C, 55 59, H, 640, N, 502 Calc C, 5585, H, 649, N, 501%) The cationic portion of this salt will subsequently be referred to as FMDMA+ FMDMA +-WL was prepared using three different methods In the first method FMDMACl(0 23 g, 1cec, 0 81 mmol) was dissolved in 50 cm3 of deionised water, and 1 g of powdered Na+-WL, dried at 120"C, was added The suspension was stirred for 8 h at 25°C before the clay was isolated and the process repeated twice more The product was then washed (5 x 120 cm3 deionised water) as above The product is sub- sequently referred to as FMDMA+-WL( 1) (Analysis Calc 100% exchange, C, 14 2 Found C, 11 O%, equivalent to 80% 850 J Mater Chem, 1996, 6(5),849-859 exchange) In the second method DMAMF (046 g, 2 cec, 162 mmol) was suspended in 50 cm3 deionised water, and 5 cm3 of 1 mol dmP3 HCl (an excess) added dropwise with stirring to give a solution of FMDMACl Ground Na+-WL powder (1 g), dried at 120"C, was added and the resulting suspension was stirred for 18 h at 25 "C The clay was isolated by centrifugation and the process repeated twice more Finally, the product [referred to as FMDMA f-WL(2)] was washed with deionised water (5x 120 cm3), and air dried at room temperature In the third method, 1 g of H+-WL, dried at at 120"C, was added to DMAMF (046 g, 2 cec, 162 mmol) dissolved in 50 cm3 of methanol The resulting suspension was stirred for 18 h at 25 "C, centrifuged and finally washed with methanol (5 x 120 cm3) The product [referred to as FMDMA+-WL( 3)] was then air dried at room temperature DMAMF (1 29 g, 3 cec, 243 mmol) was suspended in 100 cm3 of deionised water, and 1g of powdered Na+-WL (dned at 120°C) was added The suspension was left to stir for 6 h at 25 "C The clay was then isolated by centrifuga- tion and the process repeated twice more The final prod- uct, DMAMF-WL, was then washed (3 x 120 cm3 H,O, 3 x 120 cm3 MeOH, 1 x 120 cm3 H20) DMAMF (043 g, 2 cec, 1 62 mmol) was dissolved in 50 cm3 of methanol and 1 g of the calcined pillared clay, pretreated at 120°C, added The suspension was stirred for 18 h, washed (5 x 120 cm3 methanol), and collected in the normal manner The product is subsequently referred to as DMAMF-AI-PILC (Analysis Calc 100% exchange, C, 142 Found C, 42% equivalent to 29% exchange) Adsorption isotherms Methanolic solutions (20 cm3) of DMAMF-FMDMACI in the range 0-3 cec were prepared, and the absorption at 435 nm, characteristic of both DMAMF and FMDMACl, measured Clay (0 1g) dried at 120 "C was then added and the suspensions shaken overnight These were then centrifuged, and the absorb- ance of the supernatant measured using a Hitachi U-2000 double-beam UV-VIS spectrophotometer, with cells of path- length 1cm Thermogravimetry Thermogravimetry was performed using a Mettler TG50 ther- mobalance connected to a Mettler TClOA processor Samples (5-10 mg) were heated from 25 to 800 "C, at a rate of 20 "C min-l, in a dynamic atmosphere of dry N, gas flowing at 20 cm3 min-l X-Ray diffraction X-Ray diffraction traces of pressed powder samples were recorded using a Phihps PW1830 diffractometer using Cu-Ka radiation (i=15418 A) yhereas a Philips PW1050, using Co- Ka radiation (A= 1789 A) was used to study partially oriented samples on glass slides A heating stage manufactured accord- ing to Brown et a1 l2 was used to heat the partially oriented samples in the temperature range 20-400 "C X-Ray fluorescence XRF analyses of samples presented as lithium tetraborate beads were obtained using a Philips PW2400 X-ray spec- trometer calibrated using certified reference materials C,H,N analyses were obtained from Medac Ltd Mossbauer The Mossbauer spectrometer, cryostat, sample presentation and fitting routines have been described in detail elsewhere Absorbers of Mossbauer t values <1, with a maximum iron concentration of 7 mg cmP2 for FMDMACl, were studied The 100 200 300 cec offered (%) Fig.1 Isotherms for the adsorption, from methanol, of DMAMF on Na+-WL (0)and Al-PILC (0)and FMDMA' on Na+-WL (A), H+-WL (W) and on Na+-WL in the presence of [H'] (0) values of the isomer shift, 6, the quadrupole splitting, A, and the linewidths, r,quoted are relative to the source, 57C0 in a rhodium matrix at room temperature.Results Adsorption isotherms The adsorption of DMAMF onto Na+-WL and Al-PILC from methanol resulted in an uptake equivalent to only 10% of the cec (Fig. 1). The loading on the A1-PILC was disap- pointing given that when the pillared clay is calcined at 500 "C, protons are released as the aluminium oxide pillars are formed, which then migrate into the layers. These protons can be enticed into the interlayer using strong bases such as amrnonia.I3 Thus it was anticipated that interaction with these protons might provide the driving force to draw DMAMF into the galleries in the pillared clay.Formation of the hydro- chloride salt, FMDMACl, followed by contact with the clay in methanol proved successful yielding 53YO exchange. Production of the chloride salt in situ, by the addition of acid to the methanolic solutions followed by contact with the clay, was undertaken and this resulted in 48% exchange. In the final experiment of the series, DMAMF was contacted with H +-WL. Exchange was successful although the loading achieved was only 45% of the theoretical value, perhaps implying that the upper limit for exchange using methanol as solvent was near 50% cec. Elemental analyses The loadings achieved following one contact in methanol were disappointing so attempts were made to increase the level of exchange by contacting Na+- or H+-WL three times with the metallocene using water as solvent.In the main this proved more successful as the following results show. The theoretical value for the Na,O content of fully Na+-exchanged WL, given a cec of 81 mequiv (100 g clay)-', is 2.09 mass% whereas the Fe203 content should increase from 0.5 to 5.6% m/m if FMDMA' ions occupy all the exchange sites. Note, however, that the calculations for iron content assume that no oxidation or volatilisation of the metallocene occurred, in line with previous observation^.^,^,^ Table 1 lists the results of the XRF and C,H,N analyses and expresses these values as the percent- age of exchange sites occupied by the metallocene or vacated by the Na' ions.When the metallocene is adsorbed in the cationic form the number of resident Na' ions replaced should correlate with the amount of iron adsorbed given that both species carry a single positive charge. Thus the discrepancy between the number of Na' ions displaced, the amount of iron adsorbed and the C,H,N analysis was a cause of initial concern (Table 1). For example, C,H,N analysis indicated that FMDMA' ions occupied 80% of the exchange sites in FMDMA'-WL( l), a figure which agreed with the number of Na+ ions displaced (76% cec) but not with the amount of iron determined by XRF (59%). The figures for DMAMF-Al-PILC behave in a similar manner. C,H,N analysis indicated that DMAMF occupied 29% of the exchange sites, whereas XRF data suggested a value of 33%.The value of 67% Na+ displacement when DMAMF was contacted directly with Na+-WL (DMAMF-WL) was unexpected. It is proposed that there were sufficient protons present during this process to protonate enough DMAMF to cause this level of exchange. Thermogravimetry The derivative thermograms presented in Fig. 2 were obtained after each sample had been pre-conditioned in the nitrogen purge gas for 15 min. This procedure removes much of the physisorbed water, which contributes little information, and serves to emphasise the maxima associated with desorption of strongly bonded species. The derivative thermogram for Na+-WL [Fig. 2(a)] shows that the desorption of the remain- ing physisorbed water was essentially complete by 100 "C, with dehydroxylation of the structure reaching a maximum at 680"C.14 Liquid DMAMF boils at 200°C and so little infor- mation regarding its decomposition was gained.The corre- sponding chloride salt, FMDMACl, began to decompose at ca. 150°C with an associated mass loss of 27%. Further mass losses of 15, 9.4 and 14.5% occurred with associated maxima at 350, 460 and 520 "C, respectively [Fig. 2( b)]. The derivative thermogram for FMDMACl provides a useful fingerprint for the protonated moiety insofar as a number of the mass losses were also observed in FMDMA'-WL( 1) [Fig. 2(c)]. For example, a maximum in the derivative thermogram for FMDMA+-WL( 1) at 200 "C was clearly visible and there was evidence for the presence of a maximum at 350°C.The maximum at 625°C may reflect some combination of the FMDMACl maximum at 620 "C and the structural dehydrox- ylation of WL which maximised at 680°C [Fig. 2(a)]. In addition, a new maximum at 740°C which corresponded to twice the mass loss associated with the maximum at 200°C was observed, perhaps indicating that the FMDMA 'cation follows a different decomposition pathway when exchanged onto WL. The derivative thermogram for the desorption of DMAMF from A1-PILC exhibited a small maximum at 200 "C but was dominated by a peak at 550"C, which accounted for 8% of the initial mass or 43% of the total mass loss. TP-SIP-MS TP-SIP-MS was used to explore the way in which the various complexes were thermally degraded.The maxima in the total ion current (TIC) for the desorption of metallocene from WL and Al-PILC correlate quite well with those observed in the derivative thermograms, which is reassuring given the different conditions under which they are obtained." The TIC for the thermal decomposition of the incorporated metallocene, which reached maxima for FMDMA+-WL and DMAMF-A1-PILC at 225 and 250 "C respectively, represented the combination of a large number of mass fractions. In particular, peaks at m/z 214 (NCH,Fc), 200 (CH,Fc), 186 (Cp,Fe), 134 [CH,(q5-C5H4)Fe] and 121 (CpFe) [Fc= Fe(q-CSH,)(q-CSH4); Cp =$-C5H5] proved that iron was volatilised from the sample, although this process was essentially complete by 400 "C. Above 400 "C the decomposition products contained only ligand, with characteristic peaks at m/z 79 (CH,Cp) and 66 (Cp).No iron was desorbed. This behaviour is summarised in Fig. 3 where the intensity of peaks, selected to distinguish between metallocene and ligand desorption, are plotted as a function of sample temperature. It is important not to equate TIC with the amount of metallocene desorbed. Reference to the derivative thermograms in Fig. 2, where the area under the peaks is directly related to the mass loss, indicates that the amount of material desorbed below 400°C was not as signifi- cant as the TIC suggests, yet the mass spectra quite clearly J. Muter. Chem., 1996, 6(5),849-859 851 Table 1 Summary of elemental analysis data for the samples described in the text Fe contenta Na content" (+O 1)YO (fO 1)Yo (m/m) (m/m) Na+-WL 04 19 H+-WL 04 01 AI-PILC 04 00 FMDMA+-WL( 1) 34 05 FMDMA' WL(2) 41 01 FMDMA+-WL( 3) 34 01 DMAMF-WL 33 07 DMAMF-Al-PILC 20 01 " Based on XRF analysis figures Based on C,H,N analysis figures I I I I t I 1 400 Mm 800200 TI"C Fig.2 Derivative thermograms for (a) Na+-WL, (b) FMDMACl, (c) FMDMA+-WL( l),(d) A1-PILC and (e) DMAMF-Al-PILC corroborate the loss of some iron which explains the discrep- ancies noted in the elemental analyses above Mass spectral analysis of the peaks contributing to the large maximum at 680°C in the TIC for the desorption of DMAMF from Al- PILC proved that this maximum was due to dehydroxylation of the structure and the pillar Powder X-ray diffraction The quality of the X-ray diffraction traces collected using pressed powder samples is shown in Fig 4 and the index for each pattern is ogiven in Table 2 The basal spacing for Na'-WL was 12 5 A, which is commensurate with one water layer between adjacent clay layers, whlst the spacing for FMDMA+-WL( 1)[Fig 4(b)] was 15 1A The diffraction trace for the Al-PILC, after firing for 1 h at 5OO0C, exhibited a 852 J Muter Chem , 1996,6(5), 849-859 Fe adsorbed" Na desorbed" metallocene (% cec) (YOcec) adsorbedb - - - 94 - 100 59 76 72 95 59 95 57 67 33 - spacing of 18 8 A,thus confirming that the pillaring process had been successful Variable-temperature X-ray diffraction VTXRD provides the first real indication that the metallocene cation was present within the interlayer of WL At room tempFrature and humidity Na'-WL exhibits a d spaFing of 12 5 A which upon heating to 50 "C decreases to 9 6 A This latter value is diagnostic of an Na+-exchanged clay from which all the interlamellar water has been expelled Incorporation of a large species such as the FMDMA' cation between the aluminosilicate layers increases the d spacing and makes it more thermally stable than the corresponding water-expanded material The 15 1 A spacing, as evidenced by the 001 and 003 reflections, remained essentially constant until the tomposite was heated to 200 "C, whereupon it collapsed to 13 0 A (Fig 5) This reduction in the d spacing coincides with the onset of the first major mass loss in the derivative thermogram for FMDMAfWL( 1) [Fig 2(c)] and there was no evidence of a 9 6 A spacing, characteristic of a completely collapsed clay, which indicates that the decomposition products of the FMDMA' cations remained in the interlayer region The PXRD trace for the pillared clay provided little information regarding the location of tbe metallocene because the spacing remained constant at 18 8 A and no extra peaks were observed, suggesting that the metallocene was not mixed with Al-PILC in a powder form The variable-temperature Mossbauer spec- troscopic data (vide znfra) support this observation Mossbauer spectroscopy 57Fe Mossbauer data were obtained for FMDMA', as the chloride salt, and after incorporation into Westone-L, FMDMA+-WL( 1)-( 3) and A1-PILC, respectively, over the temperature range 15-300 K Selected spectra are shown in Fig 6 and 7 and the parameters derived from the fitting process are listed in Tables 3-5 FMDMACl was fitted as a resolved quadrupole doublet whereas the fitting strategies for the clay- supported complexes had to allow for the small amount of iron in the clay structure, the absorption of which became increasingly more significant as the temperature increased It is common for the recoil-free fraction of inorganic species to decrease more slowly with temperature than that for an organometallic species This was particularly evident in DMAMF-A1-PILC (Fig 7) where the combination of a low loading, equivalent to 29% cec, and the much lower recoil- free fraction meant that the contribution from the structural iron in WL dominated the spectrum above 200 K The similarity of the Mossbauer parameters, 6 and A, for FMDMA' as the chloride salt and when incorporated in the host aluminosilicate indicates that there was no substantial change in the organometallic cations upon exchange In par- ticular, the absence of a component with a reduced quadrupole splitting suggested that the ferrocene unit had not been oxidised 100 (a) -TIC 0 .c 0 200 400 600 200 400 600 E,O0 .-E VIc s .-0 200 400 600 200 400 600 TI"C Fig.3 TP-SIP-MS data for (a) (b) FMDMA+-WL( 1) and (c) (d)DMAMF-Al-PILC Table 2 PXRD data for Na+-WL and FMDMA+-WL( 1) 281degrees I h k 1 dobslA dca,clA Na+-WL" 14.17 8 0 0 2 6.24 6.25 19.87 3 1 1 0 4.46 4.45 020 28.43 27004 3.14 3.13 35.96 2 0 0 5 2.49 2.50 43.48 3 0 0 6 2.08 2.08I I FMDMA +-WL( 1)b110,020 5.86 1 0 0 0 0 1 15.10 15.10 11.65 3 0 0 2 7.59 7.55 17.65 1 7 0 0 3 5.02 5.03 19.84 8 1 1 0 4.47 4.45 0 2 0 23.62 4 0 0 4 3.76 3.78 e2 .-u) 0.0 40 I do I do ' lo I do I do 29.63 34.7 1 41.91 5 2 1 0 1 0 0 3 0 5 0 7 3.01 2.58 2.15 3.02 2.60 2.16 61.94 2 0 6 0 1.49 1.50 upon intercalation.The variation of the absorption area data with temperature, for the organometallic cation, was analysed and the resulting plots of log (area) us.temperature for the 250al! 110.020 I I samples under investigation are presented in Fig. 8. The values of OD andf, obtained using software which uses the full Debye 900. integral, are presented in Table 6. The illustrative data pre- sented concentrates on FMDMA+-WL and DMAMF-AI- PILC, but these are representative of all the samples studied as the data in Table 6 and the plots in Fig. 8 confirm. 4 I Discussion The adsorption isotherm data (Fig. 1) and the results of the elemental analyses confirm that DMAMF, and the correspond- Fig. 4 PXRD profiles, obtained using Cu-Kcr radiation, for ing FMDMA' cation, was adsorbed onto WL from both (a) Na+-WL and (b) FMDMA+-WL(1) (0s small quantity of fine methanol and water with varying degrees of !uccess to produce grained opaline silica impurity, K =kaolin) a composite with a basal spacing of 15.1 A.The method of J. Muter. Chem., 1996, 6(5), 849-859 853 2 3 4 5 6 7 8 0 10 11 12 13 14 15 16 17 18 192021 2223 242526 2eldegrees Fig. 5 PXRD profiles, obtained using Co-Ka radiation, for FMDMA+-WL(1) at (i) 20, (ii) 50, (iii) 100, (iv) 150, (v) 200 and (vi) 250°C introduction of the FMDMA' cation into the interlayer region influences the amount of cation adsorbed but, in general, acceptable levels of incorporation are achieved from multiple contacts in water. The poor uptake of DMAMF from methanol was attributed to a combination of two distinct factors. Firstly, incomplete separation of the layers due to the solvent (meth- anol), and secondly, there was little to favour incorporation of a neutral species between the layers of Na+-WL.Indeed, similar loadings of cationic half-sandwich compounds of iron were achieved using methanol as solvent.' The low loading of the DMAMF on A1-PILC, from both methanol and water, was a combination of two factors. Firstly, the protons generated during the firing process may not have been available for complexation with the dimethylamino group on the metallo- cene. Secondly, it is probable that sites at the periphery of the interlayer region were filled first thus preventing the diffusion of further molecules into the structure. The observed basal spacing of 15.1A indicates that, in contrast to the bilayer formation in a-SnP,6 MOO, and a-ZrP,7 and VOP04,* only a single layer of metallocene resides in the interlamellar region ofWL. The reduction in the basal spacing from 15.1 to 13.0A when FMDMA+-WL(1) was heated to 200°C must be attributed to the decomposition of the FMDMA cation witbin the interlayer region.Moreover, + the final value of 13.0A indicates quite clearly that the decomposition was not complete. These observations are in accord with both the derivative thermograms (Fig. 2) and the TP-SIP-MS results (Fig. 3) and suggest that some residue containing iron was left between the aluminosilicate lamellae. The volatilisation of some of the metallocene, which contrasts with recent studies of organoiron species on WL9 and AlP04- 5,394 at temperatures around 200 "C may contribute, at least in part, to the reduction in basal spacing.The data presented in Table 1 indicate that the FMDMA' cation displaced Na' ions from the exchange sites on WL; thus it is unlikely that the volatilised metallocene arose solely from surface sites, 854 J. Muter. Chem., 1996,6(5), 849-859 although the possibility is not rejected. The similarity of the desorption profiles, derived from mass spectrometry (Fig. 3), for FMDMA+-WL( 1) and DMAMF-Al-PILC suggest that the metallocene molecules are desorbed from similar sites with the proportion of strongly bonded molecules, which only lose ligand upon thermal treatment, outnumbering those which desorb near 200°C.The mass spectral data give no indication that the aminomethylmetallocene is changed upon incorpor- ation into the host structures and this is supported by the parameters for the incorporated metallocenes derived from the 57Fe Mossbauer data, Fig. 5 shows that the 15.1 A basal spacing waso stable at 200°C. The thickness of the metallocenc is 6.65 which, when added to th,e layer thickness of 9.6 A, should result in a d spacing of 16.3 A. However, the incorporation of FMDMoA+ cations in VOP04* only rFsulted in an expansion of 5.8 A, a value close to that of 5.5 A observed here. It is common for metalloc5ne expanded layered materials to display d spacings up to 1A less than the value anticipated from the molecular dimensions of the guest, particularly in swelling layer lattices such as a-Zr( HP0J2,17 VOP04,18 and V205 Given the .7319 uncertainty regarding the increase in d spacing of metallocene expanded layered hosts, conclusions regarding the orientation of the FMDMA' cation are difficult to reach.However, the observed 15.1 A d spacing of FMDMA+-WL( 1) is consistent with the cation adopting an orientation where the plane of the cyclopentadienyl ring is perpendicular to the basal surface with the side chain accommodated in the interlamellar space, thus making no contribution to the layer expansion [Fig. 9(a)]. It is more difficult to ascertain the orientation of the metallocene in DMAMF-Al-PILC because the height of the pillars, which exceed the dimensions of DMAMF, determine the interlayer spacing [Fig. 9(b)] and this spacing does not alter after the PILC has been fired.The 57Fe Mossbauer spectrum for FMDMACl consisted of a single symmetric doublet, with a quadrupole splitting, A, of 2.34 mm s-', which remained constant between 15 and 300 K, whereas the isomer shift, 6, exhibited a typical second-order Doppler shift effect, falling steadily from an initial value of 0.41 mm s-l at 15 K to a final value of 0.34 mm s-' at 300 K (Table 3). Analysis of the normalised area us temperature data (Fig. 7)yielded a Debye temperature, OD, of 144 K and a recoil free fraction,f,,, K, of 0.14 when an effective recoiling mass of 57 u was assumed. The low OD, which is typical of organometal- lic compounds, may be further reduced in this instance owing to the difference in size between the large FMDMA+ cation and the smaller chloride anion.The halfwidth at half height, r/2,of the absorption peaks varied from 0.13 mm s-' at 15 K to 0.16mm s-' at 300 K. This broadening arises owing to increased vibration within the lattice as the temperature of the solid was increased. Fig. 6 illustrates how the 57Fe Mossbauer spectrum for FMDMA+-WL varied with temperature. The sharp, outer doublet, which dominates the spectra at low temperatures, was assigned to the FMDMA' cation (uide infra). The broad, ill defined absorption seen between the wings of this sharp, outer doublet has been attributed to two components. The first is a weak, broad doublet arising from the small amount of Fe"', present owing to isomorphous substitution in the octahedral sheet of WL [Q(l) in Table 41, whilst the second is a broad singlet, characteristic of Feo [S(l) in Table41.This singlet arises from the small quantity of iron which was added to the graphite rod to aid machining when making the sample holders. This contribution is not normally observed, but owing to the low iron content of the materials under study, the absorption becomes significant. The broadness of the Fe"' doublet indicates that the iron present in the clay occupies a range of closely related sites. When exchanged into WL the FMDMA' cations exhibited isomer shifts and quadrupole splittings which were very similar to those determined for FMDMACl (Table 3) and varied little 100 f Y 97.5 !! I + 99.3 100 240 K 1 2- Ti 3 100 A8 Y c .- v) .-E chc.96.8 I 99.0 15 K 2 1 T 100 t- 3 100 96.7 98.4 -4 0 2 4-4 -2 0 2 4 vlmm s-l Fig. 6 Variable-temperature 57FeMossbauer spectra collected for FMDMA+-WL( 1) at the temperatures indicated despite the different routes by which the clay/metallocene were prepared. The similarity of these values indicates that the FMDMA cations occupied similar environments in both the+ chloride salt and in WL. This is firm evidence that WL simply expanded to accommodate the FMDMA' cation, with no oxidation of the iron centre nor distortion in the orientation of the cyclopentadienyl rings. Given that the increase in the d spacing upon incorporation of the FMDMA' cation into WL was l.OA less than anticipated, and that previous studies2' have shown that a 9" tilt in the cyclopentadienyl rings reduces the isomer shift by 0.02 mm s-' and the quadrupole splitting by 0.11 mm s-', the similarity of the observed parameters was surprising.Nonetheless, it is consistent with an earlier study where a similar low d spacing did not alter appreciably the Mossbauer parameters of half sandwich organoiron com-pounds when they were incorporated into WL.' J. Muter. Chem., 1996, 6(5),849-859 855 80K 300 K ln n2 i -3 I 00 # 99 1 995 50K 240 K 1-2 100 -3 n 100 h s v c 0 u)fn 6 *c 98 5 99 6 15 K 100 100 99 5 -4 -2 0 2 4-4 -2 0 2 4 vlmm s-l Fig.7 Vanable-temperature 57Fe Mossbauer spectra collected for DMAMF-AI-PILC at the temperatures indicated The Mossbauer spectra for the complex formed when Al- PILC was treated with DMAMF (Fig 7) were of lower quality than those recorded for the FMDMA+-WL( 1)-( 3) samples because the amount of iron present was only equivalent to 29% cec, and the recoil-free fraction fell off much more rapidly Nonetheless, the values of 6 and d determined from these spectra (Table S), together with the resistance of the incorpor- ated metallocene to the washing procedures, suggests that the 856 J Muter Chem , 1996, 6(5),849-859 incorporated species was the FMDMA’ cation The narrow doublet [Q( 1) in Table 51 became evident in the Mossbauer spectrum of the fired Al-PILC pnor to contact with DMAMF The origin of this doublet has not been studied extensively The consistency of the values of d and 6 determined for the FMDMA+ cations in WL together with their similanty to the values for the chlonde salt suggested that the organoiron species was the same in all the samples Yet the TP-SIP-MS Table 3 Isomer shifts, quadrupole splittings and linewidths derived from a variable-temperature 57Fe Mossbauer study of FMDMACl 15 0.4 1 1.18 0.13 0.13 2.04 0.504 33 0.41 1.18 0.13 0.13 1.89 0.772 50 0.41 1.18 0.14 0.14 1.82 1.074 80 0.40 1.17 0.16 0.15 1.56 0.663 100 0.40 1.17 0.14 0.14 1.35 1.065 120 0.39 1.17 0.16 0.15 1.21 0.872 140 0.39 1.17 0.15 0.14 1.05 0.810 160 0.39 1.17 0.17 0.16 0.93 0.897 200 0.36 1.17 0.17 0.16 0.70 0.799 250 0.34 1.16 0.18 0.16 0.46 0.975 300 0.34 1.16 0.20 0.16 0.3 1 1.726 Table 4 Isomer shifts, quadrupole splittings, linewidths and areas derived from a variable-temperature 57Fe Mossbauer study of FMDMA+-WL T/K phase" 6 (&0.02)/mm s-' 42 (f0.02)/mm s-' wY1) (+0.02)/mm s-' r/2 (r) (&0.02)/mm s-' normalised area relative area (&2.5%) x2 15 0.24 0.48 0.18 15 0.604 15 0.33 0.4 1 0.24 0.24 0.05 5 0.604 15 0.43 1.21 0.14 0.15 0.93 80 0.604 50 0.24 0.48 0.17 16 0.559 50 0.33 0.43 0.24 0.24 0.54 5 0.559 50 0.42 1.22 0.15 0.15 0.82 79 0.559 80 0.24 0.48 0.17 18 0.582 80 0.33 0.4 1 0.24 0.24 0.07 7 0.582 80 0.42 1.21 0.14 0.15 0.69 75 0.582 160 0.24 0.49 0.17 28 0.572 160 0.29 0.42 0.24 0.24 0.07 11 0.572 160 0.39 1.21 0.13 0.13 0.39 61 0.572 240 0.24 0.50 0.20 44 0.540 240 0.29 0.45 0.23 0.23 0.03 7 0.540 240 0.35 1.20 0.12 0.14 0.23 49 0.540 300 0.23 0.48 0.18 54 0.607 300 0.30 0.49 0.20 0.20 0.02 6 0.607 300 0.32 1.20 0.13 0.14 0.13 40 0.607 " S( 1)=Fe"' present in sample holder; Q(1)=Fe"' present due to isomorphous substitution in WL; Q(2) =Fe" present in ferrocene unit of incorporated metallocene.results suggested quite strongly that there were two adsorption (area) us. temperature data reported by Simopoulos et uLZ1for sites, one where the entire metallocene was desorbed at tem- dimethyltin species adsorbed on montmorillonite exhibited a peratures below 400°C,and a second environment where the discontinuity near 210K at which the gradient of the line metal centre was retained and only ligand was desorbed. In increased considerably, although linearity was retained.This an effort to determine whether the fitting of the 57Fe Mossbauer feature was attributed to the melting of the interlayer water absorption data would support a two site model the spectra which resulted in a lower recoil-free fraction for the Sn atoms. obtained at 15, 25, 50, 80, 100, 140, 180, 220 and 300 K for Dehydration of the samples removed the discontinuity, hence both FMDMA+-WL( 3) and DMAMF-Al-PILC were sub- corroborating the interpretation.In contrast, the Debye tem- jected to a P(Q) analysis. The P(Q) fitting program assumes perature and recoil-free fraction of the N-methyl-3-( triphenyl- a distribution of sites and an effective distribution of electric stanny1)pyridinium cation changed little upon adsorption onto field gradients and corresponding quadrupole splittings. Some the cation-exchange sites in montmorillonite,22 whereas the line broadening away from the theoretical natural linewidth is half-sandwich iron cations, tricarbonyl(~5-2,4-dimethylcyclo-expected owing to saturation effects and sample inhomogen- hexadienyl )iron ( 1+), and tricarbonyl (q5-2-methoxycyclo hex- eity. This fitting procedure indicated that there was no evidence adienyl)iron( 1 +), typically gave Debye temperatures 30 K for more than one unique site which would result in changes lower when occupying the exchange sites in WL than when in the quadrupole splitting larger than the normal line broaden- incorporated in a PF6-lattice.g This was attributed to the ing effects.This indicated that the Mossbauer data would only cations being less tightly bound when incorporated between support one type of site. Consequently, it must be considered the layers of the clay than when locked within the anionic that at temperatures below 300K the adsorption sites for (PF6-)lattice. Clearly, the more hydrophobic the incorporated +FMDMA cations were indistinguishable. organometallic species, the less influence the melting of incor- The Debye temperature, OD, and corresponding recoil-free porated solvent has on the recoil free fraction, hence no fraction,f, provide information regarding how tightly the iron- discontinuities are observed.containing species is bound within a structure. Previous varia- The values for eDandf,,, for the samples studied here ble-temperature Mossbauer studies of organometallic species are collected in Table 6. The similar Debye temperatures adsorbed in clays have shown that incorporation may result obtained for the FMDMA+ cation in WL, irrespective of in a lower Debye temperature and recoil-free fraction. The In preparation method, suggests that the recoiling mass in WL is J. Muter. Chem., 1996,6(5),849-859 857 Table 5 Isomer shifts, quadrupole splittings, linewidths and areas derived from a variable-temperature 57Fe Mossbauer study of DMAMF-AI- PILC T/K phasea 6 (f0.02)/mm s-' A/2 (+O.O2)/mm s-' r/2(1) (&0.02)/mm s-' r/2 (r) (_+0.02)/mms-l normalised area relative area (_+2.5%) ~~~ x2 14 -0.03 0.19 0.20 0.20 0.10 13 0.674 14 0.66 0.19 0.30 0.30 0.07 9 0.674 14 0.43 1.22 0.18 0.18 0.61 78 0.674 25 -0.03 0.19 0.20 0.20 0.11 14 0.586 25 0.66 0.19 0.30 0.30 0.07 10 0.586 25 0.43 1.22 0.18 0.18 0.57 76 0.586 50 -0.03 0.19 0.20 0.20 0.12 18 0.591 50 0.66 0.19 0.30 0.30 0.09 14 0.591 50 0.43 1.22 0.14 0.15 0.42 68 0.591 80 -0.08 0.17 0.20 0.20 0.09 17 0.720 80 0.63 0.19 0.30 0.30 0.09 17 0.720 80 0.42 1.20 0.21 0.2 1 0.34 66 0.720 100 -0.03 0.19 0.20 0.20 0.1 1 22 0.599 100 0.66 0.19 0.30 0.30 0.08 16 0.599 100 0.41 1.21 0.20 0.20 0.3 1 62 0.599 140 -0.05 0.19 0.20 0.20 0.10 25 0.573 1 40 0.66 0.19 0.30 0.30 0.08 19 0.573 140 0.41 1.20 0.25 0.25 0.23 56 0.573 180 -0.05 0.19 0.20 0.20 0.10 32 0.575 180 0.68 0.19 0.30 0.30 0.08 24 0.575 180 0.40 1.20 0.17 0.17 0.14 44 0.575 220 -0.08 0.18 0.20 0.20 0.09 35 0.571 220 0.66 0.19 0.30 0.30 0.07 28 0.571 220 0.39 1.20 0.18 0.18 0.09 36 0.571 300 -0.08 0.19 0.20 0.20 0.09 45 0.594 300 0.68 0.19 0.30 0.30 0.07 35 0.594 300 0.37 1.20 0.16 0.17 0.04 20 0.594 -Q(1)=Fe"' present in fired Al-PILC; Q( 2) =Fe"' present due to isomorphous substitution in WL; Q(3)=Fe" present in ferrocene unit of incorporated metallocene.0.4 Table 6 Values for the Debye temperature and recoil free fraction for samples described in the text 0.0 sample FMDMACl 144 0.14 FMDMA+-WL( 1) 139 0.13 h a FMDMA+-WL( 2) 144 0.152 -0.5 FMDMA+-WL( 3) 139 0.12 Y DMAMF-WL 138 0.12 0,-0 DM AMF-Al-PILC 118 0.06 -1 .o -1 .E 0 100 200 300 TIK Fig. 8 Variation of Mossbauer absorption line area with temperature for 0,FMDMACl; A, FMDMA+-WL(1); 0, FMDMA+-WL(2);V,FMDMA+-WL(3); H,DMAMF-WL and @, DMAMF-A1-PILC the same as that in chloride salt. In contrast, the Debye temperature, OD, and corresponding recoil-free fraction, fZg1 K, for the DMAMF-A1-PILC complex were much lower at 118 K and 0.06, respectively, revealing that the metallocene was much less tightly bound.This is commensurate with a model in which the metallocene, which is probably the FMDMAf Cation, resides in a much freer environment in the PILC where Fig. 9 Schematic illustrations of the probable orientation of the the gallery height is larger than the FMDMA' cation. This FMDMA+ cation in (a)WL and (b)Al-PILC 858 J. Muter. Chem., 1996,6(5), 849-859 contrasts with the situation in FMDMA+-WL where the organoiron cation itself determines the layer expansion. Consequently, it is envisaged that in FMDMA+-WL the rings of the metallocene unit are keyed into the aluminosilicate layer and are tightly held [Fig.9(a)]. This would account for the lower than expected layer expansion and would mean that the Fe would be the only recoiling mass. In the DMAMF-A1- PILC it is reasonable to assume that the metallocene is anchored via the dimethylamino group and that the cyclopen- tadienyl rings are not tightly held [Fig. 9(b)]. Hence the metallocene unit would enjoy considerably more freedom in the gallery space of the PILC than in the cramped, interlayer environs of FMDMA+-WL( 1)-(3). Other workers3 have found that unsubstituted ferrocen?, which is essentially spherical with an effective diameter of 7 A, appears to have almost complete three-dimensional freedom at room temperature in A1PO4-5 and AlP04-8, Poth of which have channels of diameter greater than 7.8 A.This rapid rotation of the ferrocene molecule changes the average electric- field gradient for 57Fe to zero and consequently a singlet is observed in the Mossbauer spectrum. In FMDMA+-WL and DMAMF-Al-PILC, a doublet is observed at all temperatures indicating that the aminomethylmetallocene does not rotate rapidly within these layered hosts, Fig. 9, which depicts the schematic orientation of FMDMA' ion in both WL and Al- PILC, shows that there is little room for the FMDMA' cation to rotate in WL and the bulky side chain must prevent this cation rotating within the larger gallery space in Al-PILC. The influence of bulky side chains on the freedom of organoiron species in clays has been noted previously.' Conclusions FMDMACl has been prepared and characterised using a +number of techniques.FMDMA cations have been success-fully used to displace the resident Na+ cations from the interlamellar exchange sites in WL and loadings up to 80% cec have been achieved. PXRD indicates that a single layer of metallocene is incorporated bttween the sheets and the resulting layer spacing of 15.1 A is lower than anticipated, suggesting some keying of the molecule into the aluminosilicate layer. The single-lay5r complex is stable to 200°C whereupon it collapses to 13.0A. TP-SIP-MS data clearly show that a small proportion of the incorporated metallocene is volatilised at temperatures below 40O0C, but that the majority of the metallocene degrades via loss of the cyclopentadienyl ligands.A similar thermal degradation path was observed for DMAMF-A1-PILC. 57Fe Mossbauer spectroscopy revealed that the FMDMA cation occupied a similar environment in + the chloride salt, FMDMA+-WL and DMAMF-A1-PILC, insofar as the isomer shift and quadrupole splitting of the incorporated metallocene were essentially the same in all complexes. However, variable-temperature 57Fe Mossbauer spectroscopy confirmed that the metallocene enjoyed much greater freedom in the galleries of the A1-PILC. We are indebted to Dr. Rob Brown and Gareth Parkes of the Catalysis Research Unit at Leeds Metropolitan University for the TP-SIP-MS results. References 1 K. Moller, A. Borvornwattananont and T. Bein, J. Phys. Chem., 1989,93,4562.2 G. A. Ozin and J. Godber, J.Phys. Chem., 1989,93,878. 3 A. Lund, D. G. Nicholson, R. V. Parish and J. P. Wright, Acta Chem. Scand., 1994,48,738. 4 A. Lund, D. G. Nicholson, G. Lamble and B. Beagley, J. Mater. Chem., 1994,4,1723. 5 M. Endregard, D. G. Nicholson, M. Stocker and B. Beagley, J.Mater. Chem., 1995,5,485. 6 E. Rodriguez-Castellon, A. Jiminez-Lopez, M. Martinez-Lara and L. Moreno-Real, J.Znclusion Phenom., 1987,6, 335. 7 S. J. Mason, L. M. Bull, C. P. Grey, S. J. Heyes and D. O'Hare, J.Mater. Chem., 1992,2, 1189. 8 S. Okuno and G. Matsubayashi, J. Chem. SOC., Dalton Trans., 1992,2441. 9 C. Breen, J. S. Brooks, S. Forder, A. A. Maggs, G. Marshall and G. R. Stephenson, J. Mater. Chem., 1995,5,97. 10 C. Breen, J. Madejova and P. Komadel, J. Mater. Chem., 1995, 5,469. 11 R. Schoonheydt, J. van den Eynde and W. Stone, Clays Clay Miner., 1994,41, 598. 12 G. Brown, B. Edwards, E. G. Ormerod and A. H. Weir, Clay Miner., 1972,9,407. 13 A. Molinard, PhD Thesis, The University of Antwerp, 1994. 14 C. Breen, A. T. Deane and J. J. Flynn, Clay Miner., 1987,22,169. 15 P. A. Barnes and G. M. B. Parkes, J. Thermal Anal., 1993,39,607. 16 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, 1984,p. 1236. 17 J. W. Johnson, J. Chem. SOC.,Chem. Commun., 1980,263. 18 P. Aldebert and V. Paul-Boncour, Mater. Res. Bull., 1983,18, 1263. 19 C. F. Lee, L. K. Myers, K. G. Valentine and M. E. Thompson, J. Chem. SOC., Chem. Commun., 1992,201. 20 M. Hillman and A. G.Nagy, J. Organomet. Chem., 1980,184,433. 21 A. Simopoulos, D. Petridis, A. Kostikas and N. Gangas, HyperJine Interact., 1988,41, 843. 22 K. C. Molloy, C. Breen and K. Quill, Appl. Orgunomet. Chem., 1987,1,21. Paper 5/05572E; Received 22nd August, 1995 J. Mater. Chem., 1996, 6(5), 849-859 859

ISSN:0959-9428    

DOI:10.1039/JM9960600849

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Intercalation of n-alkylamines into misfit layer sulfides Lourdes Hernan, Pedro Lavela, Julian Morales," Luis Sanchez and JosC L. Tirado Laboratorio de Quimica Inorghnica, Facultad de Ciencias, Universidad de Cbrdoba, Avda. San Albert0 Magno sln, El 4004 Cbrdoba, Spain Novel intercalation complexes of (PbS)'.'8( TiS,), and (PbS)1.14( TaS&, two representative compounds of misfit layer sulfides, have been prepared by direct reaction with n-alkylamines (C,H2,+1NH2) (n=1-9). From the observed amine stoichiometry and lattice expansions, the most likely configuration for the intercalated amine molecules is one in which the latter are located at the van der Waals gap between two adjacent TS, slabs as a monolayer with the alkyl chain oriented parallel to the layers.Thermal deintercalation of the new phases occurred at temperatures below 250 "Cand in the case of Ti intercalates, H,S was released simultaneously with the amine deintercalation process. The intercalation process was confirmed to be reversible. Thus, the product obtained from the thermal treatment recovered the main features of the original host structure and, when mixed with the amine, yielded an intercalate of similar structural characteristics to those found using the original sulfide. Composite layered sulfides of formula (MS),,,(TS,), (M = lanthanides, Sn, Pb, Bi, Sb; T =Ti,V,Cr,Nb,Ta) are a relatively new class of layered compounds that have received special attention over the last five years, because of their peculiar structures and physical properties.' Their structures consist of alternation of two-atom-thick layers of MS and rn times three- atom-thick sandwiches of TS, stacked along the c axis.The atomic arrangement of MS layers is based on a distorted rock- salt structure and the TS, units are geometrically identical to the layers found in the binary TS, layer compounds. The unit- cell axes of both sublattices MS and TS, are equal in the b direction and incommensurate in the u direction. This intro- duces a certain degree of non-stoichiometry, y, which can be determined from uTS2/uMS. The number of well known compounds belonging to the family with rn= 1 is very large, while for the family with m=2, so far X-ray single-crystal structures have only been determined for (PbS)1.14(NbS2)2,2 and(PbS)1.18(TiS2)23 (PbSe)l,12( NbSe,),.4 Recently, the single-crystal structure of the trilayer misfit compound (Gd,Sn, -&s)1.16(NbsZ)3 has been reported.' From the point of view of intercalation reactions, these two latter families of compounds have a relevant characteristic, namely, the formation of van der Waals gaps between adjacent TS, slabs of similar geometry to that found in binary metal disulfides.This feature is the accommodation of metal ions, inorganic and organic molecules between the layers.6 In this context, lithi~m,~ sodium,' hydrazine, and even cobaltocene'' can be easily and successfully intercalated into misfit layer chalcogenides. A common aspect of all these guest species is their strong reducing power. In continuing our research on the intercalation properties of these novel layered chalcogenides we have extended our study to the intercalation of organic Lewis bases as prototypical guest species.In this paper we describe the preparation and characterization of novel intercalates with linear aliphatic amines (C,-C,) as guest molecules. The comparison of the results obtained with those reported for binary intercalated dichalcogenides with the same guest molecules provides valu- able information on the interactions between the two subcells in these incommensurated intergrowths. Experiment a1 The layered host materials (PbS)1,18(TiS2)2 and (PbS)1.14(TaS2)2used in this study were prepared from the corresponding elements (supplied by Strem. Chem) as reported earlier.3*'' The materials were obtained as high-purity plate- like crystals with a metallic lustre.All the amines were of reagent grade (Merck) and were used as supplied. Intercalation was accomplished by mixing the reactants (sulfide, 70 mg; amine, 3 ml) in sealed Pyrex tubes and then heating at 50-90 "C, depending on the hydrocarbon chain length of the amine. Times for complete intercalation ranged from 1 day for methylamine (50"C) to 10 days for n-nonylamine (70"C). The absence of the (001)reflections of the host was considered as evidence for complete intercalation. Solids were filtered, washed with pure ethanol, air-dried and stored in a dry glove-box. The amounts of intercalated amine were determined by C,H,N analyses while occasionally water content was calculated by combining these data and those obtained from TG curves recorded with a Cahn 2000 thermob-alance under a dynamic nitrogen atmosphere.The chemical identity of the intercalated species was checked by temperature- programmed deintercalation (TPD) measurements that were carried out in a quartz reactor coupled to a quadrupole mass spectrometer (model VG sensorlab). The gas carrier used was nitrogen. X-Ray powder diffraction (XRD) patterns were recorded in a Siemens D500 Instrument using Cu-Ka radiation, operating at 35 kV and 20 mA. For identification purposes, the intensities were collected with steps of 0.02" (28) and times of 0.6 s per step. For the broadening analysis of the Bragg lines, the intensities were collected with the same scan step and a time of 4 s per step. Results and Discussion The amine contents of the two sets of intercalates deduced from C,H,N analysis are shown in Table 1.In most compounds the agreement between the experimental and calculated per- centages is fairly acceptable (usually <5%). The larger differ- ences appeared for the methylamine and ethylamine intercalates, in particular for the hydrogen content, probably owing to the fact that these amines are supplied in aqueous solutions (50% and 70%, respectively). From these data, the degree of water content was also calculated and is shown in Table 1. The presence of water will be discussed below. Two interesting findings deserve an additional comment.First, amine contents are low and show only small fluctuations with the amine length and there is no correlation between the composition of the intercalates and the molecular volumes or J. Muter. Chem., 1996, 6(5), 861-866 861 Table 1 Compositions of ( PbS), 18 ( TiS,), (n-alkylamme),(H,O), and ( PbS)l 14 (TaS,), (n-alkylamine), (H,O), intercalates" (PbS)l 18 (TiS,),(n-alkylamine),( H20), n-alk ylamine C H N X met h ylamine 0 28 ethylamine 0 28 prop ylamine 0 28 butylamine 0 24 pentylamine 0 23 hex ylamine 0 25 hept ylamine 0 26 oct ylamine 0 26 non ylamine 0 21 "Calculated values are shown in parentheses the amine basicity This behaviour contrasts with that found in the intercalates (n-CnH2,,+ iNH2),TiS,,i2 for which an amine content about five times greater was observed, with a small tendency to increase as the amine molecular volume increases In the case of 2H-TaS2I3 the stoichiometry (x=O 5) reported for the intercalation of methylamine and ethylamine by direct reaction is also notably higher than that found in (PbS)l 14(TaS,)2 (see Table 1) These differences in the amine stoichiometry can be discussed in terms of the role played by the MS layers on electron-accepting potential of the TS, layer Although there is some controversy concerning the evaluation of the charge transfer from MS to TS2, both spectroscopici4 and electrical transporti5 data support the occurrence of interactions between MS (which acts as the electron donor part) and TS, sublattices (which act as electron acceptors) This should reduce the electron-accepting ability of TS2, thus decreasing their intercalation properties The second finding refers to the amine content measured in the Ti compound, which is somewhat higher than that found in the Ta compound This difference in reactivity cannot be due to particle size In fact, the average particle sizes deduced from scanning electron microscopy images were 50-60 pm and 10-20 pm for the Ti and Ta compounds, respectively Similar results have been obtained with other guest species such as cobaltocene," and they have been explained on the basis of the band structure of the component sublattices l6 According to studies of the photoelectron spectra, the band structure of the misfit layer compounds is approximately a superposition of the band structure of the two component sublattices As the TS, sublattice has an atomic arrangement identical to the TS2 unit of the binary transition-metal disulfide, for the compound with Ti as the transition metal the Fermi level lies at the t2g band which is formally empty, while for the Ta compound, the Fermi level lies at the dZ2 band which is formally half filled l6 This occupancy should hinder the charge transfer from the intercalated molecule to the host lattice and hence the poorer intercalation properties of the Ta compound Taking into account the low guest stoichiometry (see Table l),the amount of guest-host charge transfer must be low and it therefore seems unlikely that the amine contents of these compounds are determined solely by the host band structure In this context, one can find a correlation between the amine content and the volume of TS2 subcells (T=Ti, Ta) Thus, the greater amine content occu~s in the Ti tompound, which has a greater cell volume (347 9 A3 us 343 3 A3) The relatively small change observed for the amount of 862 J Muter Chem, 1996, 6(5),861-866 ( PbS), (TaS,),(n-alkylamine), ( H20),, Y C H N X Y 0 21 0 36 0 19 0 20 -0 22 -0 20 0 23 -0 18 -0 19 -0 19 -0 19 -intercalated amine with the change in alkyl chain length can be explained as a consequence of two different effects First, molecular size considerations would favour the intercalation of the amines with small chain lengths Second, for the heavier amines, the increasing contribution of van der Waals forces between the hydrophobic part of the alkylamines has a tend- ency to stabilize the molecular aggregates at the interlayer space l7 This facilitates the intercalation process The XRD patterns of the amine-intercalated powdered samples of the Ta compound showed a set of multiple order reflections (001) of high intensity together with additional broad and weak lines that were indexed on an orthorhombic lattice like that of the parent compound l1 Unit-cell parameters of both sublattices of some intercalated compounds, together with those of the pristine compound, are included in Table 2 Note that the a and b axes hardly changed upon intercalation, which indicates that the deformation of the layers in the ab plane is very small However, a significant expansion along the c axis direction was observed For the Ti intercalates, the XRD patterns showed only peaks associated with (001) reflec-tions as a consequence of a strong preferred orientation of the particles, even though the spectrum was recorded by sprinkling the powder on silicone grease For this reason, we could not calculate the unit-cell constants of these intercalates and only the periodic length, which defines the thickness of a PbS-TiS2-TiS2 unit stacked along the c axis, was computed The lattice expansion of the various amine intercalates per PbS-TS2-TS, unit packing is plotted in Fig 1as a function of the chain length, together with those values of (n-alkylamine),TaS, intercalates taken from the literature A relevant feature of the data in Fig 1 is the limited change of lattice expansion on moving from methylamine to n-nonyl- amine, which implies that the hydrocarbon chains lie practically parallel to the layers The increase in the interlayer spacing can be compared wi!h the van der Waals diameter of the methyl group, ca 3 6 A The difference which appears in going from methylamine to n-nonylamine probably reflects a very small deviation of the alkylamine guest orientation from host- layer parallel A constant interlayer spacing has also been observed for TaS, n-alkylamine complexes for carbon numbers up to 4 For longer amine chains in TaS," and TiS2,l2 the interplanar separation increases gradually with n (see Fig 1) Moreover, the composition and the c-axis separation suggests that in these binary sulfides, n-alkylamines form double layers in the interlayer space Table 2 Lattice constants of (PbS)1.14 (TaS,), intercalated with n-alkylamine compound subcell PbS(PbS)1.14 (TasZ)2 TaS, ( PbS)l.14 ( Tas2 )Z (methylamine)O.Zl PbS TaS, (PbS)l.14 (TasZ)Z (ethy1amine)0.19 PbS TaS2 ( PbS)I. 14 (TasZ)Z ( hexy1amine)0.18 PbS TaS, 25 20 V V V3 15 vQ 10 4 3 0 2 46810 "C Fig. 1 Change in the lattice expansion as a function of the number of carbon atoms in the alkylamine intercalation compounds of (PbS)l.18(TiS,), (0) For comparison the and (PbS)1.14(TaS2)2 (0).alkylamine intercalation compounds of TaS, have been included [data taken from refs. 18 (A)and 19 (V )]. The different intercalation properties of misfit layer sulfides can be explained by taking into account their peculiar struc- tural characteristics. As described earlier, the stacking sequence of PbS and TS, (T=Ti,Ta) slabs along the c axis is PbS-TS2-TS2-PbS. Of the two interlayer spaces available for the location of the amine, defined as the PbS-TS2 and TS,-TS, interfaces, the latter is more favoured for two reasons. First, the distance between T!S2-TiS2 slabs is greater than that of PbS-TiS, slabs (2.934A us. 2.689 A).3 Moreover, Pb atoms protrude from the sulfur planes on both sides of the PbS double layer, and are bonded to the S atoms of the neighboring TiS, sandwiches by weakly covalent interactions.Indirect evidence of the location of the guest molecules was obtained by using ( PbS)l.18TiS, and (PbS),.,,TaS, as host materials. The compounds have a PbS-TS,-PbS stacking sequence, whose host lattice only contains PbS-TS, interfaces. We failed to intercalate n-alkylamines into these hosts under the same experimental conditions as described above. One-dimensional electron-density projections on the c axis of the Ti compounds were examined by using (001) reflections. The intensities were corrected for Lorentz and polarization effects and the structure factor phases were calculated from the known host structure.The electron density maps were then computed from the phased data. Some of them are shown as illustrative examples in Fig. 2. This plot reveals the expected sequence of atoms along Cool] in the pristine solid: (S,Pb)-S-Ti-S-S-Ti-S-( S,Pb), as the Pb atoms protrude from sulfur planes on both sides of the PbS double layer. After amine intercalation, the (S,Pb)-S-Ti-S and S-Ti-S-( S,Pb) sheets remained basically unaltered while there was a signifi- cant increase in the electron density in the region between successive TiS, layers. This means that the guest molecules are located in the S-S interlayer spaces. Moreover, the presence of a main peak between the layers is in good agreement with an orientation of the amine almost parallel to the TiS, layers a/A bfA c/A 5.820( 1) 3.303( 3) 5.771 (2) 5.778 (4) 18.00(2) 17.99(3) 5.833( 3) 3.304( 2) 5.734( 7) 5.734( 5) 21.23 (7) 21.24( 3) 5.71 1 (6) 3.3 13( 2) 5.795( 7) 5.759(3) 21.63(2) 21.61( 1) 5.809( 3) 3.307( 5) 5.785( 3) 5.754(8) 21.90( 2) 21.89( 4) I P ~ Sl I Pristine I I 1 1 1 1 1 1 1111II11111 distanceIA Fig.2 One-dimensional electron-density projection on the c axis of ( PbS)1.18(TiS,), and three representative examples of alkylamines in a monolayer arrangement. A schematic model of this arrangement is shown in Fig. 3. The different arrangement of n-alkylamines ( PbS)l + ,,( TS2)2 (T= Ti,Ta), compared with transition-metal layer disulfides, can be explained by taking into account the presence of the PbS layer between two consecutive TS2 slabs.According to SolinY2' layered solids can be classified into three groups based on the rigidity of their layers with respect to the distortions involving displacements transverse to the layer planes. Layer dichalcogenides belong to class 11. These compounds consist of three distinct planes of strongly bonded atoms and can undergo a significant transverse distortion due to the relatively high flexibility of the structure. These solids can sometimes accommodate bilayers and even trilayers o{ intercalant, and thus lattice expansions of as much as 50A have been ob- served for the neutral anisometric guest species, e.g. (octadecylamine),,,TaS, .18 According to the intercalation model described above and depicted in Fig. 3, the insertion of the PbS layers between two consecutive TS2 slabs increases the thickness of the layers that now are composed of as many J.Muter. Chem., 1996, 6(5), 861-866 863 Fig. 3 Idealised model of the structure of ( PbS)l +,,(TS,),(n-alkylam- me), complexes as five planes of strongly bonded atoms One can accept that the structures of these misfit layer sulfides will increase in rigidity against transverse layer distortions This increased host rigidity is likely to act as a kinetic barrier both to the uptake of guest molecules and to lattice expansion, and the amine should adopt a parallel orientation In fact, the dependence of lattice expansion on chain length shown in Fig 1 is similar to the expected behaviour for an infinitely rigid solid such as silicate clays The stability of the intercalates was examined from both thermogravimetric data and the results of the desorbed mol- ecules analysed by mass spectrometry The temperature range studied was 30-300°C and the base peaks of the mass spectra of amine, ammonia, carbon dioxide, dihydrogen sulfide and hydrogen compounds were recorded Of all these chemicals only the presence of amine together with H,S in the case of Ti intercalates was detected The samples may pick up some water during air handling, in particular during the transfer of the intercalated material to the TG and MS apparatus, but the water spectrum could not be recorded owing to the background of HzO in the spectrometer The TPD curves of the methylamine, ethylamine and n-butylamine intercalates are shown in Fig 4 The TPD profiles of heavier amine samples were more complicated, because of the increasing possibility of side reactions which may have taken place in the gas phase The thermogravimetric curves of all intercalation compounds showed a continuous mass loss with no sharp steps, so that it was difficult to define the temperature of the amine deintercal- ation with accuracy However, the amine profile of the TPD spectra showed a single peak centred at ca 175"C and 225 "C for Ti and Ta intercalates, respectively This is also indicative of the presence of one type of amine species at the interlayer, and the rather low temperature of deintercalation suggests that these species are weakly bonded within the layers, prob- ablj through an interaction between the nitrogen lone pair and antibonding or non-bonding empty cation states Another point of interest in the case of Ti intercalates is the simultaneous appearance of H,S with amine deintercalation The formation of H,S was recently reported in the deintercal- ation process of FePS3(CH3NH2)" and MnPS, (pyridine)?' intercalates, and was explained by assuming the presence of alkylammonium and pyridinium cations, respectively, solvated by neutral amines as the intercalated species The source of protons is probably the H,O molecules which are always associated with the amines However, in MPS, materials, charge neutrality is preserved by the loss of M2+ from the lattice, based on the ability of the lattice to exchange a fraction 864 J Muter Chem , 1996, 6(5),861-866 1 1 1 1 I I I I I I 30 100 150 200 50 100 130 200 250 300 TPC Fig.4 Temperature-programmed deintercalation spectra of different (PbS), 18(T1S2)2 and ( PbS)l 14(TaS2)2 intercalates I, methylamme, 11, ethylamine, 111, n-butylamine The profiles correspond to (a)amine base peak (m/z30) and (b)H2S (m/z 34) of the M2+ intralayer cations 23 Such an intercalation mechan- ism cannot be applied to misfit layer sulfides Two reaction models can account for the liberation of H2S First, by considering that the amines were present as alkylam- monium hydroxide, and amine and H2S are produced by reaction (I) 21 R-"H30H+S2-(*attlce)-)R-NH2+H2S+02-(latt,ce)(1) An alternative mechanism for the formation of ammonium cations in the presence of water has been outlined according to reaction (2) 24 .Y(NH3)+ TS, + xH,O+( NH4 + )Zx( NH3),-Zx( TS2 -xOx)2x-+(XlY)S, (2) This mechanism was extended by Johnson25 to the aniline intercalation reaction in TaS, in the presence of water by suggesting that some amine molecules are found as protonated amine cations while the sulfide lattice is reduced In our case, simple calculations for the methylamine intercalates, supposing that all amine molecules are protonated cations, suggest that cu 3% of the lattice sulfide should be replaced and oxidized This means that the amount of sulfur released should be < 1% of the original sulfide, too low to be unambiguously measured Unfortunately, IR spectra, which can shed light on the actual intercalated species, could not be recorded because of the high absorbance of the samples However, evidence to discard the possibility of the formation of protonated amine cations was obtained from TPD studies, as no hydrogen was evolved during the deintercalation process The appearance of this molecule can be indicative of the presence of co-intercalated alkylammonium ions as exhibited by ammonia-intercalated TiS, This Ti compound contains both ammonia and ammonium, with ammonium decomposing to ammonia and hydrogen during the thermal deintercalation process 26 Moreover, attempts to intercalate methylammonium cations from methylammonium hydroxide were unsuccessful A second scheme for the release of H2S could involve a direct attack to the lattice by water molecules [reaction (3)]: H2O +S2-(lattice) -+H2S +O2-(lattice) (3) Unfortunately, for the Ti intercalates it was not possible to estimate the water content from the TG data owing to the loss of H2S.These calculations, carried out in the methylamine and ethylamine Ta intercalates, yielded water contents in fairly good agreement with those obtained from C,H,N analyses (Table 1). Moreover, for the heavier amines, the amounts of water deter- mined were insignificant. We have no direct proof of the location of the H,O molecules in the host lattices, but both Ti and Ta compounds remained unaltered after treatment with pure water under the same experimental conditions used for the alkylamine intercalation.Moreover, the intercalated phases obtained with methylamine vapours had identical lattice expansions to those given in Fig. 1, which were prepared with the amine dissolved in water. The TPD spectra were also similar to those included in Fig. 41. Joy and Vasudevan21.22 suggested an exchange reac- tion between physically adsorbed water during the transfer of the intercalated material to the mass spectrometer apparatus and neutral amine molecules. This exchange reaction is probably enhanced for the intercalation complexes containing short-chain amines. In fact, for the n-butylamine intercalate, the intensity of the H2S peak notably decreased (see Fig.4III), whereas for heavier amines this signal practically disappeared. This feature can be correlated with the increasing hydrophobic nature of the guest species, which may limit water uptake. According to this explanation, a strong association between amine and water molecules in the intercalated state accounts for the simultaneous liberation of amine and H2S. The absence of H2S loss in the deintercalation process of Ta intercalates, in spite of their water content, should be correlated with the free energies of formation of TiO, and TaO,, -21 1.4 and -43 kcal mol- ', re~pectively.~'t Thus, the greater stability of Ti02 should favour reaction (3), and this might be the origin of the lower deintercalation temperatures exhibited by the Ti intercalates.Moreover, this behaviour is in accordance with that reported by Whittingham,' concerning the ease of preparation of A,(H20),TaS2 (A =NH,, Li, Na, K) intercalates by immersion of the sulfide in aqueous solutions. In contrast, the alkali-metal hydrates of other sulfides, particularly those of group 4, were more difficult to obtain because these sulfides are more easily hydrolysed than TaS,. The reversibility of the intercalation process was confirmed by mixing the product obtained from the TPD measurements with the intercalant under the same experimental conditions as described above. The results obtained, referred to ethylamine and (PbS)1.18(TiS2)2, are shown in Fig.5, and clearly demon- strate the formation of the intercalate of similar structural characteristics to that found using the original compound as the host lattice. This means that the release of produced H,S has little effect on the host lattice (according to XRD results), probably owing to the small extent of reaction (3). Additional information about the microstructural changes induced by the intercalation-deintercalation process in the host lattice was obtained by X-ray diffraction line-broadening analysis. The experimental X-ray powder diffraction patterns of multiple-order (001) lines were used to obtain the profiles (h)and a highly crystalline silicon standard was used to provide the instrumental broadening (g),after Ka, elimination by the Rachinger method.,' The cosines of Fourier coefficients of the pure diffraction profiles (f) were then obtained by the Stokes3' method of deconvolution [eqn.(4)]: Fn =HnIGn (4) where F, G and H are the cosines of the Fourier coefficients off, g and h profiles, respectively, and n is the order of the Fourier coefficient. t 1 calz4.184 J. I I II' 'I (C) ~ 10 20 30 40 50 60 2Wdegrees Fig. 5 XRD patterns of (a) (PbS)l,18(TiS2)2;(b) (PbS), 18(TiS2)2-(ethylamine),,,,; (c) sample (b)after thermal treatment; (d) sample (c) mixed with ethylamine, prepared under the same experimental condition as sample (b) Crystallite sizes and microstrains were then computed by the Warren-Averbach method,31 using eqn. (5): In F,( l/d) =In F,"-2n2(cn2)n2PL2/d2 (5) where F," are the size coefficients, (E,~) is the mean-square value of the component of strain, P, is the (001) periodic length, and d the (001) spacing.The crystallite size, (Do,,), was computed from the derivative dF,s/dnl,,o. Selected Warren-Averbach plots are shown in Fig. 6. The pristine misfit layer compounds showed negligible slopes for all n values, thus indicating that the microstrain content is low in the solids obtained at high temperatures. The values of crystallite size were also high (Table 3), irrespective of the exfoliation properties of these materials in directions normal to [0011. After ethylamine intercalation, a significant increase in the slopes of the plots evidenced an enhanced microstrain content.This behaviour can be interpreted in terms of a broad but symmetric distribution of interlayer spacings after the intercalation of the organic molecule. Slight changes in the angle formed between the hydrocarbon chain and the host layers could also account for the observed broadening. These changes may be either static or dynamic, as in other oxosalt intercalates. The values of the crystallite size reveal a significant decrease upon intercalation, as referred to the number of layers in each crystallite, probably as a consequence of the rupture of coherency of diffraction between consecutively stacked layered domains. On the other hand, after alkylamine thermal deintercalation the slopes decrease again, reaching values close to the pristine compounds, in good agreement with the above interpretation. A significant exfoliation also takes place on deintercalation.Conclusion We have shown that (PbS)1.1~(TiS2)2, and (PbS),.,4(TaS2), two representative compounds of misfit layer sulfides with MS :TS, =1:2, undergo a direct reaction with n-alkylamines C,H,, + ,NH2, with the subsequent formation of a complete series of intercalation compounds for n =1-9. In addition, compared with other families of related layer compounds such as transition-metal disulfides, the c axis expansion hardly changes with the chain length, and the experimental values suggest that the hydrocarbon chains are oriented parallel to the layers. This unusual finding is probably a consequence of the PbS layers that reduce the flexibility of the TS, layer structure.The intercalation reaction is reversible. J. Muter. Chern., 1996,6(5), 861-866 865 I II I References v n=3 0 05 0.10 0.1 5 0.20 2.5 1 I I n=3 2.0 I n=4 n G 1.5 c 7 1.0 0.5 - 0.0 I- 1 1 0 0.00 0.05 0.10 0.15 0.20 r I I 1 I 1 0.5 -n=l -n=O 0.0 2 1 1 1 1. 1 Fig. 6 Warren-Averbach plots of (001) reflections for (a)(PbS)l 18(T1S2)2, (b)(PbS)l 18(TiS,)2(ethy1amine)0 28, (c) (b)after thermal treatment Table 3 Results of the X-ray diffraction line-broadening analysis along [OOl] by the Warren-Averbach method intercalate/ misfit layer compound treatment (Dool)a/A n((Dool)/PL)b (PbS)I 18 (T1S2)2 pristine 242 14 ethylamine 209 10 300 "C 71 4 (PbS)114(TaS2)2 pristine 173 10 ethylamine 159 7 300 "C 120 6 "(Do,, )=size of coherently diffracting domains bn=number of layers per crystallite, P, =periodic length The authors gratefully acknowledge support from CICYT (MAT 93-1204) and Junta de Andalucia (Group 6036) and the help of Dr M A Aramendia and Mr A Porras from the Organic Chemistry Department and Mass Spectrometry Service in recording TPD spectra 1 G A Wiegers and A Meerschaut, in Sandwiched Incommensurated Layered Compounds, ed A Meerschaut, Trans Tech Pub Zurich, 1992 and references therein 2 A Meerschaut, L Guemas, C Auriel and J Rouxel, Eur J Solid State Inorg Chem , 1990, t27, 557 3 A Meerschaut, C Auriel and J Rouxel, J Alloys Comp, 1992, 183,129 4 C Auriel, A Meerschaut, R Roesky and J Rouxel, Eur J Solid State Inorg Chem , 1992,29,557 5 L M Hoistad, A Meerschaut, P Bonneau and J Rouxel, J Solid State Chem, 1995,114,435 6 A J Jacobson, in Intercalation Chemistry, ed M S Whittingham and A J Jacobson, Academic Press, New York, 1982, p 229 7 (a)C Auriel, A Meerschaut, P Deniard and J Rouxel, C R Acad Sci Paris, 1991, t313, 1255, (b)C Barriga, P Lavela, J Morales, J Pattanayak and J L Tirado, Chem Muter, 1992, 4, 1021, (c) P Lavela, J Morales and J L Tirado, J Muter Chem, 1994, 4,1413 8 L Hernan, J Morales, L Sanchez and J L Tirado, Chem Muter, 1993,5,1167 9 Y Oosawa, Y Gotoh, J Akimoto, T Tsunoda, H Sohma, H Hayakawa and M Onoda, Solid State Ionics, 1994,67,287 10 L Hernan, J Morales, L Sanchez, J L Tirado and A R Gonzalez-Elipe, J Chem SOC Chem Commun, 1994, 1081, L Hernan, J Morales, L Sanchez, J L Tirado, J P Espinos and A R Gonzalez Elipe, Chem Muter, 1995,7, 1576 11 Y Goto, J Akimoto, M Sakorai, Y Kiyozumi, K Suzuki and Y Oosawa, Chem Lett, 1990, 2057, L Hernan, J Morales, L Sanchez and J L Tirado, Electrochim Acta, 1994,39,2665 12 A Weiss and R Ruthardt, Z Naturforsch, Teil B, 1973,28,249 13 S F Meyer, R E Howard, G R Stewart, J V Acrivos and T H Geballe, J Chem Phys ,1975,62,4411 14 Y Ohno, Solid Stute Commun , 1991,72, 1081, Phys Rev B, 1991, 44, 1281, J C Jumas, J Olivier-Fourcade, P Lavela, J Morales and J L Tirado, Chem Muter, 1995,7, 1193, A R H F Ettema and C Haas, J Phys Condens Matter, 1993, 5, 3817, A R H F Ettema, G A Wiegers, C Haas and T S Turner, Surf Sci , 1992, 269/270,1161 15 G A Wiegers, A Meetsma, J R Haange and J L de Boer, Muter Res Bull, 1988, 23, 1551, G A Wiegers and R J Haange, Eur J Solid State Inorg Chem, 1991,28, 1071 16 A R H F Ettema, PhD Thesis, University of Groningen, Netherlands, 1993, A R H F Ettema, C Haas and T Turner, Phys Rev B, 1993,47,2794 17 C Rosner and G Lagaly, J Solid State Chem ,1984,53,249 18 F R Gamble, J H Osiecki, M Cam, R Pisharody, F J Disalvo and T H Geballe, Science, 1971,174,493 19 R Schollhorn, E Sick and A Weiss, Z Naturforsch Ted B, 1973, 28,168 20 S A Solin, in Intercalation in Layered Materials, ed M S Dresselhaus, NATO AS1 Series, Plenum Press, New York, 1986,p 145 21 P A Joy and S Vasudevan, Chem Muter, 1993,5,1182 22 P A Joy and S Vasudevan, J Am Chem SOC,1992,114,7792 23 R Clement, 0 Garnier and J Jegoudez, Inorg Chem, 1986, 25, 1904 24 R Schollhorn, E Sick and A Lerf, Muter Res Bull, 1975, 10, 1005, R Schollhorn, in Progress in Intercalation Research, ed W Muller-Warmuth and R Schollhorn, Kluwer Academic Publisher, Dordrecht, 1994, p 1 25 J W Johnson, Physica B, 1980,99,141 26 M J Mckelvy and W Glaunsinger, Annu Rev Phys Chem ,1990, 41,497 27 R H Shumn, D D Wagman, S Bailey, W H Evans and V B Parker, Selected Values of Chemical Thermodynamic Properties, NBS Technical Notes, Washington, 1974 28 M S Whittingham, Muter Res Bull, 1974,9, 1681 29 W A Rachinger, J Sci Instrum, 1948,25,254 30 A R Stokes, Proc Phys SOC London, 1948,61,382 31 B E Warren, X-Ray Digraction, Dover Publications, New York, 1990,p 251 Paper 5/067676, Received 12th October 1995 866 J Muter Chem , 1996, 6(5), 861-866

ISSN:0959-9428    

DOI:10.1039/JM9960600861

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Optical and EPR spectroscopic studies of silver clusters in Ag,Na-Y zeolite by y-irradiation E. Gachard,*aJ. Belloni' and M. A. Subramanian" aInstitut de Chimie et de la Matiire Condenske de Bordeaux (ICMCB), Chdteau Brivazac, Avenue du Docteur A. Schweitzer, F-33608 Pessac, France bLaboratoire de Physico-Chimie des Rayonnements, associi au CNRS, Bdtiment 350, Universitk Paris-Sud, F-91405 Orsay, France 'Dupont Central Research & Development, P.O. Box 80328, Wilmington, DE 19880-0328, USA Silver clusters in y-irradiated faujasite (zeolite Y) have been studied and characterized by optical absorption and electron paramagnetic resonance (EPR) spectroscopies. Depending upon the composition and conditions of preparation (silver and water content, irradiation dose), different silver cluster species and silver particles were observed. A new silver cluster species inside the faujasite lattice, which corresponds to a low silver content, is presented.Optical absorption bands at 265 and 305 nm have been observed and assigned to trimeric silver ions (Ag32f). In addition, EPR data of the dehydrated low-silver-content samples indicated the formation of trimeric silver clusters (Ag,') with an isotropic EPR signal (g,,, =2.034, coupling constant of 90-100 G) and no detectable hyperfine splitting. These silver clusters are probably linear. There has been increasing interest in the development of new optical, electronic and catalytic materials with precisely defined nanoparticle size, geometry and dimen~ionality.l-~ For this purpose, the synthesis and characterisation of metal clusters in different matrices (e.g.polymers, zeolites, noble gases), and in colloidal solution have been Most of the studies reported deal with the formation of metal clusters by conven- tional methods such as hydrogen reduction at moderate tem- peratures. For a better understanding of the nucleation process of cluster formation, and of the resulting chemical properties (which are different from those of the bulk), we attempted to form clusters using a radiation-induced reduction method. The method involves irradiation of zeolites with high energy y-rays to generate reducing ~0nditions.l~ When used as a support for ultra-small metal particles and clusters, zeolites appear different from other matrices, such as glasses, amorphous supports, solutions or noble gases, since the size and the diffusion characteristics of the clusters formed are determined by geo- metrical constraints induced by the size of the cages as well as their interconnections within the zeolite structure.15 The types of clusters formed, thus, can be correlated with the structure of the zeolite, the concentration of the cation exchanged or degree of ion exchange, and the type of activation and reduction.In this paper, we describe the formation of silver clusters in zeolite Y prepared by reduction induced by y-irradiation at room temperature. The aim of this work is to combine EPR and optical absorption methods in order to identify the nature of the silver clusters formed within the faujasite framework and to investigate possible correlations.Experimenta1 The silver-exchanged Na-Y zeolite (Ag,Na-Y) was pre-pared from synthetic zeolite Na-Y with an %/A1 ratio equal to 2.45 and a unit-cell composition represented by +Na,,A1,,Si,3,0,,4~242H20. The Na ions were partially exchanged by Ag' ions, by stirring the Na-Y zeolite with an appropriate volume of an AgC104 aqueous solution (0.05mol dm-3) for 24 h. After filtering, the material was washed with double-distilled water, to remove any excess Ag' ions from the zeolite lattice in the solution. The exchanged zeolite samples were placed into a Pyrex tube with purified water and propan-2-01 (CH,),CHOH (RH; 0.01 mol dm-3) in order to scavenge the OH' radicals which have oxidizing properties and to replace them with reducing radicals (CH3)2C OH (R.).The solutions contained zeolite (density 1.92g cmP3, microporous volume 0.25 cm3 g-l, particle size 2pm) at 100gdmP3 and were degassed by bubbling with argon or nitrogen prior to irradiation. The y-radiolysis was then performed at room temperature with a 137Cs source, rated at 0.240 Mrad h-' (2.40 kGy h-'). The preparation, handling and storage of all samples were performed in the dark. The exchanged silver content was obtained by atomic absorption analysis either after zeolite dissolution or by reversible exchange with the solution of silver ions. Samples with the greater silver content were prepared using a 0.06 mol dm-3 silver salt solution.The optical absorption data were collected on a Varian Cary 5.E spectrophotometer using quartz optical cells of thickness 2mm: because of the strong light scattering by the zeolite particles, the samples were diluted 20 times with deoxygenated distilled water. For EPR measurements, the samples were, after irradiation, dried thoroughly and degassed in a vacuum at room temperature. EPR spectra were then measured at 4K on a Bruker spectrometer at 9.6 GHz. Results Optical absorption observations The room-temperature optical absorption spectra for the zeo- lite sample with a composition Ag,Na,,-Y irradiated with different y-ray doses are shown in Fig. 1. The spectrum of the sample prior to y-irradiation is also included for comparison. The spectrum of the pristine sample shows a single absorption band at 210 nm, whereas, upon y-irradiation, two new absorp- tion bands appear: a narrow band at 305nm and another band at 265 nm which grow simultaneously with increasing irradiation dose.The spectra also present a shoulder at 250 nm which seems to be independent of the dose, and another weak band at 360nm (Fig. 1). On further irradiation, the samples become dark grey-brown. The spectra also clearly show the development of a very broad absorption band centred around 440 nm; the intensity of this band decreases when the sample J. Muter. Chem., 1996, 6(5),867-870 867 07 265 nmt1 I06 J 30s-05 440 nm 02 Ol'.. ;. ' :" ':.'':'...I 200 300 400 500 600 700 800 wavelengthlnm Fig. 1 Optical absorption spectra of exchanged Ag,Na,,-Y zeolite recorded at room temperature, after 20 times dilution 1, prior to irradiation, 2, sample y-irradiated with 0 7 Mrad, 3, 0 9 Mrad, 4, 1 1 Mrad, 5, 13 Mrad is exposed to oxygen In contrast, the absorptions at 305 and 265 nm are little affected by oxygen A sample with a higher silver content was also analysed The spectra of the sample, AgloNa,,-Y, show an absorption band at 440 nm from 0 3 Mrad, and no optical absorption bands at 305, 265 or 250 nm (Fig 2) However, at 0 3 Mrad an intense absorption is observed in the UV-VIS region, superimposed upon the 440nm band With increasing dose, this component decreases and the 440 nm absorbance is devel- oped, so that the total absorbance at 440nm is lower at 0 5 Mrad than at 0 3 Mrad When the sample is exposed to oxygen, the intensity of this band decreased significantly EPR results The EPR spectra (after dehydration) of the lower-silver-content sample irradiated with different y-ray doses, essentially consist of an isotropic doublet with g,,, =2 034 and a splitting constant of ca 90-100G Fig 3 shows the data for the sample y- irradiated with 13 Mrad and Fig 4 shows the evolution of these spectra with the irradiation dose O9 h I 0708!\ 4 8 06 c -f!05 0 n a 04 03 02 01 200 300 400 500 600 700 800 wavelengthlnm Fig.2 Optical absorption spectra of exchanged Ag,,Na,,-Y zeolite recorded at room temperature, after 20 times dilution 1, pnor to irradiation, 2, sample y-irradiated with 0 3 Mrad, 3, 0 5 Mrad, 4,0 8 Mrad 868 J Mater Chem ,1996, 6(5),867-870 DPFW 4 3200 3250 3300 3350 3400 3450 BIG Fig.3 EPR spectra recorded at 4 K of Ag,Na,,-Y zeolite y-irradiated with a 1 3 Mrad dose 2800 3000 3200 3400 3600 BIG Fig. 4 EPR spectra recorded at 4 K of Ag,Na,,-Y zeolite y-irradiated with 1, 0 1 Mrad, 2,O 2 Mrad, 3, 0 3 Mrad, 4,O 45 Mrad, 5, 0 75 Mrad For the sample with the higher silver content, no EPR signal was observed Therefore, this sample contains only diamagnetic species after y-irradiation Discussion The formation of silver clusters with different nuclearities inside zeolite (A, X or Y type) frameworks has been studied by various authors using the changes observed in the optical absorption spectra during dehydration, reduction and oxi- dation processes l2 Different reduced cluster species have been tentatively identified in silver zeolite As mentioned earlier, the optical absorption spectra of lower-silver-content samples (Fig 1) contain different bands (1) two bands at 265 and 305 nm which grow simultaneously with irradiation dose and which are very little affected by exposure to oxygen, (2) a broad band which peaks at 440 nm with an intensity increasing with irradiation dose, and (3) two shoulders at 250 and 360nm which are too weak to be attributed The bands at 265 and 305 nm are correlated because they grow simultaneously with irradiation dose and they are not affected by oxygen They both seem to belong to the same species It is known that optical absorption data observed for y-irradiated aqueous silver solutions with low irradiation dose showed two optical absorption bands at 265 and 310nm, which were assigned to Ag32+ species16 The formation of Ag32+ clusters in solution corresponds to pseudo-first order reactions of two successive additions of Ag' to the primary atom, the excitation coefficient being ca 1 8 xlo8 dm3 mol-'cm-' in water" Therefore, we assign the two bands observed at 265 and 305 nm in Y zeolite to the same Ag32f species These bands do not correspond to Ag, or Ago as previously identified in Ag-Y zeolite (at 285 and 308 nm, respectively) If we consider that the zeolite concentration is 100 g dmP3 and that the cavity volume which contains Ag+ (2 2 mol dm-3) is ca.50% of the zeolite volume, we derive that the radiolytic yield, as a preliminary evaluation, would be G =22 species per 100eV. Since the total yield of radiolytic species in zeolite cavities is ca. G=6 species per 100 eV,20 it is clear that the silver reduction is more efficient, as follows. (i) The local concentration of Ag' in the zeolite cavity, irrespective of the type of cavities occupied by silver, is quite high (3.6 mol dm-,), hence these result in higher reduction yields. In addition, Ag+ may even undergo direct interaction with radiation. (ii) The strong coupling of Ag+ with the zeolite structure allows scavenging of additional electrons produced in the zeolite lattice which, owing to its electronic density, receives a radi- ation dose 6 times higher than the cavity volume.Other authors 21 found a similar electron yield, three times higher in hydrated zeolite than that observed in free water. In the case of the silver zeolite mentioned above, the irradiation dose rate is also low and the formation of positively charged clusters predominates owing to the presence of excess silver ions. The stability with respect to coalescence of the cationic Ag,'+ clusters is due to their interaction with the anionic sites of the aluminosilicate lattice. The absence of reaction with oxygen is possibly because of the non-diffusion of oxygen molecules into the smallest cages.After dehydration under vacuum, the EPR spectra of Ag,NaS1-Y showed an isotropic doublet with g,,, =2.034 and a splitting constant of ca. 90-100 G. We suggest that the EPR signals shown in Fig. 3 and 4 correspond to the signal of the trimeric silver cluster species. The trimeric silver clusters may have different charges and structures, both of which are paramagnetic: Ag32+ and Ag,'. Several configurations are possible for trimeric clusters with different point-group sym- metries such as Dooh(linear configurations), D,, (equilateral triangles) and CZL(isosceles triangle^).^^,^^ According to the literature," the Ag3'+ species produced in zeolite Y are characterized by an EPR twin signal, with a g factor equal to 1.974 and a large coupling of 515 G.The Ag,' species obtained after y-irradiation at 77 K for the dehydrated Ag,,Na-A zeo-lite,24 are characterized by an isotropic quartet with glso= 2.043 and a hyperfine coupling constant of 102G. The line intensity ratio (close to 1:3 :3 : 1) and the isotropic nature of the EPR spectrum imply interaction with three equivalent silver nuclei with a triangular or bent structure. By comparing our EPR results (for low-silver-content samples) with the above-mentioned EPR parameters for Ag," and Ag,' species, the paramagnetic silver clusters observed in this study could be better assigned to linear Ag,' with three non-equivalent silver nuclei. In this case the EPR spectrum should show a doublet, as was observed for our samples (Fig.4). The reduced splitting in the signal observed suggests that spin population is probably not distributed evenly within the cluster. The fact that no EPR signal corresponding to Ag3'+ clusters was observed after dehydration leads us to assume that Ag,' observed by EPR is formed by the complete reduction of Ag,2+ during the dehydration step. As shown in Fig. 4, the intensity of the EPR signal assigned to Ag,' increases with the irradiation dose. This is in agreement with our observations that as the irradiation dose increases the concen- tration of Ag32+ also increases before dehydration (Fig. 1). The Ag,' species absorbs optically around 360 nm.24 However, the signal that we observed at this wavelength was too weak to be analysed quantitatively.As mentioned previously, the optical absorption spectra of the irradiated zeolite with the higher silver content exhibited only one broad absorption band in the range 380-460nm. The intensity of this band increased with the absorbed irradiation dose (Fig. 2) and decreased after exposure to oxygen. This band may be associated with diamagnetic silver centres as indicated by the absence of an EPR signal. The absence of the EPR signal was already observed" for clusters containing more than six nuclei and for larger clusters. An absorption band at 380 nm for silver clusters in zeolite Y was assigned by Kellerman and Texter'' to species containing approximately seven nuclei. We suggest that this absorption at 440 nm may be due to clusters of n 3 6 atoms which develop only in the larger cage of the zeolite framework.Moreover, these clusters are corroded by oxygen. It is likely that their size does not exceed 8-10 atoms7 and that they are located in a site readily accessible to oxygen molecules such as the supercage of the zeolite lattice, and this explains why the intensity of this band decreases upon exposure to oxygen. The colour evolution from brown to darker grey during y-reduction may again confirm the formation of larger cluster^.^' The formation of silver clusters in the y-irradiated zeolite sample can probably be explained by a mechanism involving migration and subsequent recombination of Ag + ions and/or Ago atoms as shown in eqn. (1)-(9).The electrons involved in the reduction steps are generated by the interaction of y-rays with water m01ecules~~ located in the zeolite channels, H' and OH' radicals being scavenged by alcohol molecules in the supercages only. H20 eaq-, OH', H30+,H', H,, H 20, (1) RH+OH' (H') -+ R'+H,O (H2) (2) Ag+ +eaq- + Ago (3) Ag'+Ag+ + Ag,+ (4) Ag2+ +Ag+ +. Ag,'+ (5) At increasing irradiation: (9) According to the discussion, a seemingly direct effect of the radiation on Ag+ , or the scavenging of electrons produced in the lattice by Ag' can also occur. Ag + + ezeol--+ Ago (10) Reaction (10) shows that the cluster formation during irradiation depends not only on the presence of water, but also on the electron scavenging probability of Ag+ ions as well as Ago atoms or other intermediate species, if any.This is in agreement with the observed EPR signal (single signal which can be assigned to Ag,' silver clusters) for the low-silver- content zeolite sample. Among diamagnetic species it is known, for instance, that Ag3+ absorbs at cu. 440 nm.24 These species containing two reduced atoms must be formed with higher radiation doses [reactions (6), (7) or (S)]. Conclusions y-Radiolysis in aqueous solutions is a powerful method for the in situ production of metal clusters in zeolites. The reduction of silver-exchanged faujasite induced by y-irradiation has been studied by optical absorption and EPR spectroscopies. The absorption bands at 305 and 265nm observed in the optical data have been assigned to the Ag,2+ species, and the broad absorption band at 440 nm has been assigned to diamagnetic species such as Ag, + (for low-silver-content samples) or clusters with higher nuclearities (for higher-silver-content samples).We suggest that the paramagnetic silver species observed by EPR spectroscopy in zeolite Y (with low silver content) are trimeric linear silver clusters (As,') which are formed by the reduction of Ag3'+ during the dehydration step. Because of their ultra- small size, location, reactivity and interaction with their J. Muter. Chem., 1996, 6(5),867-870 869 environment, clusters in zeolites pose a greater challenge in terms of their characterization and identification Extended X-ray absorption fine structure (EXAFS) and high-resolution microscopy studies are under way to obtain further information on the location and near-neighbour interactions in these materials, and the complete studies will be published soon The authors gratefully acknowledge Dr J Michalik, Dr J F Silvain and Dr J Khatouri for helpful discussions and sugges- tions We would also like to thank Dr M Turner for critically reading the manuscript References T Wasowtcz and J Michalik, Radiat Phys Chem ,1991,37,427 R A Schoonheydt, J Phys Chem Solids, 1989,50,523 R Janes, A D Stevens and M C R Symons, J Chem SOC Chem Commun ,1988,1454 G A Ozin, F Hugues, D F McIntosh and S Mattar, ACS Symp Ser , 1983,58,1351 A Tonscheid, P L Ryder, N I Jaeder and G Schulz, Surf Sci, 1993,281,51 M Mostafavi, N Keghouche, M 0 Delcourt and J Belloni, Chem Phys Lett, 1990,167,193 0 Platzer, J Amblard, J L Mangnier and J Belloni, J Phys Chem ,1992,96,2335 E Janata, A Henglein and B G Ershov, J Phys Chem, 1994, 98,10888 9 B G Ershov, E Janata and A Henglein, J Phys Chem, 1993, 97,339 10 J Baumann, R Beer, G Calzafern and B Waldeck, J Phys Chem , 1989,93,2292 11 M Narayama, A S W Li and L Kevan, J Chem Phys, 1985, 83,2556 12 A S W Li and L Kevan, Radiat Phys Chem, 1982,20,199 13 A Abou-Kais, J C Vedrine and C Naccache, J Chem Soc Faraday Trans 2,1978,74,959 14 J Khatouri, M Mostafavi, J Amblard and J Belloni Chem Phys Lett ,1992,191,351 15 J Michalik, T Wasowicz, A Van der Pol, E J Reijerse and E de Boer, J Chem SOC Chem Commun ,1992,29 16 E Janata, A Henglein and B G Ershov, J Phys Chem, 1994, 98,10888 17 E Janata, Radiat Phys Chem , 1996,47,29 18 R Kellerman and J Texter, J Chem Phys ,1979,70,1562 19 D R Brown and L Kevan, J Phys Chem, 1986,90,1129 20 M Mostafavi, M 0 Delcourt and G Picq, Radiat Phys Chem, 1996,41,453 21 X Liu, G Zhang and J K Thomas, J Phys Chem, 1995, 99, 10024 22 W S Scruple and H Abe, Faraday Discuss Chem SOC, 1980, 14,87 23 B G Ershov, G V Ionova and A A Kiseleva, Russ J Phys Chem , 1995,69,239 24 J Michalik and L Kevan, J Am Chem Soc , 1986,108,4247 25 C Ferradini and J Pucheault, in Bzologze de 1 action des rayonne- ments ionisants, Masson, Pans, 1983, p 25 Paper 51066386, Received 9th October, 1995 870 J Muter Chem, 1996,6(5), 867-870

ISSN:0959-9428    

DOI:10.1039/JM9960600867

出版商:RSC    

年代:1996

数据来源: RSC