摘要:

LuBa,Cu30, -x thin films prepared using MOCVD Sergey V. Samoylenkov, Oleg Yu. Gorbenko, Igor E. Graboy, Andrey R. Kaul and Yury D. Tretyakov Chemistry Department, Lomonosov Moscow State University, 119899 Moscow, Russia LuBa,Cu,O,-, thin films with T,= 86-88 K were prepared by flash evaporation MOCVD on LaAlO,, SrTiO,, Zr0,(Y,03) and NdGaO, single-crystal substrates (deposition temperature 795 "C, partial oxygen pressure 1.35 Torr). Values for the critical current density,j, (77 K, H = 100 Oe), of 9 x lo5, 1.1 x lo6and 1.2 x lo6 A cm-, were measured for films on ZrO,(Y,O,), SrTi0, and LaA10, respectively. The calculated magnitude of IAj,/ATl for LuBa,Cu,O,-, films on coherent substrates was found to be higher than that of YBa2Cu3O7-, films, indicating a more efficient magnetic flux pinning mechanism.The temperature dependence of conductivity fluctuations was considered in terms of a Lawrence-Donniach model and a 3D +2D dimensional crossover was registered for the films on SrTiO,. The LnBa2Cu30, -,family of compounds (Ln = lanthanide attempted to determine the real reason for the stabilization element) has not attracted as much interest as YB~,CU,O~_~, effect. since the properties of LnBa,Cu,O, -,were often considered to be similar to those of the yttrium phase. All attempts to synthesize single-phase LuBa,Cu,O, -ceramics were unsuc- cessful, resulting in the remarkable scarcity of literature on the subject.1-7 The published data are in good agreement with the theory of LnBa,Cu,O,-, phase stability described in ref.3, which states that the stability of LnBa2Cu,O7-, is lowered signifi- cantly as the ionic radius of Ln3+ decreases. The decrease of the LnBa2Cu30, -,peritectic melting temperature on moving to the heavier lanthanide elements' is good evidence for this decrease in stability. In view of this theory one can assume that the po,-T area of LnBa,Cu,O, -,thermodynamic stability (Po, =oxygen partial pressure) is narrowest when Ln = Lu; thus, under the standard ceramic synthesis conditions, the LuBa,Cu,O,-, phase turns out to be unstable. However, the partial substitution of Lu by lanthanides with larger ionic radii stabilizes this phase. Superconducting solid solutions Lu, -,Ln,Ba2Cu307 -,,have been prepared as a single phase with Ln = Y, Sm, Dy and even Pr and Tb.3*476,9910 It was noted that the single-phase ceramics only forms if the (Lu,Ln),+ radius exceeds some critical value, which was found to be nearly equal to the radius of Yb3+ ., In contrast with these results, according to recent publi- cation~"-~~LuBa2Cu30,-,single crystals can be successfully grown by a self-flux technique.The untwinned single crystals of LuBa,Cu,O,-, prepared are of high quality and have outstanding superconducting characteristics compared with YBa2Cu,07-,~rysta1s.l~However, the thermodynamic stab- ility of LuBa,Cu,O,-, single crystals was not studied, and the growth conditions were not described. It was suppo~ed~~~'~ that the LuBa,Cu,07-, phase can also be obtained as a thin film.There are basically two reasons to expect this stabilization: (1) the lower po, and temperature of the phase formation in comparison with the ceramic synthesis, and (2)the gain in free energy for a highly oriented film on a coherent substrate. Thin films of LuBa2Cu307-, prepared recently by laser ablation16-18 were single phase, highly c-axis oriented and demonstrated excellent superconducting properties (critical temperature, T,= 86 K; critical current density, j, (77 K) = (2-5) x lo6 A ern-,, surface resistance, R, (77 K, 10 GHz)= 320 @). Nevertheless, the phase stability of LuBa2Cu307-, in the thin film state was not characterized. In this work we tried to prepare LUB~~CU,O,-~ thin films by another prospective deposition technique, namely metal- organic chemical vapour deposition (MOCVD) and we Experimental Thin films of LuBa,Cu,O, -,on ZrO,( Y203),SrTiO,, LaAlO, and NdGaO, single-crystal substrates were prepared by a flash evaporation technique as described else~here.'~.~~ The exper- imental setup is shown in Fig.1, and the main parameters of the deposition process are summarized in Table 1. The volatile precursors used were Lu( thd),, Cu(thd), and Ba(thd), or Ba( thd),.2phen (thd = 2,2,6,6-tetramethylheptane-3,5-dionate, phen = o-phenanthroline). X-Ray diffraction (XRD) studies of the films including 8-26 scans and 4 scans were performed with a DRON-3M diffractometer to determine the phase composition, orientation and lattice parameters of LuBa,Cu,O, -,.Sputtered neutral atom mass spectrometry (SNMS) depth 6 Fig. 1 Experimental MOCVD setup: 1, vibration device of the feeder; 2, powder mixture of precursors; 3, evaporator; 4, heated transport lines; 5, quartz reactor; 6, resistance furnace; 7, substrate; 8, substrate holder with rotation gear; 9, pressure gauge; 10, gas supply; 11, pumping out line Table 1 Main parameters of the deposition process powder evaporation temperaturePC 230-240 argon flow/l h-' 14-17 oxygen flow/l h-' 2-4 deposition temperature/oC 780-830 pulse duration/s 10 pulse rate/pulses min -3 powder feed rate/mg min-' 0.5-1 reactor pressure/Torr 8-10 substrate rotation rate/cycles min-' 5 J. Mater Chem., 1996,6(4), 623-627 623 profiling was carried out using an INA-3 system. A single crystal of LuBa2Cu307-, grown by the self-flux technique was used as a standard for the quantitative analysis of the film composition.Scanning electron microscopy (SEM) with a Jeol JEM-2000 FXII instrument was used to study the surface morphology of the films. Ac susceptibility x( T)measurements were performed in the temperature range 100- 10 K using an EPD-cryogenics magnet- ometer. Temperature dependences of the critical current den- sity, jc(T),were found from the imaginary part of x(T).The calculation technique based on the Bean critical state model is described in detail in refs. 21 and 22. Electrical resistance R(T) was measured using the standard ac four-probe technique with silver-plated contacts in the temperature range 77-300 K.High-temperature electrical resistance measurements were accomplished using the ac (1kHz) two-probe technique with gold contacts. Ceramic synthesis was performed by calcination for 1h of a BaO,, Lu203 and CuO mixture at 900°C in air followed by regrinding and pressing. The procedure was repeated twice. Final sintering was accomplished in the oxygen flow at 850-940°C over 5 h. The cooling step was carried out in both oxygen and argon atmospheres. Results and Discussion The initial conditions for the deposition of Lu-Ba-Cu-0 thin films were chosen according to the results of the optimization of YBa,Cu,O, -x film preparation by MOCVD.23,24 Further definition of deposition parameters and variation of the com- position of the precursor powder mixture was accomplished using XRD and SNMS data.As a result, mainly c-oriented LuBa2Cu,0, -x thin films containing no admixture phases were prepared from precursors mixed in the mole ratio Lu(thd), : Ba(thd), : Cu(thd), = 1.0: 3.2 : 2.3 at the deposition temperature 795-800 "C and po2 = 1.35 Torr. If Ba(thd),.2phen was used instead of Ba(thd),, po2 = 1.5 Torr was necessary to prepare films of the same quality. At lower po2 values, strong interactions between LuBa2Cu307-, and ZrO,(Y,O,) sub-strate were observed, which indicates a partial melting in the film. This effect is probably due to the additional consumption of oxygen to oxidize phenanthroline. The appearance of a peak corresponding to reflections (1 10) and (103) of LUB~,CU,O,-~ was observed in the pr$sence of admixture phases and for films thicker than 3000A, which suggests the formation of randomly oriented crystallites.The peak had a rather high intensity when the deposition tempera- ture was decreased to 780 "C; apart from this peak, only (001) peaks were simultaneously registered. So in this case the film consisted of two single-axis textures, namely (001) and (110). No (hOO)reflections were observed for the prepared films, so under the deposition conditions used the a orientation well known for YBa2Cu307-, films did not form in LuBa2Cu307-films. The c parameter was found to pe equal to 11.67 A for the films on ZrO,(Y,O,), and 11.69A for the films on SrTiO,, LaA10, and NdGaO, (calculations were made for films of thickness 3000-4000 A), the latter value being close to that of single-crystal LuBa2Cu307-, reported in refs.12 and 13. An additional 2 h annealing of LuBa2Cu307-, films on different substrates was carried out at 450°C in the oxygen flow to provide films with the same oxygen non-stoichiometry, thus removing any oxygen effects on the lattice parameters. The difference in the lattice parameters was not affected by the annealing. The reason for the result seems to be connected with the larger lattice mismatch between LuBa,Cu,O,-, and Zr02(Y,03) as compared with the other substrates used. Using 4 scans for reflections (107) and (105) of LuBa2Cu3O7-,, and (102) and (220) of SrTiO,, the following epitaxial relations were established for LuBa,Cu,O, -./SrTiO,: [lOO]f//[lOO], and [OOl]f//[OOl], (f and s denote film and substrate respectively).In the case of LuBa,Cu30,~,/Zr02(Y,03), scans for4 reflections (106) and (113) of LuBa2Cu307-, and (222) of ZrO,(Y,O,) were used. With [OOl]f//[OOl],, two types of orientation in the ab plane were registered, namely[loo],//[ 1101, and [ l0Olf//[ 1001,; the former was predomi- nant. The intensity ratio of the corresponding peaks on 4 scan patterns correlated with the j, values. In fact, the coexistence of two types of orientation in the ab plane indicates the formation of larger angle grain boundaries in the film (45" boundaries) which are responsible for the decrease in j,. The typical surface morphology of LuBa,Cu,07 -thin films prepared by MOCVD is shown in Fig.2. Separate crystallites revealing numerous growth steps and spiral elements on their surface protrude above the dense matrix. Such a surface morphology is the most pronounced for LuBa2Cu,07 -,films on Zr02(Y203), starting to develop at film thicknesses as low as 600-800 A. The dimensions and the number of outgrowths increase and their shape becomes more imper-fect when the film thickness is increased. It is rather difficult to deduce surface roughness characteristics from the SEM in-plane view, but an indirect estimation can be obtained from the broadening of the film-substrate interface in the SNMS profiles. From Fig. 3, the surface roughpess of the LuBa,Cu,OJ-,/SrTiO, film of thickness 3000 A does not exceed 600 A.A comparison of the surface morphology features of LuBa2Cu307-,and YBa2Cu,07 -$s-27 thin films prepared by MOCVD reveals some distinctions: (1) an absence of a-oriented crystallites in LuBa,Cu,O, -x films, irrespective of film thickness aned deposition conditions used; and (2) dis- turbance of the superconducting matrix flatness for the thinner LuBa,Cu,O, -x films, compared with YBa,Cu,O, -films. The distribution of constituents in the films was rather Fig. 2 Surface morphology of LuBa,Cu,O, -film of thickness 3000A on SrTiO,; (a)magnification 2 x lo4;(b)magnification 8 x lo4 624 J. Muter Chem., 1996, 6(4), 623-627 1 u AI I ma 400 6Qo 800 1ooo etching time/s Fig.3 SNMS LuBa,Cu,O, - depth JSrTiO,, profiles foro film thickness 3000 A constituents of uniform according to the SNMS depth profiling (Fig. 3).Nevertheless, a significant broadening of the film-substrate interface was registered for LuBa,Cu,O, -JNdGaO, with marked penetration of Ga into the film. So, concerning the film-substrate interactions, the behaviour of LuBa,Cu,O, -films is quite similar to that of YBa,Cu,O,-, films. The results of measurements of the temperature dependences of the ac susceptibility and the electrical resistance are summar- ized in Table 2 and Fig. 4. The small widths of the diamagnetic superconducting transitions correspond to highly c-oriented films. LuBa,Cu,O,~, films on Zr02(Y203), LaAlO, and SrTiO, are characterized by T,=87-88 K, which agrees well with values for LUB~,CU,O,-~ films prepared by pulsed laser deposition (PLD).14*16-’8 Ev idently, the lower T, of the film on NdGa0, is due to the chemical interaction between the film and substrate detected by SNMS.All LuBa,Cu,O, -films prepared demonstrated deviation from the linear behaviour of the electrical resistance in the temperature range T,<T <(T,+ 15 K) [Fig. 4(a)]. The enhancement of the electrical conductivity near T, (‘paraconductivity’)28is caused by the fluctuation phenomena. The effect was first considered by Aslamazov and Larkin as the acceleration of short-lived superconducting charge-carrier pairs formed in the thermal non-equilibrium above T, in an electrical field. The paraconductivity, Aa(t), defined as l/p(t)-l/po(t), where p(t) is the measured resistivity (am), po(t)is the extrapolation of the linear part of the p(t) depen-dence to the lower-temperature range, and t =(T/T,)-1, is described for a two-dimensional layered superconductor by the Lawrence-Donniach formula: where e is the electronic charge, [,(t) is the coherence length in the c direction and d is the distance between superconducting layers (equal in the case of LnBa,Cu,O, -x, to the c parameter).Two limiting cases of the formula are two-dimensional fluctu- ations [C,(t)<<d] and three-dimensional fluctuations in the Aslamazov-Larkin theory. As far as p(t) is derived from R(T) II ‘5Y -0.4 n1 I I Fig.4 (a) Temperature dependences of electrical resistance of LuBa,Cu,O,-, films on: 1, SrTiO,; 2, ZrO,(Y,O,); 3, NdGaO,; (b)temperature dependences of ac susceptibility x( T)of LuBa2Cu,0, -films on: 1, SrTiO,; 2, ZrO,(Y,O,); 3, LaA103 (in an excitation field of 0.1 Oe) multiplied by the geometry parameters, the consideration of R(T) curves in the coordinates of Fig. 5 differs from the standard plot, ln[Aa(t)] -In@), only by a constant. The temperature range of fluctuation seems to be broader than that for YB~,CU,O,-~ which is why the crossover from a three-dimensional fluctuation to a two-dimensional mode (3D-2D) first assumed for LuBa,Cu,O,-, in ref. 11 on the basis of specific heat measurements can be easily observed from the proper consideration of the resistivity curves (Fig. 5).The values of j, (77 K, H= 100 Oe) for the prepared LuBa2Cu307-, films are normally > lo5 A cm-2, except for LuBa,Cu,O, -JNdGaO, where chemical interactions with the substrate took place. The results correspond to the reported properties of the films prepared by PLD.14 Maximal j, values achieved are listed in Table 2. The value of j, correlates with the lattice mismatch between LuBa,Cu,O,-, and substrate material. Of the substrates studied, LaA10, possesses the smallest lattice mismatch with LuBa,Cu,O,-, judging by the a and b lattice parameters from ref. 12, thus j, is the highest for LuBa,Cu,O, -JLaA10,. LuBa2Cu,0, -thin films of superior quality were also prepared by PLD on the substrate.16-18 LuBa,Cu,O, -thin films on coherent substrates (LaA10, Table 2 Superconducting characteristics of LuBa,Cu,O, -x films AT,“/K T,IK ATb/K j, (77 K)/ substrate on X(T) (R=0) on R(T) 10’ A cm-, LaA10, 87 --12 SrTiO, 87 87 1 11 Zr02 (y203) 87 88 1 9.7 NdGa03 85 86 2 0.8 a Determined by 10% and 90% of the maximal diamagnetic response.Determined by 10% and 90% of the resistance directly before transition. J. Mater Chem., 1996,6(4), 623-627 625 31 I In (T-TJT,) Fig. 5 Temperature dependence of excess conductivity in LuBa2Cu,0, ./SrTiO,, revealing crossover of fluctuation mode 3D +2D R,( T), resistance determined by extrapolation of the linear part of the R(T)curve to the lower-temperature range and SrTi03) show higher IAjJATJ values than epitaxial YBa2Cu307-, film prepared by MOCVD also (Fig 6) Such behaviour indicates the more efficient magnetic flux pinning mechanism in LuBa,Cu,O,-, films These new pinning centres can be (1) precipitates of secondary phases formed during cooling owing to the possible eutectoid decomposition in the temperature range where LuBa2Cu30,-, is metastable, (2) extended defects of higher density formed during the film growth The surface morphology features described above correspond to the latter assumption We could not find any evidence for the former assumption Moreover, no observation of such precipitates was reported in the literature available Detailed high-resolution electron microscopy investigations may be useful for solving the problem The appearance of secondary phases during the ceramic synthesis of LuBa,Cu,O, -2 suggests that possibility (1) may be the case One might expect that the decomposition of LuBa,Cu30, -x takes place at the low-temperature or the high- temperature boundary of the thermodynamical stability area We observed no signs of low-temperature decomposition, neither during cooling of the prepared films nor after additional annealing at po,= 1 and 0 3 bar in the temperature range 550-780 "C for 0 5-1 h (according to electrical resistivity and XRD measurements) High-temperature decomposition of LuBa,Cu,O, -,occurs at temperatures 2900 "C (Po, =0 21 bar) 29 Below 900 "C the 30 \4 y 20 6 a (D .z 0-10 0.' 30 5'0 7'0 b? phase formation in the bulk proceeds too slowly The goal of the ceramic syntheses in this work was to prove that these incompatible limitations are responsible for the synthetic difficulties We observed no XRD peaks of LuBa,Cu,O,-, after annealing for 5 h of the ceramic precursor in an oxygen atmosphere at 850-920 "C Thus, the successful preparation of LuBa2Cu307-, is possible from the melt only This is the case for single-crystal growth l1 l3 However, because of the differ- ence between the melt and crystallizing phase compositions, the preparation of the single-phase ceramic material is practi- cally impossible 30 During zn sztu growth from the vapour phase, kinetic obstacles observed in the solid-state reaction do not take place Thus the synthesis of LuBa,Cu,O,-, is possible inside the area of thermodynamic stability where the single-phase mate- rial is forming Conclusion In this study, the optimum conditions for the preparation by MOCVD of single-phase c-oriented LuBa,Cu,O, -thin films with high superconducting characteristics were found The basic peculiarities of LuBa,Cu,O, -,thin films as compared to YBa,Cu,O, -,ones are higher IAjJATI values revealing more efficient magnetic flux pinning, a broader temperature range of fluctuations near T,, and a stronger disturbance of the surface morphology The behaviour of LuBa,Cu30,-, thin films at enhanced temperatures, still within the metastability range, is of great interest By initiating advanced precipitation under such con- ditions, one could bring about the formation of new numerous pinning centres This research was supported by ISF grant MAT300 and by the grant of the Russian National program 'High temperature superconductivity' (N 93-458) The authors would like to acknowledge their colleagues from the Chemistry Department of MSU P E Kazin, S A Pozigun, V V Lennikov, V Putljaev and J A Rebane, for many useful collaborations in the study of films The authors are also grateful to Dr V I Voronkova for providing single-crystal standards References 1 P H Hor, R L Meng, Y Q Wang, L Gao, Z J Huang, J Dectold, K Forster and C W Chu, Phys Rev Lett, 1987, 58,1891 2 J M Tarascon, W R McKinnon, L H Green, J W Hull and E M Vogel, Phys Rev B, 1987,36,226 3 P Somasundaram, A Mohan Ram, A M Umarji and C N R Rao, Mater Res Bull, 1990,25,331 4 R Balakrishnan, U V Varadaraju and G V Subba Rao, Modern Phys Lett B, 1989,3,653 5 T Itoh, M Uzawa and H Uchikawa, J Cryst Growth, 1988, 91,397 6 A Oota, Y Sasaki, M Ohkubo and T Hioki, Jpn J Appl Phys, 1988,27, L1425 7 E Hodorowicz, S A Hodorowicz and H A Elick, J Alloys Compd, 1992,181,445 8 V K Janovskyi, V I Voronkova, I V Vodolazskaja, L N Leont'eva and T P Petrovskaya, SPhChT, 1989, 2, 30 (in Russian) 9 K I Gnanasekar, A S Tamhane, R Pinto, R Nagarajan, M Sharon, L C Gupta and R Vijayaraghavan, Physica C, 1994, 219,183 0 G Markandeyulu, K V Gopalakrishnan, A K RaJaraJan, L C Gupta,R Vijayaraghavan,A S Tamhane,K I Gnanasekar and R Pinto, Phys Rev B, 1993,47,1123 1 B Zhou, J Buan, S W Pierson, C C Huang, 0 T Valls, J I Liu and R N Shelton, Phys Rev B, 1993,47, 11631 2 J Buan, B P Stojkovich, N E Israeloff, A M Goldman, C C Huang, 0 T Valls, J I Liu and R N Shelton, Phys Rev Lett, 1994,72,2632 13 J.Buan, B. Zhou, C. C. Huang, J. I. Liu and R. N. Shelton, Phys. Rev. B, 1994,49,12220. 22 P. E. Kazin, T. E. Os’kina and Yu. D. Tretyakov, Appl. Supercond., 1993, 1, 1007. 14 R. Pinto, S. P. Pai, A. S. Tamhane, P. R. Apte, L. C. Gupta, R. Vijayaraghavan, K. I. Gnanasekar and H. V. Keer, Phys. Rev. 23 0.Yu. Gorbenko, V. N. Fuflyigin, Yu. Yu. Erokhin, I. E. Graboy, A. R. Kaul, Yu. D. Tretyakov, G. Wahl and L. Klippe, J. Muter. B, 1992,46,14242. Chem., 1994,4,1585.P. Schwab, X. Z. Wang and D. Bauerle, Appl. Phys. Lett., 1992, 24 0.Yu. Gorbenko, A. R. Kaul, S. A. Pozigun, E. V. Kolosova, 60,2023. R. Pinto, K. I. Gnanasekar, H. V. Keer, A. S. Tamhane, L. C. Gupta and R. Vijayaraghavan, Physicu C, 1994,227,120. R. Pinto, A. G. Chourey and P. R. Apte, Appl. Phys. Lett., 1994, 25 S. N. Polyakov and V. I. Scritny, Muter. Sci. Eng. B, 1993, 17, 157. L. Luo, M. E. Hawley, C. J. Maggiore, R. C. Dye, R. E. Muenchausen, L. Chen, B. Schmidt and A. E. Kaloyeros, Appl. Phys. Lett., 1993,62,485. 64, 2166. K. I. Gnanasekar, P. Selvam, H. V. Keer, R. Pinto, S. C. Purandare, 26 J. Hudner, 0. Thomas, E. Mossang, P. Chaudouet, F. Weiss, D. Boursier, J. P. Senateur, M. Oestling and A. M. Gaskov, J. Appl. A. S. Tamhane, L. C. Gupta and R. Vijayaraghavan, Appl. Phys. Lett., 1994,65, 1296. 27 Phys., 1993,74, 4631. Y. Q. Li, J. Zhao, C. S. Chern, P. Lu, B. Gallois, P. Norris, B. Kear 19 I. S. Chuprakov and A. R. Kaul, J. Chem. Vup. Deposition, 1993, and F. Cosandey, Physicu C, 1992,195,161. 2, 123. 28 W. Lang, C. Heine, P. Schwab, X. Z. Wang and D. Baeuerle, Phys. 20 W. J. Lackey, W. B. Carter, J. A. Hanigofsky, D. N. Hill, E. K. Barefield, G. Neumeier, D. F. O’Brien, M. J. Shapiro, J. R. Thompson, A. J. Green, T. S. Moss, R. A. Jake and 29 30 Rev. B, 1994,49,4209. G. V. Bazuev, SPhChT, 1992,5171 (in Russian). N. Guskos, V. Likodimos, A. Koufoudakis, C. Mitros, H. Gamari- K. R. Efferson, Appl. Phys. Lett., 1990,56, 1175. Seale and M. Wabia, Phys. Status Solidi, B, 1994, 182, K19. 21 J. Z. Sun, M. J. Scharen, L. C. Bourne and J. R. Schrieffer, Phys. Rev. B, 1991,44, 5275. Paper 5/05483D; Received 17th August, 1995 J. Mater Chem., 1996, 6(4), 623-627 627

ISSN:0959-9428    

DOI:10.1039/JM9960600623

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Unit-cell symmetries and Raman spectra of calcium- and neodymium-doped barium cerate proton-conducting ceramic electrolytes Robert C. T. Slade,*" Sara D. Flint," Alison Holloway," Narendra Singh,"*b Lubomir Smrcok' and Daniel Tunega' aDepartmentof Chemistry] University of Exeter, Exeter, UK, EX4 4QD bDepartment of Physics, Gaya College, Gaya 823001, India 'Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava, Slovak Republic Combined X-ray powder diffraction and Raman spectroscopic investigations of the systems BaCe, -,Ca,O, -a and BaCe, -,Nd,O, -o: at ambient temperature are presented. X-Ray studies involving in-depth investigation of diffraction profiles show that the unit cell (a crystal average) is truly orthorhombic throughout the composition ranges of both systems.The Raman spectra for BaCe, -xCax03-a are similar to that for BaCeO, itself, indicating little distortion of the structure when calcium is dopant. A feature at around 610 cm-' in the spectra for BaCe, -,Nd,O,-,, and not evident in the spectra of Ca-doped samples, is associated with modes involving the rare-earth-metal dopant. Studies of proton-conducting ceramics are comparatively recent. In 1981 Iwahara et al.' found that certain perovskite- type oxides exhibit appreciable ionic conductivity (crzlop2S cm-l), at temperatures in the range 600 <T/"C <800, in atmospheres containing hydrogen or water vapour. Protonic conduction in such materials is a result of reactions of oxygen vacancies (which are produced by partial substitution by an aliovalent cation, e.g.YIrl on Ce" sites) and/or holes with moisture or H, to generate OH- ions, the protons of which can migrate by jumping to a neighbouring (unprotonated) oxygen. The first such materials studied were of the type SrCel-xM1ll,O,-o: (MI1'= trivalent metal and a= number of oxide ion vacancies in the lattice per formula unit).' Further studies have included other systems with trivalent dopants (BaCe, -xM111x03 CaZr, -xM111x03 -a, -a, BaZr, -xM111x03-a, BaTh, -xGd,032-12 and SrZr, -xM111x03-a, also the system BaCel-,Ca,0,-a13-16 (Ca is a divalent dopant). Recent developments have included novel complex perovskites of the types A2BC06 and A,BC209 (A =Sr, Ba; B, C =metals, e.g.BC, =CaNb,), with oxide vacancies introduced by varying the B :C atomic These materials exhibit protonic conductivities greater than those of previous systems. The unit-cell symmetries appropriate to the structures of BaCeO, and the derived protonic conductors BaCe, -,M,03 -a have been the subject of much confusion in the literature, this confusion being consequent, in part, on possible slight distor- tions (rotations of CeO, octahedra) which would result in a symmetry lower than cubic. Early studies proposed a cubic (Pm3m),199" tetragonal (P4/mbm)21 or orthorhombic (Pbnm)22*23cell. The results of X-ray powder diffraction studies are determined largely by the positions of the 'heavy' (high atomic number) atoms present and are 'insensitive' to distor- tions originating largely in the repositioning of 'light' atoms (0in cerates). The details of X-ray diffraction profiles do not, however, appear to have been the subject of studies at high resolution.A single-crystal X-ray diffraction study of SrCeO, showed that compound to belong to the space group Pbr~rn.,~ In experimental studies of cell symmetries in the systems BaCe, -xMx03-a no studies combining high-resolution diffraction techniques and spectroscopic (Raman) measure- ments on the same samples have been reported. Confusion in the previous literature could have arisen from comparison of the results of X-ray and Raman studies carried out by separate teams on different samples with possibly differing thermal histories.Combined studies provide definition of unit-cell symmetry (from diffraction) prior to interpretation of the appearances of Raman spectra, and we now present such studies of the systems BaCe, -,CaXO3-a and BaCe, -xNd,O, -a at ambient temperature. Experimenta1 Materials Doped barium cerates BaCe, -,Ca,O, -a (x =0.02, 0.05, 0.10, 0.15) and BaCe, -xNd,O,-a (x =0.02,0.05,0.10) were prepared from dry powders by standard ceramic routes as described previo~sly.~*~*'~Sample compositions were verified by both X- ray fluorescence (XRF) and energy dispersive analysis by X- rays (EDAX) techniques, with BaCeO, as standard. Intended (by design of reaction mixtures) and empirical compositions agreed well within experimental error (at the relatively high mass percentages of Ba and Ce in these materials, and with the isostructural parent BaCeO, as standard, uncertainties in elemental percentages are small fractions of one percent of the empirical values; derived uncertainties in x are small, even for x =0.02).X-Ray powder diffraction studies Initial X-ray powder diffraction studies (in Exeter) wete per- formed using Ni-filtered Cu-Ka radiation (A= 1.54178 A) and a Philips PW1050 goniometer adapted in-house for step- scanning and digital acquisition of diffraction profiles at ambi- ent temperature. Initial profiles, recorded with a step size of 28=0.01" and a dwell time of 8 s, were as reported in the literature (e.g.refs. 6,15), consistent with the formation of single perovskite phases.Close examination of these profiles (by computer expansion of the axes in 28) revealed evidence for possible structure in the 'lines' at 28>40". Further profiles were recorded over the range 39 <20 <61", with a step size of 0.01" and a dwell time of 160s. Fig. 1 and 2 show sections of these profiles for single phases in the systems BaCe, -,Ca,O, -a and BaCe, -,NdXO3 -a respectively. Each 'line' in the range 39 <28 <61" is at least a triplet, and it follows that, as for the BaCeO, parent, all the phases studied are of lower symmetry than cubic or tetragonal at ambient temperature. High-resolution powder diffraction studies were performed in Bratislava using a Stoe STADI-P diffractometer in trans- mission mode and configured with a curved Ge(ll1) primary beam monochromator and linear position-sensitive detecto:.Strictly monochromated Cu-Ka, radiation (A=1.540598 A) J. Mater. Chem., 1996, 6(4), 629-633 629 I I I I 1 400 40.5 41 0 41.5 420 500 505 51 0 515 520 5815 59'0 595 600 605 26/degrees Fig. 1 Sections of the X-ray powder diffraction profiles (Exeter) for the system BaCe, ,$2aXO3 for (top to bottom) x =O 15,O 10 0 05,O 02,O 0 I I I I I 400 405 41 0 41 5 420 500 505 51 0 51 5 520 58 5 590 595 60.0 605 2Megrees Fig. 2 Sections of the X-ray powder diffraction profiles (Exeter) for the system BaCe, xNd,O, a for (top to bottom) x =O 10, 0 05, 0 02, 0 0 (see above) Full structure determinations, and was used to scan the diffraction profiles at ambient temperature and Niel~en~~ in the angular range 20 <28 <120" in steps of 0 02" All samples in particular the fine detail of 0-atom positions, would require thus analysed were strong absorbers and the orthorhombic neutron powder diffraction data The purpose of this study unit-cell symmetry led to profiles which, relative to those for was to examine symmetry and determine lattice constants a related cubic cell, had additional fine structure and a plethora of new peaks of low intensity, rendering it difficult to maintain Raman spectra an equal precision for all estimated peak positions Rietveld Spectra were collected in Bratislava at ambient temperature techniquesz5 were therefore applied (giving the advantage of using a JEOL JRS-SZ Raman spectrophotometer equipped an additional, structural, constraint in lattice parameter with a double-grating monochromator An argon ion laser refinement procedures) via a local version of the program was used as the source, providing excitation at 488 0 nm (blue DBW3226 Atomic positions, taken to be those determined line) The power of the beam at the sample was ca 350mWfrom single-crystal data for SrCeO, (space group Pbnrn) by and the scattered light from the powder samples was collected Ranlrav and Niel~en,~~ were held constant The lattice param- at the right-angle scattering geometry eters, the so-called profile parameters and the overall isotropic temperature factors were allowed to vary, and the background was modelled with a polynomial in 28 Rietveld refinements Results and Discussion converged with R, =8-1 1% Fig 3 shows a typical empirical diffraction profile obtained Results of previous studies -a) and the subsequent fit The use of neutron powder diffraction techniques might have in Bratislava (for BaCeo 98Nd0 0203 to the profile after Rietveld refinement The profile at high been expected to resolve the problem of unit-cell symmetry in angle (95 d28 d 120") is of particular interest, clearly showing these systems A neutron study by Jacobsen et showed splittings that would not arise from a cubic cell In fitting the the structure of BaCeO, itself at ambient temperature to profile, atom positions were constrained to be those of Ranlrav belong to the orthorhombic space group Pbnrn, the later studies 630 J Muter Chem , 1996,6(4), 629-633 15K 1OK 5000 7 I v) v)C s 0Y .-a v)t o 0.0 40.0 L 60.O 80.0 c-. 100.- i s c.- II In 111 11 1111 II 1111 111 I I ni I I 111 I II u ir IIIP 11 I 111 1111 111 144 II 111 III II *0° I 0 0.o 95.0 100. 105. 110. 115. 2Wdegrees Fig. 3 X-Ray powder diffraction profile (Cu-Ka, radiation) for BaCeo,,,Nd,~o,03~a at ambient temperature. The solid lines show the fit and difference plot obtained after constrained Rietveld refinement (see text) of the data. Vertical bars denote the positions (2O/degrees) corresponding to predicted d spacings. DMPLOT software was used for plotting.36 by Longo et aLZ7 were, however, interpreted in terms of a tetragonal cell at ambient temperature, and those of Predu and Dinescu2* in terms of a tetragonal cell at T< 427 "C and a cubic cell at higher temperatures. A recent neutron diffraction study by Knight29 has indicated that the correct interpretation could be the following sequence of structural changes: Pbnm (orthorhombic) at ambient-290 "C, Incn (orthorhombic) at 290-400 "C, F32/n (rhombohedral) at 400-900 "C, Pmjm (cubic) at T >900 "C.There is also confusion as to the unit-cell symmetries appro- priate to doped perovskites of the type BaCe, -xMx03-a. Early studies assumed cubic cells (e.g. refs. 6,20), but neutron diffrac- tion studies of Y-and Gd-doped samples (x=O.lO) at ambient temperature demonstrated the same Pbnm (orthorhombic) structure found for the parent BaCeO, under the same con- dition~.~~Scherban et al. 31-33 measured Raman spectra as functions of dopant, temperature and composition. They inferred a variable unit-cell symmetry (but did not report diffraction studies), the symmetry observed depending on temperature, dopant identity and level of doping (Pbnrn=>P4/rnbrn*Prn3rn with increasing temperature and/or x).It follows that the ambient-temperature defect structure may not be linked directly to the high-temperature conduction mechanism (applying in a phase at high temperature) and, further, that conductivity studies pertaining to different tem- perature or composition regimes should not be inter-related in too simplistic a manner (structural changes have generally been neglected).Scherban et inferred a simple composition dependence of unit-cell symmetry at ambient temperature for Nd as dopant (uiz.x=O.O2, Pbnrn; 0.05, P4/mbrn; 0.10, Prngrn), but Knight and B~nanos~~ found no evidence in neutron diffraction studies for Nd-concentration-dependent phase transitions. Raman spectra for SrCeO, and for the phases SrCe,-,Yb,O,-, have been interpreted on the basis of the factor group D2,, for the space group Pbnrn by Scherban et a1.31,33 and Kosacki et aL3' respectively. Scherban et aL3, found no apparent temperature dependence of unit-cell sym- metry for SrCeO,. Unit cells determined in this work For all materials studied in this work, the ambient temperature structures have orthorhombic unit cells (see above).The lattice parameters determined by Rietveld refinement techniques in the space group Pbnm (see above) for compositions in the systems BaCe, -,Ca,O, -,and BaCe, -,Nd,O, -a are given in Table 1. The parameters for the system BaCe,-,Nd,O,-, fit closely into the trends evident in the neutron diffraction work of Knight and Bonano~,~~ who worked at intervals of 0.04 in x in the range O.O<x<O.2. Raman spectra with Ca as dopant Raman spectra for phases in the system BaCe,-,Ca,O,-, are presented in Fig. 4. The general appearances of the spectra are very similar for all values of x and closely resemble those in spectra given by Scherban et al. for BaCeO, at ambient temperature,, and for other systems they assigned as ortho- rhombi~.~~,,~This is not surprising in view of the chemical J.Muter. Chem., 1996, 6(4), 629-633 631 Table 1 Lattice parameters for the orthorhombic unit cells" character- istic of the doped cerates BaCe, *CaXO3 a and BaCe, ,Nd,03 a Ca 0 02 0 05 0 10 6 2320( 2) 6 2297 (2) 6 2294(4) 6 2156(2) 6 2149( 3) 6 2171(4) 8 7774( 3) 8 7760(3) 8 7776(4) 0 15 6 2295( 3) 6 2143(4) 8 7733(4) Nd 0 02 0 05 6 2318(2) 6 2319(2) 6 2163(3) 6 2166(3) 8 7782( 3) 8 7791(3) 0 10 6 2294( 3) 6 2129(3) 8 7737(4) "All observed XRD lines are indexable on the basis of the space group Pbmn, numbers in parentheses denote the standard deviation in the last figure 150 250 350 450 550 650 750 wavenumber /an' Fig. 4 Raman spectra at ambient temperature for the system BaCe, ,CaXO3 a for (top to bottom) x =0 15, 0 10,O05, 0 02 similarity to those materials and our demonstration (above) of orthorhombic cells in this study The multiplet around 350cm-1 arises from the Ce-0 vibrational modes associated with the CeO, octahedra of the perovskite structure 31 35 The broad band previously reported3' at ca 200 cm-' is also evident, but peaks at lower frequencies are not observable The latter is a consequence of the available instrumentation, which employed a double monochromator (Scherban et a1 had access to a triple mono~hromator~~) Raman spectra with Nd as dopant Samples containing Nd are brown and are much stronger absorbers in the visible region than are the off-white samples containing Ca as dopant In the case of Nd-doped samples, a new feature at ca 610cm-' is seen in the spectra (Fig 5) A further difference is the higher background and signal-to-noise levels in the Nd case, possibly arising simply from higher dielectric screening in the Nd case (this would be consistent both with the brown colouration of the samples and with the disappearance of the 300 cm-' feature into the background at high Nd levels) The diffraction studies in this work demonstrated ortho- rhombic unit cells for all compositions in the system BaCe, -,Nd,03 at ambient temperature, in agreement with -GI the work of Knight and Bonanos34 The Raman spectra in Fig 4, nonetheless, have the same general appearance as those reported by Scherban et a2 32 33 Their inference, from interpret- ation of the Raman spectra, of a composition dependence of unit-cell symmetry at ambient temperature is, therefore, not supported I I 250 350 450 550 650 750 wavenumber Icm-' Fig.5 Raman spectra at ambient temperature for the system BaCe, ,Nd,O, a for (top to bottom) x=O 10, 005, 0 02 The feature at around 610cm-', not evident in the spectra of Ca-doped samples (above), is, as stated by Scherban et a1 ,32 33 associated with modes involving the rare-earth-metal dopant, a similar band and assignment have been reported by Kosacki et al 35 for SrCe1-,Yb,O3-, Consistent with that assignment, the relative intensity of that band increases with increasing dopant level in this work Conclusions Combined X-ray diffraction and Raman spectrcscopic studies have been carried out on samples within the systems BaCe, -,Ca,03 -a and BaCe, -,Nd,03 at ambient tempera- --a ture Confusion in the previous literature could have arisen from comparison of the results of separate X-ray and Raman studies of different samples with possibly differing thermal histories The following conclusions can be drawn As shown by high-resolution powder diffraction, the unit cell (a crystal average) for all compositions in the systems (with either Ca or Nd as dopant) in this study is truly orthorhombic at ambient temperature The Raman spectra for BaCe, -,Ca,03 -oL are similar to that for BaCeO, itself, indicating little distortion of the structure when Ca is dopant The postulate of Scherban et a1 ,33 that unit-cell symmetry in the system BaCe, -,Nd,03 at ambient temperature depends on x,is not supported by this study Raman spectroscopic investigations of single crystals within these systems could remove some of the ambiguities inherent in interpretation of the Raman spectra of powders We thank referees for constructive comments concerning the Raman spectra We thank the EPSRC and British Gas plc for a CASE studentship for SDF We thank the University of Exeter for a travel grant (to Exeter) for NS We thank the Earth Resources centre (University of Exeter) for analyses using XRF and EDAX techniques We thank the Royal Society for a travel grant (to Bratislava) for R C T S The programme in Bratislava is funded by the Slovak Grant Agency for Science References 1 H Iwahara, T Esaka, H Uchida and N Maeda, Solid State Ionzcs, 1981,314,359 2 A Mitsui, M Miyayama and H Yanagida, Solid State iunics, 1987, 22,213 3 H Iwahara, H Uchida, K Ono and K Ogaki, J Electrochem SOC,1988,135,529 632 J Muter Chern, 1996, 6(4), 629-633 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 H Iwahara, H Uchida and K Morimoto, J Electrochem Soc, 1990,137,462 T Yajima, H Kazeoka, T Yogo and H Iwahara, Solid State Zonics, 1991,47,271 R C T Slade and N Singh, Solid State Zonics, 1991,46, 11 1 R C T Slade and N Singh, J Muter Chem, 1991,1,441 T Hibino, K Mizutani, T Yajima and H Iwahara, Solid State Zonics, 1992,57, 303 H Iwahara, T Yajima, T Hibino, K Ozaki and H Suzuki, Solid State Zonics, 1993,61, 65 H H Huang and M Ishigame, Solid State Ionics, 1991,47,251 A F Sammells, R L Cook, J H White, J J Osborne and R C Macduff, Solid State Ionics, 1992, 52, 11 1 R L Cook, J J Osborne, J H White, R C Macduff and A F Sammells, J Electrochem Soc, 1992,139, L19 T Yajima, H Iwahara and H Uchida, Solid State Zonics, 1991, 47,177 J F Liu and A S Nowick, in Solid State Zonics IZ,eds G A Nazri, D F Shriver, R A Huggins and M Balkanski, MRS Symp Proc , 1991 210,675 R C T Slade, S D Flint and N Singh, J Muter Chem, 1994, 4,509 S D Flint and R C T Slade, Solid State Zonics, 1995,77,215 K C Liang and A S Nowick, Solid State Zonics, 1993,61,77 K C Liang, Y Du and A S Nowick, Solid State Ionics, 1994, 69,117 A J Smith and A J E Welch, Acta Crystallogr, 1960, 13,653 N Bonanos, B Ellis, K S Knight and M N Mahmood, Solid State Zonrcs, 1989,35, 179 S Shin, H H Huang, M Ishigame and H Iwahara, Solid State lonics, 1990, 40/41,910 22 A J Jacobsen, B C Tofield and B E F Fender, Acta Crystallogr Sect B, 1972,28,956 23 K S Knight, M Soars and N Bonanos, J Muter Chem, 1992, 2,709 24 J Ranlarv and K Nielsen, J Muter Chem , 1994,4, 867 25 H M Rietveld, J Appl Crystallogr ,1969,2,65 26 The Rietveld Method, ed R A Young, Oxford University Press, Oxford, 1993 27 V Longo, F Ricciarcello and D Mmichelh, J Muter Sci, 1981, 16,3503 28 M Predu and R Dinescu, Rev Roum Chrm ,1976,21,1023 29 K S Knight, Solid State Zonics, 1994,94, 109 30 K S Knight, M Soar and N Bonanos, J Muter Chem, 1992, 2,709 31 T Scherban, R Villeneuve, L Abello and G Lucazeau, Solid State Commun ,1992,84,341 32 T Scherban, R Villeneuve, L Abello and G Lucazeau, Solid State Ionics, 1993,61, 93 33 T Scherban, R Villeneuve, L Abello and G Lucazeau, J Raman Spectrosc , 1993,24,805 34 K S Knight and N Bonanos, Solid State Zontcs, 1995,77, 189 35 I Kosacki, J Schoonman and M Balkansi, Solid State Zonics, 1992,57,345 36 H Marciniak, S Gierlotka and B Palosz, Advanced graphics tools for the analysis of powder difraction data XVIEW and DMPLOT, in Collected Abstracts of the 4th European Powder Difraction Meeting, Chester, 1995, p 103 Paper 5/04231C, Received 30th June, 1995 J Muter Chem, 1996, 6(4), 629-633 633

ISSN:0959-9428    

DOI:10.1039/JM9960600629

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Magnetic, electrical and lsrEu Mossbauer properties of EuPt Ge Rainer Pottgen,"" Reinhard K. Kremer," Walter Schnelle," Ralf Mullmand and Bernd D. Moselb "Max-Planck-Institut fur Festkorperforschung, Heisenbergstrasse I, 0-70569 Stuttgart, Germany bInstitutfur Physikalische Chemie, Universitat Miinster, Schlossplatz 417, 481 49 Miinster, Germany The title compound was prepared from the elements in a tantalum tube at 1170 K. EuPtGe [single-crystal X-ray data: space group P2,3,Z =4, a =654.63( 9) pm, R, =0.0208,378 F2 values and 11 parameters] crystallizes with the LaIrSi type structure, an ordered ternary version of the SrSi, type. Magnetic susceptibility and '"Eu Mossbauer measurements show no magnetic order down to 4.2 K. The experimentally determined magnetic moment, peXp=7.80(5) p,JEu compares well with the free ion value, peff=7.94 pu,/Eu for Eu2+, in accordance with the isomer shift 6 = -10.4 mm s-' which is typical for divalent Eu.EuPtGe is a metallic conductor with a specific resistivity at room temperature of 145 pt2 cm. Recently, the crystal structure and properties of EuAuGe' and the related intermetallics EuAgGe, EuPdGe and EuZnGe have been In the course of our systematic studies on crystal structures and physical properties of equiatomic europium transition-metal germanides and indide~,~ we have now investigated EuPtGe. In the present paper we report on single-crystal X-ray data, magnetic susceptibility, electrical resistivity, specific heat and '"Eu Mossbauer investigations of this intermetallic compound.X-Ray powder data of EuPtGe have been published previously by Evers et aL6 Experimental Starting materials for the preparation of EuPtGe were ingots of europium (Johnson Matthey), platinum powder (Degussa) and germanium lumps (Wacker), all with stated purity >99.9%. The elements were mixed in the ideal atomic ratio and sealed in a tantalum tube under an argon pressure of about 800 mbar. The argon was purified over molecular sieves, titanium sponge (at 900K) and an oxisorb ~atalyst.~ The tantalum tube was sealed in a quartz ampoule to prevent oxidation and heated at 1320 K for five days; then cooled to 1170 K and annealed for a further four weeks. Modified Guinier powder patterns' of the samples were recorded with Cu-Ka, radiation using silicon (a=543.07 pm) as an internal standard.The indexing of the diffraction lines was facilitated by intensity calculations9 taking the positional parameters of the refined structure. The lattice constant (Table 1) was obtained by a least-squares fit of the powder data. It is in good agreement with the data obtained on the four-circle diffractometer [a =654.00(1) pm] as well as the powder data from Evers et al. of a=655.1(1) pm.6 Single-crystal intensity data were collected on a four-circle diffractometer ( Enraf-Nonius CAD4) with graphite-monochro- mated Ag-Ka radiation and a scintillation counter with pulse height discrimination. The magnetic susceptibilities of polycrystalline pieces of EuPtGe were determined with an MPMS SQUID magnet-ometer (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 (0.8 x 1.0 x 1.2 mm3 cut with a diamond saw from a larger ingot) with a conventional four-point setup. Four very thin gold wires (diameter 0.1 mm) were glued to the block using a silver epoxy resin. Cooling and heating curves measured at a constant current between 4.2 and 300 K were identical within error bars. The specific heat capacity of a EuPtGe powder sample (about 600mg) was measured between 20 and 270K in an adiabatic scanning calorimeter designed for small samples." The powder was sealed in a Duran glass ampoule under 4He exchange gas. The resolution of the measurement was 0.15% of C, at 100 K.'"Eu Mossbauer experiments were performed between 4.2 and 300K in a conventional Mossbauer spectrometer with a "'Sm :EuF, source at 300 K. Results Powderous EuPtGe is dark grey and stable in air for several months. No decomposition was visible. Single crystals are light grey with a metallic lustre, with an irregular platelet-like shape. Structure refinement Single crystals of EuPtGe were selected by mechanical fragmen- tation from the annealed sample. They were investigated by Buerger precession photographs in order to establish both symmetry and suitability for intensity data collection. The precession photographs indicated a primitive cubic cell. The systematic extinctions (hOO only observed for h =2n) led to the non-centrosymmetric group P2,3, in agreement with the results reported for the prototype LaIrSi" as well as the X-ray powder refinement for EuPtGe.6 Details on the data collection and crystallographic data are listed in Table 1.The atomic positions of the previous powder investigation6 were taken as starting values. The structure was refined with SHELXL-93,12 assuming anisotropic displacement parameters for all atoms. The refined atomic parameters are listed in Table 2, interatomic distances in Table 3. A final difference Fourier synthesis revealed no significant residual peaks. Additional data on the structure refinement are available.? In order to check for the correct handedness (since P2,3 is a non- centrosymmetric space group), SHELXL-93I2 automatically calculates the absolute structure parameter, which should be 0 for the correct and + 1 for the inverted absolute structure.Our refinement (see Tables 1-3) resulted in an absolute struc- ture parameter of 0.01(3), indicating the correct handedness. A refinement of the inverted structure, on the other hand, resulted in an absolute structure parameter of 0.97(6), higher standard deviations (by a factor of 2) for the positional parameters and higher residuals of R, =0.0399 and R2=0.0939. t Details may be obtained from: Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen (Germany), by quoting the Registry No. CSD-401561. J. Muter. Chem., 1996, 6(4), 635-638 635 Table 1 Crystal data and structure refinement for EuPtGe empirical formula formula mass T/KWavelengthlpm crystal system space group unit-cell dimensions/pm cell volume/nm3 z Dclgcmcrystal size/pm3 absorption correction transmission ratio (max, min) absorption coefficient/mm F (000) 8 range for data collection/degrees scan mode range in hkl total no reflections independent reflections reflections with I >241) refinement method data/restraints/parameters goodness-of-fit on F2 final R indices [I >20(1)] R indices (all data) extinction coefficient largest difference peak and hole/e nm absolute structure parameter Table 2 Atomic coordinates and anisotropic displacement parameters' (pm2) for EuPtGe (Ueq is defined as one third of the trace of the orthogonalized U,, tensor) atom P2,3 X u11 u12 Eu 4e 0 13207( 6) 114(1) 11(1)Pt 4e 041819(4) 123( 1) -10( 1) Ge 4e 0 83351( 12) 135(3) 4(3) 'The anisotropic displacement factor exponent takes the form -2n2 [(ha*)2U,,+ +2hka*b*UI2] x=y=z U,,=U22=U33=Ueq, U12= u13=u23 Table 3 Interatomic distances (pm) in the structure of EuPtGe, all distances shorter than 600 (Eu-Eu), 535 (Eu-Pt, Eu-Ge), 470 (Pt-Ge) and 405 (Pt-Pt, Ge-Ge) are listed (standard deviations are all equal to or less than 0 1 pm) Eu 1 Pt 3244 Pt 3 Ge 2380 3 Pt 327 7 1 Eu 3244 3 Ge 3328 3 Eu 3277 1 Ge 3385 3 Eu 3879 3 Ge 3746 3 Pt 3879 Ge 3 Pt 2380 6 Eu 401 1 3 Eu 3328 1 Eu 3385 3 Eu 3746 Magnetic susceptibility The inverse magnetic susceptibility of EuPtGe as a function of temperature is displayed in Fig 1 Above 100K EuPtGe follows the Curie-Weiss law No magnetic order could be observed down to 4 2 K The experimental magnetic moment peXp=7 80( 5) pg/Eu, calculated from the slope of the 1/x us T plot according to pexp=283[~(T-@)]~/~,is close to the free ion value peff=7 94 pB for Eu2+ The paramagnetic Curie temperature (Weiss constant) of 0=20( 1)K was determined by linear extrapolation of the high temperature part of the 1/x us T plot to 1/x=O At about 70 K, the 1/x us T plot deviates from linearity This slight deviation is most probably due to a minor amount of an impurity of ferromagnetic EuO (T,= 69 K),13-" although the Guinier powder patterns showed single-phase EuPtGe In order to quantify the nature and 636 J Muter Chem, 1996,6(4), 635-638 EuPtGe 419 64 293 (2) 56 086 cubic P213 (no 198) a =654 63( 9) 0 28054( 7) 4 9 936 25 x 50 x 75 from $-scan data 0 855,O 471 44 14 692 2-26 a19 O,<h,<10,O,<k,<10,-10,<1< 10 1256 378 (R,,,=O0436) 371 (R,,,,,=O0291) full-matrix least-squares on F2 378/0/11 1138 R, =0 0202, R, =0 0425 R1 =0 0208, R2 =0 0427 0 0079(7) 1630 and -1328 OOl(3) 0 50 100 150 200 250 300 T/K Fig. 1 Temperature dependence of the reciprocal magnetic susceptibil- ity of EuPtGe measured at a magnetic flux density of 2 T amount of this impurity, specific heat measurements were performed A similar magnetic behaviour was recently also observed for isotypic EuPdSi by Adroja et a1l6 Specific heat measurements The specific heat capacity C,(T) shows a small, but clearly visible A-type anomaly with a peak at 69 0 K The shape and temperature of this anomaly is consistent with the presence of a small amount of a EuO impurity in the EuPtGe sample l3 l5 A rough estimate of the size of the anomaly and a comparison with the data in ref 15 yields a content of about 1-2 mass% of EuO in the EuPtGe powder No further anomalies are visible in the CJT) curve '"Eu Mossbauer spectroscopy The Mossbauer spectrum of EuPtGe at 4 2 K and the theoreti- cal fit is shown in Fig 2 The curve can be fitted well by a single Eu" site at 6 = -10 40( 5) mm s-l subject to quadrupole interaction with an axially symmetric field gradient e2qeffQ = 4( 1)mm sC1, and an Eu"' impurity of area 5% at 0 5 mm S".l The parameters are nearly constant up to room temperature, I I-15100.0 I I I I J -37.a -18.9 0 t18.9 t37.a vlmm s-1 Fig. 2 '"Eu Mossbauer spectrum of EuPtGe at 4.2 K relative to EuF, 6(300 K) = -10.46(3) mm s-'. Between 300 and 4.2 K no magnetic order can be detected. Very similar "'Eu Mossbauer properties have recently been reported by Adroja et ~1.'~for isotypic EuPdSi. The latter silicide has an isomer shift of 6(300 K)= -10.15 mm s-' and shows no magnetic ordering down to 4.2 K. Electrical conductivity In Fig. 3 the temperature dependence of the specific resistivity of EuPtGe is presented.The specific resistivity decreases with decreasing temperature as is typical for metallic conductors. In agreement with the magnetic susceptibility and '"Eu Mossbauer investigations, no phase transition is observed in the temperature range investigated. At room temperature EuPtGe has a specific resistivity of 145 pR cm. For comparison, europium and platinum have specific resistivities at room temperature of 90 pQ cm and 10.9 pQ cm, re~pective1y.l~ Discussion The present single-crystal investigation on EuPtGe fully con- firms the previous powder investigation.6 EuPtGe (see Fig. 4) crystallizes with the LaIrSi-type structure," an ordered ternary version of SrSi2.'s-20 The LaIrSi structure" is also formed by the related compounds NdIrSi,'l RPtSi (R=Ca, Sr, Ba, RPdSi (R =Sr, Ba, Eu),~~ EU),'~,'~ RPtGe (R =Ca, Sr, Ba, Eu)~ and RIrP (R =Ba, Eu).~~ Among these intermetallics, the rare- earth-metal containing compounds exhibit interesting physical properties.While LaRhSi and LaIrSi2' become supercon- ducting at low temperatures, NdIrSi" and EuIrPZ4 order ferromagnetically. A detailed description of this structural family was already given by Klepp and ParthC for the prototype LaIrSi" as well as by Evers et al. for the silicide series with Pd and Pt.6*22323 The europium atoms in EuPtGe have the high coordination number (CN) of 20 with six Eu, seven Pt and seven Ge atoms 145 r 135 5 125 G.d Q 115 105 I 1 I I 1 I95 1 0 50 I00 1% 200 250 300 TIK Fig.3 Temperature dependence of the specific resistivity of EuPtGe Fig. 4 Perspective view of the cubic EuPtGe structure. The three- dimensional [PtGe] polyanion is outlined. in their coordination shell. This is typical for such intermetallic compounds. The coordination polyhedron for this position was already shown and discussed in detail by Klepp and ParthC for the prototype La1rSi.l' The Eu-Eu distances of 401.1 pm are about twice the metallic radius [r(Eu) for CN 12=204.2 pm2']. The Eu-Pt distances range from 324.4 pm to 387.9 pm with an average distance of 353.0 pm, somewhat longer than the sum of the metallic radii [r(Pt) for CN 12= 138.7 pm2'] of 342.9 pm. The Eu-Ge distances (332.8-374.6 pm) have an average value of 351.5 pm, which is also longer than the sum of the metallic radii [r(Ge) for CN 12=136.9 pm''] of 341.1 pm.Both Pt and Ge atoms have CN 10 with three Ge (Pt) and seven Eu atoms in their coordination shell. The Pt-Ge distance of 238.0 pm is significantly shortened when compared to the average Pt-Ge distance of 260.1 pm for the four Ge neighbours of the Pt atoms in CaPtGe' with the TiNiSi-type structure. Similar short Pt-Ge distances have recently been observed in the cerium compounds Ce,Pt,Ge, (240.6-277.8 pm)26 and Ce,Pt,,Ge,, (232.0-264.0 ~m).'~ A comparison of the interatomic distances indicates strong Pt-Ge bonding in EuPtGe. Considering that the europium atoms are by far the most electropositive component in EuPtGe, they will have largely transferred their valence elec- trons to the three-dimensional [PtGe] polyanion.Therefore the compound can be described to a first approximation as Eu2+ [PtGe]'-. The three-dimensional [PtGe] polyanion is outlined in Fig. 4. At this point it is interesting to compare the structure of EuPtGe to that of the binaries EuG~,~,~~ sinceand EuP~~,~~ very often the equiatomic RTX compounds adopt ordered structures of the binary border phases. The latter germanide may be considered as a Zintl compound and the formula may be written as Eu2+(Ge-),. Considering the 8-N rule, the germanium anions are thus isoelectronic with arsenic. Indeed, the germanium sublattice in EuGe, consists of puckered hexa- gons with triply-connected germanium atoms, like in a-arsenic.30 These two-dimensional networks are separated by the europium atoms.In EuPtGe, half of the germanium atoms are replaced by platinum. Although the size and electronegativ- ity of Pt and Ge are similar, the structure is different to that of EuGe,. Each platinum and germanium atom remains triply- connected; however, the network is now three-dimensional and the europium atoms are embedded in the [PtGe] polyanion (see Fig. 4). If all the germanium atoms in EuGe, are replaced by platinum, the situation is totally different. EuP~,~~ is a Laves phase with the MgCu,-type structure. In EuPt, each platinum atom has six platinum neighbours within the tetra- hedral Pt sublattice. J.Mater. Chem., 1996, 6(4), 635-638 637 We thank Prof Dr A Simon and Prof Dr H Eckert for their 14 D B McWhan, P C Souers and G Jura, Phys Rev, 1966, 143, interest and steady support of this work We are also grateful to W Rothenbach for taking the Guinier powder patterns, to E Brucher for the susceptibility measurement, to N Rollbuhler for the electrical conductivity measurement and to Dr W Gerhartz (Degussa AG) for a generous glft of platinum powder The Stiftung Stipendienfonds des Verbandes der Chemischen Industrie supported our research by a Liebig grant to R P 15 16 17 18 19 385 B Stroka, J Wosnitza, E Scheer, H v Lohneysen, W Park and K Fischer, Z Phys B -Condens Matter, 1992,89,39 D T Adroja, B D Padalia, S K Malik, R Nagarajan and R Vijayaraghavan, J Magn Magn Mater, 1990,89,375 Handbook of Chemistry and Physics, ed R C Weast, CRC Press, Boca Raton, FL, 66th edn ,1985 K H Janzon, H Schafer and A Weiss, Angew Chem 1965, 77, 258 P I Kripyakevich and E I Gladyshevskii, Sou Phys Crystallogr , 1966,11,693 20 G E Pringle, Acta Crystallogr Sect B, 1972,28,2326 References 21 B Chevalier, P Lejay, A Cole, M Vlasse and J Etourneau, Solid State Commun ,1982,41,801 1 2 3 4 5 6 7 8 9 10 R Pottgen, J Mater Chem ,1995,5,505 R Pottgen, Z Naturforsch B Chem Scz , 1995,50, 1071 R Pottgen, Z Naturforsch B Chem Scz , 1995,50,1181 R Pottgen, 2 Kristallogr , 1995,210,924 R Pottgen, J Mater Chem ,1996,6,63 J Evers, G Oehlinger, K Polborn and B Sendlinger, J Alloys Compd, 1992,182, L23 H L Krauss and H Stach, Z Anorg Allg Chem ,1969,366,34 A Simon, J Appl Crystallogr ,1971,4, 138 K Yvon, W Jeitschko and E Parthe, J Appl Crystallogr, 1977, 10,73 W Schnelle and E Gmelin, Thermochimica Acta SpeciaZ Issue -ESTAC6, in press 22 23 24 25 26 27 28 J Evers and G Oehlinger, J Solid State Chem ,1986,62, 133 J Evers, G Oehlinger, K Polborn and B Sendlinger, J Solid State Chem ,1991,91,250 C Lux, A Mewis, S Junk, A Gruetz and G Michels, J Alloys Compd, 1993,200,135 E Teatum, K Gschneidner Jr and J Waber, Rep LA-2345, US Department of Commerce, Washington, DC, 1960 A V Gribanov, 0 L Sologub, P S Salamakha, 0 I Bodak, Yu D Seropegin, V V Pavlyuk and V K Pecharsky, J Alloys Compd ,1992,189, L11 A V Gribanov, Yu D Seropegin, 0 I Bodak, V V Pavlyuk, L G Akselrud, V N Nikiforov and A A Velikhovski, J Alloys Compd, 1993,202,133 E I Gladyshevskii, Dopov Akad Nauk Ukr RSR, 1964,2,209 11 K Klepp and E Parthe, Acta Crystallogr Sect B, 1982,38, 1541 29 A Iandelli and A Palenzona, J Less-Common Met, 1981,80, P71 12 G M Sheldrick, SHELXL -93, Program for Crystal Structure Refinement, University of Gottingen, Germany, 1993 30 A F Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 5th edn ,1984 13 B T Matthias, R M Bozorth and J H Van Vleck, Phys Rev Lett ,1961,5, 160 Paper 5/06719G, Received 10th October, 1995 638 J Mater Chem, 1996,6(4), 635-638

ISSN:0959-9428    

DOI:10.1039/JM9960600635

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Synthesis and crystal structures of two metal phosphonates, M(HO,PC,H,), (M = Ba, Pb) Damodara M. Poojary,' Baolong Zhang," Aurelio Cabeza,' Miguel A. G. Aranda,b Sebastian Bruque' and Abraham Clearfield*" 'Department of Chemistry, Texas A & M University, College Station, TX 77843, USA 'Departamento de Quimica Inorganica, Cristalografia y Mineralogia, Universidad de Malaga, 29071-Malaga, Spain Divalent metal phosphonates, Ba( H03PC6H5), and Pb( H03PC6H5)', have been synthesized and structurally characterized [crystal data: a=32.18( l),b=5.546(4), c=8.495(4) A,p= 103.21(3)", space group C2/c and 2=4 for Ba(H03PC6H5)2; a= 31.8302( lo), b =5.5997(2), c =8.2935( 3) A,p= 101.875(2)", space group C2/c and 2=4 for Pb(Ho3Pc6HS),]. Their structures are isomorphous. The structure of the barium compound was solved from single-crystal data which was then used to refine the structure of the lead compound by Rietveld methods.In these compounds the metal :phosphonate ratio is 1:2 and the phosphonates use all their oxygens to bridge the metal atoms, which are arranged in two-dimensional layers. One of the phosphonate oxygens is protonated. The phosphonate oxygens are involved in both chelation and bridging interactions. The metal atoms are eight-coordinate; four of the binding sites are due to symmetry-related positions of a single oxygen atom and two each from the remaining two oxygen atoms. Layered metal phosphonate compounds have attracted a great deal of research activity in recent years.'-4 These compounds have potential applications in sorption, catalysis and ion exchange.The layered metal phosphonates can also be easily pillared to yield materials similar to pillared clays, which are amenable to incorporation of specific and selective species and a wide variety of materials properties.'*' Recently it was shown that surface-bound multilayer metal phosphonate structures, analogous to Langmuir-Blodgett films, can be grown through stepwise absorption of metal ions and bisphosphonic acid on metal substrates., Earlier work on metal phosphonates concen- trated on zirconium compounds of composition Zr (O,PR), .2,6 The layered structure of these compounds is built up by oxygen-bridged metal octahedra which are separated by the hydrophobic regions of the organic moieties.The phosphonate groups lie above and below the plane of metal atoms. Recently, studies have shown that a variety of metal ions, including Group 4 and 14 element^,^ divalent'-" and tri~alent'','~ ions form several variations of this type of layered compound. The phosphonate oxygens in these compounds act as unidentate, chelating or bridging ligands depending on the metal :phos-phonate ratio and the coordination requirement of the metal involved. Note also that a variety of organic groups with or without active moieties can be organized in between the inorganic layers of the layered compounds, which is important in designing materials for specific physical properties. The recent discovery of porous structures in metal phosphonate compounds further intensified research activities in this Only a few studies of alkaline-earth-metal phopshonates have been carried out.Magnesium phosphonatesZ2 of the type Mg(03PR)*H,0 have structures similar to the now well known zinc' or rnangane~e'~ phosphonates. Magnesium was also shown to form a different structure type containing [Mg(Hz0)6]2f ions, when the phpsphonic acid is too bulky to fit in the space required (28 A') by the more common layered type compound.22 Calcium was found to form two different layered structures.'* The calcium phosphonates of the type Ca(HO,PR), have a structural motif very similar to the layer structure of zirconium ph~sphonate.~They contain charge-balancing protons bonded to the phosphonate oxygens, making the phosphonate ligand: metal ratio 2: 1, as in the zirconium or titanium compounds.Calcium methylphosphon- ate, Ca(O,PCH,)-H,O is monoclinic, space group P2,/c. The Ca atom is seven-coordinate in the form of a peatagonal bipyramid wit! six Ca-0 bond distances of 2.4A and a seventh of 2.7 A." To gain further insight into the chemistry of alkaline-earth-metal phosphonates we have isolated the barium phenylphosphonate whose synthesis and structure are presented in this paper. We also describe the structural charac- terization of lead@) phenylphosphonate, a first example of a non-transition metal, non-alkaline-earth divalent metal phos- phonate. Interestingly, its structure is isomorphous to the barium compound.Experimental Materials and methods Chemicals of reagent quality were obtained from commercial sources and were used without further purification. Thermal analysis was carried out on a Du Pont thermal analysis unit (model no. 951) and Rigaku Thermoflex apparatus at a heating rate of 10 K min-'. IR spectra were recorded on Digilab Model FTS-40 FTIR and Perkin Elmer 883 spectrometers in the spectral range 4000-400 cm-', using the KBr disk method. Synthesis of barium phenylphosphonate, Ba (HO3PC6H5)2 Phenylphosphonic acid (1.6 g) was first dissolved in deionized distilled water to which 2.5 g of BaCl, .2H20 was added. The cloudy solution thus formed was heated at reflux for one day in a mineral oil bath. The solid formed was filtered off, washed and air dried.Analytical data: C, 31.97; H, 2.63. Calc. for Ba(H0,PC6H5),: c, 31.90; H, 2.66%. Synthesis of lead phenylphosphonate, Pb(HO3PC6H5)2 Lead hydrogen-phenylphosphonate was synthesized by slow addition of a solution of lead acetate (0.1 mol drn-,) over a solution of phenylphosphonic acid (1 mol dm-3) to a final P :Pb molar ratio of 12 :1. The precipitate was heated at reflux for 30 days. The resulting solid was filtered off, washed with water, and air dried. This compound may also be obtained by hydrothermal synthesis. A suspension (prepared as previously) was stirred at 200°C for 4 days in a PTFE-lined reactor at J. Muter. Chem., 1996, 6(4), 639-644 639 autogenous pressure The two synthetic methods led to the same product but single crystals were not obtained A shorter reaction time led to the same product but with slightly lower crystallinity The sample was dissolved in a solution containing an equimolar ratio of HNO, (67% m/m) and H202 (30% m/m) Then, the lead content was determined by atomic absorption spectroscopy (AAS) Carbon and hydrogen con-tents were determined by elemental chemical analysis in a Perkin Elmer 240 analyser The phosphorus content was deduced from the carbon percentage found, assuming a C P molar ratio of 6 1 The results were Pb, 38 0, P 11 67, C, 27 11, H, 2 39 Calc for Pb(H03PC,H,)2 Pb, 39 73, P, 11 90, C, 27 65, H, 2 30% IR spectroscopic study The Ba and Pb compounds display similar characteristics in their IR spectra The spectrum of Pb(Ho,PC,H,), is shown in Fig 1 There are no bands in the 0-H stretching region (3500-3200 cm-') which is consistent with the absence of water molecules in the structure The C-H stretching vibrations of the phenyl ring are present near to 3000crn-' The bands around 2750 and 2340cm-' are characteristics of the 0-H stretching frequencies of the monohydrogen phos- phonate groups The C-C aromatic stretching bands appear near to 1400 cm-' The intense band at around 1160 cm-' is due to the P-C stretching vibrations and the strong bands in the region of 1OOOcm-' are due to the P-0 stretching vibrations of the tetrahedral CP03 group The bands at 695 and 750 cm-' are characteristic of the out-of-plane (monosub- stituted) phenyl ring vibrations Thermal analysis study A thermogravimetry (TG) curve for the barium compound is shown in Fig 2(u) The compound loses 4% of its mass at about 400"C, corresponding to the release of one water molecule due to the condensation of hydrogen phosphonate groups (calculated mass loss 3 990/) The release of the organic group starts around 500°C and is complete at 600°C except for some residual carbon The observed mass loss (3229%) I Fig.1 IR spectrum of Pb(H03PC6H,), 1 I I I I I 11 100 300 500 700 T1°C Fig. 2 (a)TG curve for Ba(H03PC6H5)2 and (b)TG and DTA curves for Pb(HO,PC,H,), agrees well with a calculated value of 34 1% The total mass loss up to 1000°C is 35% which is consistent with the theoretical value of 34 6%, calculated for the conversion of Ba(HO,PC,H,), to Ba(P03)2 The TG curve for Pb(HO,PC,H,), is shown in Fig 2(b) As in the case of the Ba compound, the Pb compound shows two distinctive mass losses in its TG curve, due to the release of a water molecule and the organic groups The release of water in this case, however, takes place at a lower temperature (257 "C) when compared to that from the Ba compound The observed mass loss of 3 5% is in excellent agreement with a calculated value of 345% The second mass loss, due to the release of the phenyl groups, can be seen at around 550°C The observed mass loss for this process is 27 7% (calculated mass loss 2955%) The final thermal decomposition product is an amorphous glass of composition Pb( The overall mass loss, 31 2%, agrees well with the theoretical loss of 29 93% for the decomposition of Pb( HO,PC,H,), to Pb( Note also the thermal behaviour of the Pb compound, as seen in its DTA curve [Fig 2(b)] The curve shows no exotherm and the temperature does not increase abruptly as is usual for the combustion of organic matter There is an endotherm centred at 275 "C with an associated mass loss of 3 5% due to the release of the water molecule During the second mass loss two endotherms take place The first endotherm, centred at 450"C, which is very sharp, is probably due to a phase transition The second endotherm, centred at 550°C, is very broad and intense, and is due to the release of the organic matter (phenyl groups), but not due to combustion 640 J Muter Chem, 1996, 6(4), 639-644 X-Ray structure analysis of Ba (HO,PC,H,), A colourless plate-like crystal with dimensions 0.30 x 0.30 x 0.015 mm3 was mounted on a glass fibre.Most of the crystals scanned were twinned and showed broad reflection profiles. The crystal used for data collection was, however, free from twin contribution. All crystallographic measurements were carried out on a Rigaku AFC5R diffractometer with graphite-monochromated Mo-Ka radiation (A=0.71069 A) and a 12 kW rotating anode generator operated at 50 kV and 180 mA. Cell parameters for data collection were obtained from least-squares refinement of 24 reflections chosen from the 28 =8-35' shell immediately preceding data collection.Intensity data were collected at -110°C using w28 scan mode in shells to a maximum value of 28 =50". Three intensity standards were measured every 150 reflections to monitor crystal decay. Scans of (1.84-tO.3 tan 0)"were made at a rate of 16 degrees min-' in co. The weak reflections [I <10.00(1)] were rescanned (maximum of 3) for good counting statistics. Of the 1491 reflections which were collected, 1462 were unique (Rin,=0.015). A total of 1179 reflections were observed with 1 >341). The data were corrected for Lorentz and polarization effects. Pertinent crystallographic data are presented in Table 1. The structure was solved by the Patterson method.23 The position of the heavy atom was located by deconvolution of the Patterson function.Other non-hydrogen atoms were found in the successive difference Fourier maps. The hydrogen-atom positions were calculated on the basis of geometrical consider- ations and included in the least-squares refinement, but were not refined. The final cycle of full-matrix refinement was based on 1179 reflections and 96 structural parameters. An empirical absorption correction, based on a $-plot, was applied which resulted in transmission factors ranging from 0.36 to 1.0. The weighting scheme was based on counting statistics and included a factor (p=0.03) to downweight the intense reflections. The maximum and minimum peaks on tFe final difference map corresponded to 1.63 and -2.73 e AP3 respectively. These residuals were found near the heavy atom.Neutral scattering factors were taken from Cromer and Waber.24 Final atomic and isotropic thermal parameters are given in Table 2; bond lengths and angles in Table 3. Coordination about the metal atom along with atom labelling is given in Fig. 3. Arrangement of atoms in the layer and the packing down the b axis are shown in Fig. 4 and 5, respectively. Data collection and structure refinement of Pb(HO3PC6H5), Diffraction data for the powder were collected with a Siemens D-501 automated powder diffractometer using graphite-mon- ochromated Cu-Ka radiation. The powder pattern of the sample showed a very high degree of preferred orientation. To reduce this effect on the data, the sample was mixed with Table 1 Crystallographic data for Ba( HO3PC,H5), formula mass 451.51 cqstal system monoclinic 44 32.18( 1) bl+ 5.546 (4) CIA 8.495 (4) PldFgrees 103.21 (3) VIA3 1476(2) Z' 4 space group c2/c Dc[g cm-3 2.04 i/A 0.71069 T/"C -110 absorption coeff/cm-' 29.24 observed data [I >3.00(1)] 1179 no.of parameters 96 R(Fd 0.040 RW(F0) 0.050 GOF 2.6 Table 2 Positional parameters and B(eq) for barium phenyl-phosphonate 0 0.10236(8) 0.25 1.21(2) -0.06690( 5) -0.0554( 2) -0.0582( 2) -0.0423(1) -0.1228( 2) -0.1528( 2) -0.1961(3) -0.2096( 2) -0.1804( 3) -0.1370( 2) -0.1897 -0.3341(3) -0.2973(8) -0.6084( 7) -0.1706( 7) -0.291(1) -0.446( 1) -0.412( 1) -0.224( 2) -0.070( 1 ) -0.101(1) 0.0594 0.0644( 2) 0.2445( 5) 0.0332( 5) -0.0212(5) -0.0173( 7) 0.0227(9) -0.043( 1) -0.147( 1) -0.187( 1) -0.1228( 9) -0.2598 1.25(6) 1.9(2) 1.7(2) 1.5(2) 1.6(2) 2.4( 3) 3.2(3) 3.1(3) 3.1(3) 2.3( 3) 3.8 -0.1170 0.0089 -0.1496 2.7 -0.1433 -0.5780 0.0957 2.9 -0.2164 -0.5222 -0.0142 3.8 -0.2396 -0.2007 -0.1932 3.8 Table 3 Bond lengths(& and angles(degrees) for Ba( H03PC,H,), Ba( 1)-O( 1) 2 x 2.839(5) C( 1)-C(2) 1.39( 1) Ba(1)-0(2) 2 x 2.811(4) C(2)-C(3) 1.39(1)Ba( 1)-0(3) 2 x 2.641(4) C(3)-C(4) 1.38( 1) Ba(1)-0(3)2 x 2.832(4) C(4)-C(5) 1.37( 1) P(1)-O( 1) 1.504(4) C(5)-C(6) 1.39(1)P(1)-0(2) 1.580(4) C(6)-C( 1) 1.39(1)P(1)-0(3) 1.498(4) P( 1)-C( 1) 1.791(7) O(1)-Ba( 1)-O( 1) 77.3(2) O(i)-P( 1)-O(2) 106.8 (3) O(1)-Ba( 1)-0(2)2 x 1414 1) O(1)-P( 1)-0(3) 112.2( 3) O(1)-Ba( 1)-0(2)2 x 96.5( 1) O(1)-P( 1)-C( 1) 111.5( 3) O(1)-Ba( 1)-O( 3)2 x 121.8( 1) O(2)-P(1)-O(3) 11 1.6( 2) 0(1)-Ba(l)-O(3)2x 72.4(1) O(2)-P( 1)-C( 1) 105.4(3) 0(1)-Ba(l)-O(3)2x 52.1(1) O(3)-P( 1)-C( 1) 109.1(3) O(1)-Ba( 1)-0(3)2 x 77.2( 1) C(2)-C(l)-C(6) 118.9(6) O(2)-Ba( 1)-0(2) 110.4(2) C( 1)-C( 2)- C( 3) 120.3( 7) O(2)-Ba( 1)-0(3)2 x 91.3( 1) C( 2)- C( 3)- C( 4) 120.1 (7) O(2)-Ba( 1)-0(3)2 x 79.3( 1) C(3)-C(4)-C( 5) 120.0( 7) 0(2)-Ba(l)-0(3)2 x 164.3(1) C(4)-C( 5)-C( 6) 120.8( 7) O(2)-Ba( 1)-0(3)2 x 69.4( 1) C( 1)-C(6)-C(5) 119.9( 7) O(3)-Ba( 1)-0(3) 163.5(2) O(3)-Ba( 1)-0(3)2 x 73.1(2) O(3)-Ba( 1)-0(3)2 x 116.3(2) O(3)- Ba( 1)- O(3) 11 5.4(2) 02c 02b Fig.3 Coordination geometry about the metal atom (Ba, Pb) with atom labelling spherical particles of silica, Cab-oil M-5 (from Fluka), with an approximate size range of 12-45 nm.25 The pattern was scanned over the angular range 28= 12-loo", with a step size of 0.03" at a rate of 15 s per step.The data were transferred to a VAX computer for Rietveld analysis by the GSAS suite of programs.26 The powder pattern was auto-indexed by using the Lattparm program27 based on the Visser algorithm,28 from the position of the first 2,O reflections. The result wasoa monoclinic unit cell [~=31.1$1 A, b=5.592 A, ~=8.285 A, p=93.239", Vr 1441.23 A3, Z=4, and V(non-hydrogen atom)= 17.15 A3 J. Muter. Chem., 1996, 6(4), 639-644 641 Fig.4 Arrangement of the inorganic framework in the layers The carbon atoms are represented by small circles, oxygens by large open circles, P by hatched circles and metal atoms by filled circles Other atoms of the phenyl groups are omitted for clarity Fig.5 Projection of the structure down the b axis showing the arrangement of phenyl groups in the interlayer space atom-1] with a figure of merit M,, of 4429The systematic absences were consistent with the non-standard monoclinic space group 12/u This unit cell was transformed to the standardosetting C2/5 and the oresulting unit-cell dimensions (a=31 80 A, b=5 59 A, c=829 A, p=lOl 92") are very similar to those of Ba( H03PC6H5)2 given above Rietveld refinement was started with the atomic positions of the Ba compound as starting model in space group C2/c After the initial refinement of the overall parameters (scale factor, unit-cell parameters, zero-point error, background function, preferred orientation coefficient and peak-shape parameters), the atomic positions were refined with soft constraints consisting of P-C and C-C bonds, with values of 180(2) and 140(1)A, respectively Finally, one thermal vibration parameter was refined with one parameter for each type of atom The final refinement con- verged with RWP=5 7%, R, =4 O%, and RF=1 8% Positional parameters are given in Table 4, bond parameters in Table 5 and the final observed, calculated and difference profiles are given in Fig 6 642 J Muter Chem, 1996, 6(4), 639-644 Table 4 Atomic positions and thermal parameters for Pb( H03PC6H5)2" atom X Y Z u,,,/A2 0 00 0 0774( 2) 0 25 0 0173(3) -0 0647(2) -0 3267(8) 0 0746( 7) 0 010( 2) -0 0554(3) -0 2736( 15) 0 2569( 12) 0 0086(20) -0 0552(3) -0 5965(22) 0 0449( 10) 0 0086 -0 0406( 3) -01708(16) -00147(11) 0 0086 -0 1213(2) -0 2953(25) -0 0093( 18) 00133(25) -0 1509(4) -04609(20) 0 0280( 16) 0 0133 -0 1942(4) -0 41 57( 27) -0 0404( 20) 0 0133 -0 2083(4) -0 2236(27) -0 1460(21) 0 0133 -0 1777(4) -0 0623( 25) -0 1811(16) 0 0133 -0 1340(4) -0 1007( 25) -0 1142( 18) 0 0133 a Crystal datao space group C2/c, a=31 8302( A,b=5 5997(2) A, c=8 2935(3) A, /I=101 875(2)' V= 1446 60( 12) A3 and 2=4 Table 5 Bond lengths (A)and angles (") for Pb(H03PC6H5)2 Pb-O( 1) 2x 2650(8) Pb-O(2) 2x 2843(10) O(1)-Pb-O( 1) 84 2(4) Pb-0(3) 2x 2605(9) O(1)-Pb-0(2)2 x 142 7(3) Pb-0(3) 2x 2693(9) 0(1)-Pb-O(2)2~ 99 l(3) O(1)-Pb-0(3)2 x 126 9(3) 0(2)-Pb-0(3)2 x 162 3(3) 0(1)-Pb-O(3)2~ 724(3) 0(2)-Pb-O(3)2~ 73 2(3) O(1)-Pb-0(3)2 x 54 9(3) 0(3)-Pb-0(3) 156 8(4) 0(1)-Pb-0(3)2 x 79 O(3) 0(3)-Pb-0(3) 1179(4) 0(2)-Pb-0(2) 100 l(4) 0(3)-Pb-0(3)2 x 1190(3) 0(2)-Pb-0(3)2 x 88 8(3) 0(3)-Pb-O(3)2~ 73 8(3) 0(2)-Pb-0(3)2 x 76 3(2) P-O(1) 1 510( 10) O(l)-P-0(2) llOO(6) P-O(2) 1571(11) O(l)-P-0(3) 1120(6) P-0(3) 1461(9) O(1)-P-C(1) llOO(7) P-C( 1) 1 800(22) 0(2)-P-0(3) ll09(6) 0(2)-P-C(1) 104 l(6) 0(3)-P-C(1) 109 5(7) 20 30 40 50 60 28/10 degrees Fig.6 A portion of the observed and calculated profiles for the Rietveld refinement for Pb(HO,PC,H,), The lower curve is the difference plot on the same intensity scale Results and Discussion The crystal structures of barium and lead phenylphosphonates are isomorphous The structures are layered with the metal atoms (Ba, Pb) located on the two-fold axis and lie in the bc plane at x=O and 1/2 The coordination about the metal atoms is shown in Fig 3 together with the numbering scheme used in the tables The phosphonate group P(l) chelates the metal atom through 0(1) and O(3) There are two such groups related by symmetry (two-fold axis) bonded to the same metal atom The third oxygen of each phosphonate group bridges to the adjacent metal atom along the b direction (Fig 4) To complete the coordination sphere, each O(3) donates an electron pair to the adjacent metal atoms along the c direction In the bc plane the metal atoms are separated by about 5.6 A along the b direction and 4,4A in the c direction. Along the c axis they are arranged in the form of a zigzag chain as shown in Fig.4. Adjacent atoms in this chain are bridged by oxygen atoms [0(3)] which creates four-membered rings. Oxygens 0(1) and O(2) are involved in binding to a single metal atom while O(3) binds to two metal atoms. Thus, 0(1) and O(2) provide two binding sites each to the metal while O(3) provides four sites leading to eight- coordination for the metal atoms in a distorted dodecahedra1 geometry, The Ba-0 distances range between 2.641(4) and 2.839(5) Ab Among the four Ba- O(3) distancesotwo are shorter [2.641(4)A] than the other two [2.832(4)A]. The longer bond lengths correspond to atoms [0(3) and O(3a) in Fig.31 that are involved in chelation to the metal atom. This type of long and short bond distances are common in metal phosphon- ates where phosphonate oxygens are involved in both chelation and bridging. For example, in the structure of Zn(O,PCH,Cl)," the two longest bonds are those that form the chelate ring, and the two shortest are those formed by the same oxygens donating to adjacent zinc atoms. The Ba-0 bond lengths with the :ther two oxygens, O(1) and 0(2), are 2.839(5) and 2.811(4) A respectively. The P-0 bond length (Table 2) jnvolving the O(2) atom is significantly longer [1.580(4)A] than the other two P-0 distances [1.501(4)A], indicating that the proton is bonded to the O(2) atom. The phenyl groups display a regular planar geometry with normal bond parameters.In the case of the lead compound, the Pb-O(1) and Pb-0(2) distances are 2.650(8) and 2.84( 1)A respectively. The twoo types of M-0(3) bond lengths [2.605(9) and 2.693(9) A] have much more similar values in this case when compared to the Ba structure. Again, the proton is bonded to the O(2) atom in the Pb compounds oas indicated by the greater P-0(2) bond length [1.57(1)A]. The geometry of the phosphonate group is regular. The arrangement of the organic groups in the interlayer space is shown in Fig. 5. Th,e phenyl groups are separated by one unit cell or about 5.6A along the b axis. The adjacent rings along the c direction are not in the same plane, instead they are shifted by half a unit along the unique axis, b.This fact is best illustrated in Fig. 4 where the carbon atom [C( l)] bonded to phosphorus is represented by a small circle. The phenyl groups of one layer are pointing towards the centre of two neighbouring phenyl groups of the adjacent layers, as shown in Fig. 5. The phenyl groups are tilted away from the normal to the metal oxygen plane by about 30". The arrange- ment of phenyl groups with respect to the metal-phosphonate layers is similar to that observed for zirconium phenylphos- phonate compo~nd,~ Zr(o3PC6Hs),, that also crystallizes in space group C2/c. However, in the Zr compound the metal atoms are six-coordinate and all the oxygen atoms are bonded to a single metal atom. The Zr atoms in the plane are arranged in a regular hexagonal manner and the P atoms lie above and below this plane.This arrangement of metal and P atoms is slightly distorted in the Ba and Pb compounds owing to the geometrical requirements for higher coordination number for the metals. Nevertheless, there are six metal atoms surrounding each metal atom as in the Zr compounds but they do not occupy the corners of a regular hexagon. Similarly, there are six phosphonate groups about each of the metal atoms, among which three are above the plane and the other three are located below the plane. All of the 1:2 metal phosphonates cited above have unit- cell dimensions within the plane that are very similar. In this regard they mimic the parent compound a-zirconium phos- phate, Zr( HP04), H2O3' whose cell dimepions within the layer are a =9.6060(2) and b=5.297(1)A.The interlayer dimension is variable and depends upon the nature of the organic pendant group and its orientation to the plane. To the best of our knowledge the lead phenylphosphonate represents the first example of a non-transition, non-alkaline- earth-metal layered phosphonate to be structurally charac- terized. Among the alkaline-earth-metal compounds, as already mentioned, two calcium phosphonate structures have been reported,15 one of which is that of Ca(O,PCH,)*H,O, the other is of Ca(HO3PC6HI3),. The former crystallizes in the monoclinic space group P2,/c, with a =8.8562( 13), b= 6.6961( lo), c=8.1020( 10)A, and p=96.910( 11)'.The structure is layered, and the phosphonate oxygens are involved in both chelation and bridging. The calcium atoms have an approxi- mately pentagonal-bipyramidal coordination. In terms of met-al :phosphonate ratio, the compound Ca( HO$C6H13)2 resembles very closely the Ba structure presented here, although the organic group in the Ca structure is a primary alkane. This calcium compound crystallizes in the sFace group Pi, with a= 5.606(2), b= 7.343(3), c =21.158(7) A, a =97.31(3), ,8=96.98(3), y= 90.43(4)". Again, this structure is layered and the phosphonate oxygens are involved in both bridging and chelation, as in the Ba and Pb compounds. Unlike the title compounds, the calcium atoms in this structure are octa-hedrally coordinated.One further difference between the pre- sent structures and this Ca structure is that the phosphonate oxygen carrying the proton does not coordinate to the Ca atom. The alkyl chains are oriented towards the interlayer space, as the phenyl groups in the Ba and Pb compounds. We thank the National Science Foundation (grant no. DMR- 9107715) and DGICYT (Ministerio de Educacion y Ciencias, research project PB-93/1245) for financial support. References 1 A. Clearfield, in Design of New Materials, ed. D. L. Cocke and A. Clearfield, Plenum, New York, 1986. 2 A. Clearfield, Comments Znorg. Chem., 1990,10, 896. 3 G. Cao, H. Hong and T. E. Mallouk, Acc. Chem. Res., 1992, 25, 420. 4 M. E. Thompson, Chem. Muter., 1994,6,1168. 5 M. B.Dines and P. DiGiacomo, Inorg. Chem., 1981, 20, 92; M. B. Dines, P. DiGiacomo, K. P. Callahan, P. C. Griffith, R. Lane and R. E. Cooksey, in Chemically Modijied Surfaces in Catalysis and Electrocatalysis, ed. J. Millar, ACS Symp. Ser. 192, American Chemical Society, Washington, DC, 1982,p. 223. 6 G. Alberti, U. Costantino, S. Allulli and N. Tomassini, J. Znorg. Nucl. Chem., 1978, 40, 113; G. Alberti and U. Costantino, in Inclusion Compounds, ed. J. L. Atwood, J. E. D. Davis and D. D. MacNicol, Oxford University Press, London, 1991, vol. 5, ch. 5. 7 D. M. Poojary, H-L. Hu, F. L. Campbell and A. Clearfield, Acta Crystallogr., Sect. B, 1993, 49, 996; D. M. Poojary, C. Bhardwaj and A. Clearfield, J. Mater. Chem., 1995, 5, 17; D. M. Poojary, B.Zhang and A. Clearfield, Angew. Chem., Znt. Ed. Engl., 1994, 33,2324. 8 K. J. Martin, P. J. Squattrito and A. Clearfield, Znorg. Chim. Acta, 1989, 155,7. 9 Y. Zhang and A. Clearfield, Znorg. Chem., 1992,31,2821. 10 C. Bhardwaj, H-L. Hu and A. Clearfield, Znorg. Chem., 1993, 32, 4294. 11 D. M. Poojary and A. Clearfield, J. Am. Chem. Soc., 1995, 117, 11278. 12 D. Cunningham, P. J. Hennely and T. Deeny, Znorg. Chim. Actu, 1979,37,95. 13 G. Cao, H. Lee, V. M. Lynch and T. E. Mallouk, Znorg. Chem., 1988, 26, 63. 14 G. Cao, H. Lee, V. M. Lynch and T. E. Mallouk, Znorg. Chem., 1988,27,2781. 15 G. Cao, V. M. Lynch, J. S. Swinnea and T. E. Mallouk, Znorg. Chem., 1990,29,2112. 16 R-C. Wang, Y. Zhang, H-L. Hu, R. R. Frausto and A. Clearfield, Chem.Muter., 1992,4, 864. 17 G. Alberti, U. Costantino, F. Marmottini, R. Vivani and P. Zappelli, Angew. Chem., Int. Ed. Engl., 1993,32, 1357; G. Alberti, F. Marmottini, S. Murcia-Mascaros and R. Vivian, Angew. Chem., Znt. Ed. Engl., 1994,33, 1594. 18 K. Maeda, Y. Kiyozumi and F. Mizukami, Angew. Chem., 1994, 106,2429;Angew. Chem., Znt. Ed. Engl., 1994,33,2335. J. Muter. Chem., 1996, 6(4), 639-644 643 19 J L Bideau, C Payen, P Palvadeau and B Bujoli, Inorg Chem, 1994,33,4885 25 M C Morris, H F McMurdie, E H Evans, B Paretzkin, J H deGroot, R Newberry, C R Hubbard and S Carnel, Natl Bur 20 21 22 23 24 D M Poojary, D Grohol and A Clearfield, Angew Chem, Int Ed Engl, 1995,34,1508 D M Poojary, A Cabeza, D Grohol, M A G Aranda, S Bruque and A Clearfield, Znorg Chem ,in the press G Cao, H Lee, V M Lynch and T E Mallouk, Solid State Ionics, 1988,26,63 TEXSAN, TEXRAY Structure Analysis Program, Molecular Structure Corporation, The Woodlands, Texas, 1987 (revised) D T Cromer and J T Waber, International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, UK, 1974, vol IV, Table 2 2A (present distributors Kluwer, Dordrecht) 26 27 28 29 30 Stand (US) Monogr 25,1977,14 A C Larson and R B Von Dreele, Report No LA-UR-86-748, Los Alamos National Laboratory, 1987 R Garvey, LATTPARM autoindexing program, Department of Chemistry, North Dakota State University, Fargo, ND See also R Garvey, Powder Difraction, 1986,1, 114 J W Visser, J Appl Crystallogr, 1969,2, 89 P M Wolff, J Appl Crystallogr, 1968,1, 108 J M Troup and A Clearfield, Inorg Chem , 1977,16,3311 Paper 5/05464H, Received 16th August, 1995 644 J Mater Chem, 1996, 6(4), 639-644

ISSN:0959-9428    

DOI:10.1039/JM9960600639

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Protonation and olation of 2,2'-bipyridyl and 1,lO-phenanthroline in y-titanium phosphate dihydrate? Carla Ferragina,a M. Antonietta Massucci' and Anthony A. G. Tomlinson' aI.M.A.I., Rome Research Area, C.N.R., P.O. Box 10, Monterotondo Staz., 00016 Rome, Italy bUniversita di Roma 'La Sapienza', P.le A. Moro 5, 00185 Rome, Italy 'Istituto di Chimica dei Materiali, Rome Research Area, C.N.R., P.O. Box 10, Monterotondo Staz., 00016 Rome, Italy 2,2'-bipyridyl (bpy) and 1,lO-phenanthroline (phen) intercalate topotactically into Ti( H2P04)( P04).2H,0 (y-titanium phosphate, y-Tip), the aromatic rings undergoing hydration owing to the strongly acidic character of the >P(OH), groups. For protonated phen, a 'slotted' assembly in the interlayer phosphate surface is given, via NH+ HO-P hydrogen bonding (from indirect spectroscopic results and TG/DSC).Instead, protonated bpy adopts a slanted orientation within the layers, as does 2,9-dimethyl- 1,lO-phenanthroline (dmphen) which remains unprotonated. These differing assemblies lead to differing accessibilities of intercalated amine to metal ion (dmphen >bpy >phen) as confirmed by Cu2+ uptake diffusion rates. Cation exchange of Co2+, + +Ni2 and Cu2 -exchanged composites do not give coordination pillaring, even after dehydration of the materials. Ag + exchanges with Ti( H2P04)( PO4)( phen),.,,.2.5H20 and Ti( H2P04)( P04)(dmphen)o~,, .2.2H20 to give highly expanded composites (dOo2=20-21.3 A). The former has high thermal stability, decomposing only at >450 "C,and may be considered a pillared + + +material.Low Ag -loaded forms give time-dependent staging, ascribed to intralayer Ag diffusion. Ag -phen.y-TiP materials show enhanced selectivities for Cu2+ (Cu2+ >>Ni2+ =Co2+) from binary, ternary and quaternary mixtures and the anhydrous Ni' -exchanged material has a square-planar coordination. Intralayer coordination models are suggested. + The chemistry and applications of metal ion complex moieties supported in/on restricted media interfaces is now well devel- oped' and, as interfacial arrays and geometries are clarified via modern techniques,' there is renewed interest in the supra- molecular chemistry of composite^.^ For zeolites and clays, metal ion surface (or interface) complexes thus supported are of most intere~t.~ However, in situ complex construction in a layered material can also provide a host of 'engineered' small-pore metal-complex pillared materials, promising for applications in fields ranging from optoelectronics5 to pseudo- metalloenzyme catalysk6 We recently found that the well characterised ion-exchanger y-zirconium phosphate gives com- posites different from those of the aform, although details of interface assemblies could not be rationalised in the absence of a structure determinati~n.~ A Rietveld X-ray powder diffrac- tion (XRPD) structure of the y-titanium analogue has since shown that it is best formulated as Ti(H,P04)(P0,)~2H,0, a structure in which a highly acidic, hydrophilic -P(OH), channel alternates with a hydrophobic >P=O channel.' We now report composites formed by the same ligands as before in this matri~,~ with the objective of probing how the surface influences subsequent metal-complex formation and induction of micropores, and site-selectivity.Experimenta1 Materials CrystaJline Ti( H2P04)( P04).2H,0 (basal spacing dm2 = 11.62 A; referred to as y-TiP for convenience) was prepared by treating dried, amorphous titanium phosphate (obtained from reaction between TiC14 dissolved in 2mol dmP3 HC1 and 2mol dm-3 phosphoric acid at room temperature) with 10 mol dm-3 H3P04 under hydrothermal conditions at 300 "C for 48 h. The diamines and metal ion salts were commercial products (Aldrich) and were used as received.t Part 6 of a series entitled 'Pillar Chemistry'. For Part 5, see ref. 7. Intercalation of diamines y-TiP was first pre-swelled with EtOH by agitation of a mixture of y-TiP (1 g) in anhydrous EtOH (100 ml) for 24 h, and centrifugating the suspension at <2000 rpm (to avoid layer collapse). The product was contacted whilst still wet with 100 ml of a 0.01 mol dm-, H,O-EtOH solution of the amine at 40-45 "C (preliminary experiments using 0.1 mol dm -3 concentration solutions gave the same, but less crystalline, solids). After varying contact times, the solids were filtered off and air dried. Table 1 lists stoichiometries, preparation con- ditions and interlayer distances (average of the do,, reflections, which usually go to 1= 3). Phen (but not bpy and dmphen) was subsequently found to intercalate into y-TiP (again at 45°C) without the need for pre-swelling, to give the same final product.Metal ion exchange Batch methods were used, as before." In a typical preparation, a solution of 5mmol dmW3 metal acetate (Co, Ni, Cu) or 10 mmol dm-, AgN03 was agitated with 1mmol of diamine- intercalated material, such that [Amine]:[M"+] =1, where the overbar signifies 'intercalated' ([Amine]: [M"' ]=2 were also investigated for the Ag +-containing suspensions) at 45 "C for 10 days. After filtration, the supernatant was analysed [atomic absorption (AA) spectrophotometry] and the solid dried in air. Ion selectivity experiments were carried out using y-TiPH1.,' [Ago,,,( phen)o.44]*2.5H20 and binary, ternary and quaternary mixtures of transition-metal ions (Co, Ni, Cu, Pd).In a typical experiment, 1 mmol of the Ag-phen derivative was suspended with stirring in 50 ml of water containing 1 mmol in each component (acetate salt) at 45°C for 7 days. The resulting solids were filtered off, air dried and analysed. Analyses and physical methods Chemical analyses [C, H, N; AA spectrophotometry, thermo- gravimetry-differential thermal analysis (TGA-DTA)] and physical measurements [X-ray powder diffraction (XRPD), optical spectra, EPR] were carried out as described pre- J. Muter. Chem., 1996, 6(4), 645-651 645 Table 1 Stoichiometries, preparation conditions and interlayer distances material' contact time/days T/"C d002IA do02 '/A y-TiP(bpy), 43 04H20 15 9 y-TiP(phen), 44 2 5H20 15 8 y-TiP(dmphen), 23 2 2H20 15 9 Stoichiometries deduced from TG and C, H, N analyses Ag+-exchanged viously lo Uptakes in ion-selectivity experiments were followed by UV-VIS spectral methods Several resulting solids were also analysed uza atomic absorption spectroscopy, the two methods gave identical results (within experimental error) Results and Discussion All three amine-intercalated materials give XRPD patterns as expected for grossly topotactic amine diffusion (simple layer expansion) and the limiting products have stoichiometries similar to those in the zirconium analogue' Dmphen is the exception, giving only a single phase-pure product containing ca 1 dmphen per 4 basal units (y-zirconium phosphate, y-ZrP, also gives a single phase, although it contains ca 1 dmphen ligand per 3 basal units7) In addition, the interlayer expansions in y-TiP(phen), 44 2 5H,O and the y-ZrP analogue are similar (6 4 A), although the former differs from the latter in being obtainable from unswelled starting material and also shows high crystallinity even at 400 "C (alFeit with a marked decrease in the interlayer distance, to 14 85 A) Both results suggest that the amines are assembled differently in the two materials Inspection of the y-TiP structure' shows that the distance betyeen the OH groups of adjacent >P(OH), groups is ca 3 5 A The lower icpic radius of Ti4+ (068 A) with respect to that of Zr4+ (0 90 A) suggests that in the former the >P(OH), groups are more laterally flexible than in the latter (as also indicated by the thermal amplitudes'), allowing an interlayer assembly for phen in which one aromatic benzene ring is slotted into the layers via strong hydrogen bonding with an adjacent >P(OH)2 group The IR spectrum does indeed show evidence for the presence of both protonated phen and specifi- cally bound water molecules A band at 3537cm-1 with a shoulder at 3487 cm-' are assignable to vsym and vaSym modes of lattice H20 and they virtually disappear on dehydration to be replaced by a single band at 3350cm-', ascribable to v(NH+) modes [Fig l(b)] There is also significant splitting of the broad v3(P02-)band on intercalation, further evidence for specific interaction of phen with Po43-groups More interesting is the appearance of bands at 1240 and 1474 cm-I which are due neither to matrix nor to phen modes, they can be assigned to the 0-H and C-H vibrations of a C(H)(OH) group," suggesting that a covalent hydration has occurred in one aromatic ring The UV spectra of bpy- and phen-loaded materials provide further support for both diprotonation and olation, since UV spectra are very sensitive to ring modifi- cation12 Thus, apart from the expected shifts of the phen bands to 224 and 277 nm and the appearance of low-energy shoulders characteristic of [H,phen12+ ,I2 there is also a large low-energy shift of the characteristic [H,phen12+ shoulder (which lies at 318nm in [H2phenI2') to 357nm which is ascribable to loss of aromatic character in a ring [see Fig 2(b)] Turping to y-Tip( bpy), 43 0 4H@, the interlayer distance (14 5 A) is similar to that in the y-zirconium analogue, suggest- ing that a slanted interlayer orientation is given (The layer 'thickness' in ?-Tip, which includes the bimetal-phosphate layer, is cu 903 A, which leaves a van der Waals distance of Ad,,, =5 2 A for the bpy composite ) Again, UV spectral evidence suggests that bpy is also present in the diprotonated 646 J Muter Chem, 1996,6(4), 645-651 25 14 45 1447 (0 18) 43 25 18 00 20 50 (044) 43 25 17 40 21 30 (0 23) 43 Mole ratio in parentheses vlcm ' Fig. 1 FTIR spectra of (a) y-TiP(bpy), 43 0 4H20, (b) y-TiP(phen),,, 2 5H2O -, As-prepared, ---, after dehydration at 120°C for 2 h A/nm Fig.2 UV-VIS spectra TiP(phen), 44 2 5H20 -, 120°C for 2 h of (a) y-TiP(bpy), 43 04H,O, (6) y-As-prepared, --, after dehydration at form [H2bipy12+.Intense bands characteristic of [H2bipy12+ are observed at 220 and 285 nm, with a further band at 315 nm and a much less intense band at 390nm, and an indistinct plateau between 450 and 600 nm [see Fig. 2(u)]. Bands at 1240 and 1474 cm-' are again present in the IR spectra. We conclude that bpy also undergoes ring hydration, and Scheme 1rational-ises the results for both materials. Details from the TG-DTA results also support the sugges-tion that olation occurs on intercalation, and they are particu-larly clear for bpy-loaded materials.Gross thermal losses are analogous to those in a-and y-ZrP: zeolitic water is lost between 25 and 250"C, and organics loss overlaps with final water loss due to phosphate-to-pyrophosphatecondensation. Final, small losses at 600-1 100"Care probably due to carbon-isation of remaining organic residues, and the sharp exotherm at 810°C is due to a phase change to cubic TiP207 (as confirmed by XRD).I3 Turning to the mid-temperature range, between 300 and 450 "C,y-Tip(bpy),~,,-0.4H20 (but not phen and dmphen) gives two exothermic peaks [see Fig. 3(a)] and the corresponding mass losses are in very close agreement with scission of bpy, the C(H)(OH) product being lost first (Scheme 2). This result is not surprising if proton-assisted scission of the H,2+bpyC(H)(OH) by close-lying hydrogen bonds is involved.Analogous scission of bpy after coordination to acidic groups has been reported for alumina surfaces.14 Conversely, y-Tip(phen),.,,.2.5H20 loses zeolitic water in a series of steps up to 200"C, (some water being lost only at ca. 330 "C, as expected for specific hydrogen-bonded H20). $!--40-v) IIIIllllII TIT Fig. 3 TG (-) and DTA (---) curves of (a) y-TiP(bpy)043-0.4Hz0; (b)y-TiP(phen), ,,-2.5H20 Scheme 1 Slotted assembly of modified phen in y-TiP (a) and orientation of (Hzbpy)'+ (b) J. Muter. Chern., 1996, 6(4), 645-651 647 weak H bonding I OH I H+ OH /[\ / Scheme 2 Decomposition pathway of bpy in y-TiP Anomalies in the acid-base characteristics of bpy and phen have long been known,15 and their presence in y-TiP under- lines the strongly acidic nature of the =P(OH), groups The formulations agree with the low water content in Ti(H2P04)( PO,)( bpy), 43 0 4H20, the lengthwise orientation means virtually all =P(OH), sites are occupied, leaving only hydrophobic PO, channels y-TiP(dmphen), 23 2 2H20 shows no spectroscopic or TG evidence for other than a vertically slanted onentation, as expected because p-aromatic sites are occupied by Me groups Dmphen assembly seems to be controlled by the steric hindrance of Me groups Metal-ion uptake Differing amine assemblies and the presence of an extra intralayer coordinating moiety [the C(H)(OH) group] in y-TiP(phen), ,, 2 5H20 are expected to lead to large changes in cation-exchange behaviour Cu2+ uptake shows that exchange is fastest in yTiP(dmphen), 23 2 2H20 (though somewhat slower than in the y-ZrP analogue, shown in Fig 4 for compari- son) as expected, naively, for a higher intercalant interlayer density in the former Dmphen is also the only ligand giving rise to a solid with final [Cu] [L] = 1, yTiP(phen), ,, 2 5H20 gives a final material with [Cu] [L] <O 4, even after 12 days of exchange (see Fig 4) Closer inspection of Fig 4 shows that (I) Cu2+ uptake rates for bpy and phen are inverted between y-TiP and y-ZrP matrices, and (11) the uptake rates follow different laws l6 Initial stage for intercalation is as expected (eg as in the well investigated dichal~ogenides'~) We suggest that the inversion is due to the inaccessibility of strongly hydrogen-bonded P-OH groups in slotted phen, whereas the (al (b)...dmphen 08 differing slopes are due to changes in pore accessibility brought about by amine orientation Cavity vs. pillaring in metal-ion exchanged phases It is of interest to monitor whether metal-exchanged materials are pillared (or become so on dehydration) and, if so, how this influences microporosity For Co2+-, Ni2+- and Cu2+-exchanged matenals, dOo2remains constant after exchange (so coordination to ligand and/or layer must be inferred from spectroscopic data, as before") For all three metal ions, the only clear evidence for N-coordination is obtained for y-TiCu, 23(dmphen), 23 2 2H20, where d-d bands at 725 and 1025 nm follow diagnostics for N-coordination [The band at 450nm with a shoulder at 525nm in y-TiCu, 23(dmphen), 23 2 2H20 is attributable to Cu+-N metal- to-ligand charge-transfer transitions 18] We recall that tetra- gonal 0-coordinated Cu2+ is expected to give rise to relatively low-energy d-d bands (in a-ZrP itself, the d-d bands are at ca 13000 cm-') l9 Time effects are also evident in the Cu2+- exchanged dmphen composite After 4 h contact, the material is green and slowly changes (24 h) to orange-red, the optical spectra indicating that the change is probably due to changing Cu2+ Cu+ ratio, both present simultaneously Although Co2+, Ni2+ and Cu2+ can form complexes with protonated highly basic amine ligands,20 it appears that the protonated N-groups in bpy and phen in y-TiP are not readily accessible The strongly acidic interlayer in y-TiP may also be responsible the modified bpy is fixed in the trans configuration of Scheme 2, ze the exchanged cation cannot bring about the tram+czs rotation necessary for formation of a complex pillar lo Ag exchanges readily (within minutes) into all three mate- + rials, the accompanying solid-state changes being both loading- and time-dependent (Table 1) Most work has concentrated on y-TiP(phen), 44 2 5H20 because of its interesting assembly For example, at [As'] [phen] mole ratios of 1 2 or above, only y-TiPAg, 22(phen)0 44 1 8H20 IS formed whatever the con- tact time (always at 45 "C), and the intensity ratio IdoO4 Idoo2 > 1 Instead, at [Ag'] [phen]=l 1 (6 days contact), y-TiP[Ag, 37(phen), 44] is given, in which Idoo4 Idooz is now ca 1 y-TiPAg, 44(phen), 44 2 5H2O could be obtained only by successively contacting y-Tip( phen), ,, 2 5H20 with AgN03 at high [Ag'] [phen] mole ratios and it was found that Id,, 2~oo,< 1 These trends are reminiscent of staging behav- iour, and since interlayer distances do not change during the XRD changes and no phase de-mixing was detected, they involve changes in intralayer occupation alone The large layer expansion on exchange may be rationalised assuming that Ag' ions exchange = P(OH), protons, being simultaneously positioned close to the C(H)(OH) group, as shown in Scheme 3 (Despite layer expansion, the optical spectrum shows that the Ag+ does not coordinate to N atoms nor does it give I I I 1 I I timeh Scheme 3 Staging behaviour of Ag+ In y-TiP(phen), 44 2 5H20, show-Fig.4 Cu2+ uptake in (a)y-TiP(amines), (b)y-ZrP(amines) (from ref 6) ing coordination to ligand and layer phosphate 648 J Muter Chem, 1996, 6(4), 645-651 rise to Ag clusters,,, see Fig. 5.) As shown by the TG-DTA curves, decomposition of phen occurs at the same temperature whatever the Ag+ exchange, which supports this suggestion (stepwise, lower temperature, decomposition of amine in the dmphen analogue instead probably reflects the higher degree of freedom of the interlayer dmphen; Fig. 6). The XRPD pattern of y-TiPAg0~,,(phen),~,,~2.5H,0 tends further towards a stage 1 behaviour after the material is left to stand for six months.Conversely, on leaving the only half- exchanged material, i.e. y-TiPAg,.,,( phen),,,,.1.8H20, to stand, the XRPD pattern tends slowly towards that expected for stage 2 behaviour, reaching a limit after four years, as seen in Fig. 7. This implies that Ag+ ions are mobile throughout the 400 800 1200 Alnm Fig. 5 Diffuse reflectance optical spectra :(a) y-TiPCu,,,,(dmphen), 23.2H20;(b) y-TiPAg,,,,( phen), ,,.2.5H20, before (---) and after (-) dehydration Fig. 6 TG-DTA curves of (a) y-TiPAg,,,,( phen),.,,.2.5H2O; (b) y-TiPAg,,,,( phen),.,,-1.8H20; (c) y-TiPAg,,,,(dmphen),.,, .2.9H20 1 1 1 I 5 10 15 20 2Oldegrees (Cu-Kct) Fig. 7 XRPD patterns of (a) y-TiPAg,,,,( phen),,,,.2.5H20; (b) y- TiPAg,.,,( phen),,,,.1.8H,O after 4years standing at room temperature interlayer, presumably via a proton exchange mechanism involving free protons.Pore selectivity and geometry in Ag -exchanged phases + The high interlayer expansion in y-TiPAgo.22(phen)o.44.1.8H,0 (11.2 A) and the presence of residual interlayer protons are expected to enhance cation uptake compared with starting amine-intercalated materials or y-TiP itself. This is indeed the case; the Ag +-exchanged material gives higher selectivity and higher loading levels than the unloaded material, as shown in Fig. 8. Inspection of Fig. 8 shows that for binary solutions, Cu2+ exchange is highly favoured over Co2+ and Ni2+ exchange, whilst Co2+=Ni2+ (i.e.each is exchanged to roughly the same extent from a Co2+/Ni2+-containing solution) and the same is the case for Co2+ /Ni2+/Cu2+ ternary solutions. Further, the solids produced after exchange all still have layer structures, although greatly amorphized. The Co2+-and Cu2+- exchanged materials give optical spectra expected for all-0 coordinated species (i.e. cavity coordination alone is present and although pores are larger, there is little evidence for migra tion/coordina tion to the pro tona ted amine). Water molecules are coordinated, as deduced from the (small) band- energy changes on dehydration. On dehydration, Co2+- exchanged materials give optical spectra characteristic of distorted tetrahedral [COO,] coordination (see Fig.9 for example). More interesting is the geometry change on dehy- dration of Ni2+-exchanged y-TiP(dmphen),,,,.2.2H20(again, despite a large change in the optical spectra, do,, remains unchanged). A pseudo-octahedral interlayer species is present (~,=8400cm-~)at 25"C, but after dehydration there is no absorbance at <10000 cm-l, i.e. interlayer tetrahedral, five- coordinate, or pseudo-octahedral NiO, geometries can be excluded [see Fig. 9(b)]. A square-planar geometry fits the optical spectrum (d-d band centred at 14 000 cm-l. [NiN4] chromophores with mainly o-bonding N have a d-d envelope at ca. 22 500 cm-' and [NiO,] ones at 18 000-20000 cm-' 23). We attribute the low energy both to the presence of weak P-0 ligands from the host and to distortion of the NiO, due to the constrained environment. Modelling the position is J.Mater. Chem., 1996, 6(4), 645-651 649 co cu 10 .:.,;...; ........:$.;.:... 5 ..... ...0 ........I co cu ...-.-*.' . ' , ' , ' , ' -*:.. ..*:'.I.: , ,' ;:::.):*.:................................;+:.....: ...... ....................... 500 lo00 I500 2000 hlnm : 1. , ' '....-.,p.:. ' ................. , . ..:p.."..:;:'' . , , . , Co Ni ................. .................................................................................... 0 ......................... -Ni Cu Co Ni Cu 40, 35 30 25 20 15 Fig. 9 Diffuse reflectance optical spectra : (a) NiZ+I I.i-,. TiP(dmphen),,,,.2.2H20; (b)Co2+/.~-TiP(dmphen),,,,.2.2H20.-, AS-.................. prepared; ---, after dehydration at 120°C for 2 h.......................... Co Ni A 501 Ni CU Co Ni Cu 1 Fig. 10 Cation selection from six-component mixtures. A, (a) y-TiP,2H2O; (b) y-TiP(bpy)o,43.0.4H20;(c) ~-TiP(phen),,,,~2.5HzO; (d) j~-TiP(dmphen)~.,,~2.2H,O.(a) y-TiPAgo,,3(bpy)o.,3~0.4HzO;B, (b)y-TiPAg0,,,(phen),,,,.1.5H,0; (c) y-TiPAgo.23(phen),,,4.1.8H20. more difficult because the distance between P(OH)2groups is too large to allow normal Ni-0 bonds (a full molecular modelling of the sites is underway). In multicomponent cation exchange, underivatised ?-Tip selects against a mixture of Ag', Rh3+ and Pd" in the 10 5 0 Co Ni Cu Ag Fig.8 Cation uptakes for A, binary; B, ternary; and C, quaternary mixtures: (a) y-TiP(phen),,,,~l.5HZO;(b)y-TiPAgo.,,( phen)o,,4.1.8H20 650 J. Muter. Chern., 1996, 6(4), 645-651 cationic radii order. However, there are differences in exchange selectivity which are less easily rationalised, such as the enhanced selectivity of y-TiP(phen)o.,4-2.5H20 against Cu2+ , Ni2+ and Co2+ compared with y-TiP(dmphen),.,,-2.2H2O (Fig. 1OA). As expected, the Ag -exchanged materials show consider- + ably enhanced selectivity compared with the amine-intercalated materials themselves, against Ag+, Rh3+ and Pd2+ (i.e. in a multicomponent mixture containing Co2+,Ni2+ and Cu2+, partially Ag -exchanged y-Tip( phen),.,,.2.5H20 preferentially + exchanges Cu2+) (Fig.1OB). It is not immediately obvious why this selectivity should be so high, because the pores are large and there is no clear 'blocking' of pathways (as is found in cation-exchanged pillared clays24). Presumably, the presence of further coordinating groups on the bpy and phen can give rise to synergistic effects because of the variety of geometries which may be adopted by the metal ions. Simulation studies are under way to clarify the site geometries. Conclusions The combination of highly acidic >P(OH)2 and hydrophobic >P=O channels in y-TiP leads to large modifications in protonation and assembly of simple aromatic amines in the interlayer, and also unusual ion-exchange behaviour of the pores generated. Since ring hydration ofbpy and phen to produce intralayer C(H)(OH) groups may lead to the gener- ation of chiral centres, a possible use of the materials in the chiral separation of organics is being investigated.M. A. M. thanks the 'Progetto Finalizzato Chimica Fine 11' of C. N. R., and A. A. G. T. thanks the E.U. (contract no. BRE2-CT93-450) for support. We thank Dr. A. De Stefanis for technical help. References 1 For reviews, see: S. L. Suib, Chem. Rev., 1993, 93, 803; N. J. Turro and M. Garcia-Garibay, in Photochemistry in Organised and Constrained Media, ed. V. Ramamurthy, VCH, New York, 1990, ch. 4. 2 G. A. Ozin, New Muter., 1992,4, 612. 3 Y. Yan and T. Bein, Chem. Muter., 1993, 5, 905 and refs.therein; Supramolecular Architecture: Synthetic Control in Thin Films and Solids, ed. T. M. Bein, ACS Symp. Ser., no. 499, Washington, DC, 1993; The Lock and Key principle, Perspective in Supramolecular Chemistry, vol. I, ed. J. P. Berh, Wiley, New York, 1994, p. 176. 4 K. J. Thomas, Chem. Rev., 1993,93,30; X. Lin and J. K. Thomas, Langmuir, 1993,9,722 and refs. therein. 5 M. E. Katz, M. L. Schilling, S. Ungashe, T. M. Putvisnki and C. E. Chidsley, in Supramolecular Architecture: Synthetic Control in Thin Films and Solids, ed. T. M. Bein, ACS Symp. Ser. no. 499, Washington, DC, 1993, p. 25. 6 A. A. G. Tomlinson, in Pillared Layered Structures, ed. I. V. Mitchell, Elsevier, Amsterdam, 1990, p. 91; D. Mansuy and P. Battioni, in Metalloporphyrins in Catalytic Oxidations, ed.R. A. Sheldon, Marcel Dekker, New York, 1993, p. 99. 7 C. Ferragina, M. A. Massucci and A. A. G. Tomlinson, J. Chem. SOC., Dalton Trans., 1990, 1191. 8 A. Norlund Christiansen, E. Krogh-Anderson, I. G. Krogh- Andersen, G. Alberti, M. Nielsen and M. S. Lehmann, Acta Chem. Scand., 1990,44, 865. 9 A communication has appeared: C. Ferragina, M. A. Massucci and A. A. G. Tomlinson, in Recent Developments in Zon Exchange 2, ed. P. A. Williams and M. J. Hudson, Elsevier Applied Science Press, New York, 1990, p. 103. 10 C. Ferragina, M. A. Massucci, A. La Ginestra, P. Patrono and A. A. G. Tomlinson, J. Phys. Chem., 1985,89,4762. 11 A. Albert and W. L. F. Armarego, Adv. Heterocycl. Chem., 1965, 4, 1.12 R. G. Gray, J. Ferguson and C. J. Hawkins, Aust. J. Chem., 1969, 22, 209. 13 A. Clearfield, Inorganic Exchange Materials, CRC Press, Boca Raton, FL, 1982, ch. 1. 14 S. A. Bagshaw and R. P. Cooney, J. Muter. Chem., 1994,4,557 and refs. therein. 15 R. D. Gillard, Coord. Chem. Rev., 1975,16,67. 16 E. V. Boldyreva and K. M. Salikhov, React. Solids, 1985,1,3. 17 G. V. S. Rao and M. W. Schafer, J. Phys. Chem., 1975,79,557. 18 C. Ferragina, M. A. Massucci, A. La Ginestra, P. Patrono and A. A. G. Tomlinson, Muter. Res. Bull., 1987,261. 19 C. Ferragina, M. A. Massucci, A. La Ginestra, P. Patrono and A. A. G. Tomlinson, J. Chem. Soc., Dalton Trans., 1988,851. 20 N. F. Curtis, in Comprehensive Coordination Chemistry, ed. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon, London, 1987, vol. 2, p. 899. 21 E.g. C. Delmas, Y. Borthomieu, C. Faure, A. Delahaye and M. Figlarz, Solid State Zonics, 1989, 32/33, 104; T. Kobayashi, H. Kuratas and N. Uyeda, J. Phys. Chem., 1986,90,2231. 22 G. A. Ozin and S. A. Mitchell, in Inorganic Chemistry, Towards the 21st Century, ed. M. H. Chisholm, ACS, Washington, DC, 1987, p. 303. 23 R. Stomberg, I-B. Svensson and A. A. G. Tomlinson, Acta Chem. Scand., 1973,27, 1192. 24 A. Molinard and E. F. Vansant, Separation Technology, ed. E. F. Vansant, Elsevier, Amsterdam, 1994, p. 423. Paper 5/04318B; Received 4th July, 1995 J. Muter. Chem., 1996, 6(4), 645-651 651

ISSN:0959-9428    

DOI:10.1039/JM9960600645

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Computer modelling of V20,:surface structures, crystal morphology and ethene sorption Dean C. Sayle,"" David H. Gay," Andrew L. Rohl," C. Richard A. Catlow," John H. Harding,b Marc A. Perrid and Patrice Nortierc a Royal Institute of Great Britain, 21 Albemarle Street, London, UK WlX 4BS AEA Technology, B424.4 HarweEl Laboratory, Didcot, Oxfordshire, UK OX11 ORA Centre de Recherche de RhGne-Poulenc, 52 rue de la Haie Coq, 93308 Aubervilliers Cedex, France We report simulations of the surface structures and crystal morphologies of V205. The V205(OOl) surface is calculated to be the most stable, dominating the morphology. The low-energy adsorption sites of ethene on the V205(OOl), (200)and (301) surfaces are identified. The ethene molecule is observed to approach closer to the exposed ions on the (301) surface compared with those on the (001) or (200) surfaces, which is reflected in the binding energy of the ethene on the V,O5(301) surface being 44 kJ mol-' greater than on the V205(OOl) surface and 54 kJ rno1-l greater than on the V205(200) surface.Vanadium oxides have received considerable attention owing to their important role in partial oxidative catalysis.'-3 Substantial efforts are presently being made to understand these material^,^ especially their surface structure and proper- ties.' However, because of the inherent difficulty in characteris- ing mixed oxide/oxide interfaces, little is known of the detailed atomic structure and active sites on these oxides and of the reaction pathways of adsorbed molecules.In this study we employ atomistic simulation methods which are excellent tools with which to study such problems. Indeed, the development of the surface codes MIDAS6 and, more recently, MARVINY7 has led to a clearer understanding of the atomic processes that occur at surfaces and interfaces of inorganic The focus of this study is the investigation of the energetics and relaxed structures of the low index faces of V205. We predict the crystal morphology of the material based on the energies of the various crystallographic faces; our predictions can then be compared directly with experimental observations of the crystal habit. Finally we examine the low-energy sorption sites of ethene on the dominant crystallographic faces.The characterisation of the relaxed surface structures and sorption configurations of small molecules on the unsupported V205 surfaces is the first step in gaining a greater understanding of vanadia catalysts. In view of the activity and selectivity of supported V, O5monolayers as partial oxidative catalysts, we will extend our studies in future publications to consider the effect of supporting the material on Ti02 with respect to the V205 thin film surface structure. V,O, Crystal Structure The structure of V205 was reported by Bystrom et a1.l' with a refinement to the structure proposed in 1986 by Enjalbert and Galy.I2 V20, is orthorhombic with space gr9up symmetry Pmmn with u = 11.512, b = 3.564 and c = 4.368 A. V205 has a layered structure, built up from VO, square pyramids sharing edges and corners, with V205 sheets held together via weak vanadium-oxygen interactions (see Fig.1). The structure of the crystal has been successfully modelled by computational techniques as described in greater detail below. Simulation Methods and Potential Models The MARVIN program is a new code for the investigation of surfaces and interface^.^ The methodology, closely related to the earlier MIDAS code,6 considers the crystal as a stack of planes periodic in two dimensions with ions interacting via specific interatomic potentials. This stack is divided into two regions: I, where the ions are allowed to relax explicitly; and 11, where the ions are held fixed relative to each other.Region I1 is included to ensure the potential of the ions at the bottom of region I is correctly represented. The top of region I is the free surface unless two blocks are placed together, enabling the cohesive energy of the perfect crystal, grain boundary or hetero-interface to be calculated. The surfaces are created by MARVIN by cleaving the crystal at a particular crystallographic plane, which results in the formation of one of the three types of s~rfaces,'~ designated Fig. 1 (a)Diagrammatic representation of the V205 crystal structure illustrating the corner-and edge-sharing V05 square pyramids. (b) Enlargement of three square pyramids indicating the individual vanadium and oxygen species: vanadium ions are represented by filled circles, vanadyl oxygens are hatched and bridging oxygens are open circles.J. Muter. Chem., 1996,6(4), 653-660 653 types I, I1 and I11 All the surfaces in this work are created so as to fall into either type I or type I1 surface categones, where any dipole normal to the surface is quenched uzu an appropnate cleavage of the crystal surface or appropriate arrangement of surface ion species In several cases this is achieved by manipul- ating the charge distribution to form a surface where the dipole has been quenched More details of this procedure are given in the Appendix Potential Models The interionic potentials used in the present study are based on the Born model of the ionic solid, which includes a long- range Coulombic interaction and a short-range term to model the Pauli repulsions and van der Waals attractions between the ions The shell model14 describes the electronic polaris- ability of the component ions The potential parameters for V205 were taken from Dietrich et ul ,I5 whose model was able to reproduce accurately the crystal structure of the material For the ethene molecule, we use the potential parameters from the cvff frc forcefield of BIOSYM Technologies16 and potentials between ethene and V205 are from Vetrivel et all7 The potential parameters are listed in Tables 1-6 Table 1 Short-range potential energy terms between the component ions of V,O, [the analytical function is of the oform V(r)= A exp(-r/p) -0-6 with a short-range cut-off of 10 0 A], O-vanadyl oxygen, O(1)=bndging oxygen species A/eV PIA C/ev A6 v-0 2549 73 0 34115 00 v-O( 1) 5312 99 0 26797 00 0-0 22764 3 0 149 23 0 0-O( 1) 22764 3 0 149 23 0 O(1 )-O( 1 ) 22764 3 0 149 23 0 Table 2 Potential parameters of the analytical form V(r)= ~,{1-e~p[-l,(r-r,,)]}~ species DJeV AlA ro/A cut-off/A V-0 100 230217 1584 50 C-H 39201 20 1 09 14 C-C 7103 20 133 16 Table 3 Potential parameters describing the short-range potential- energy terms between ethene and V20, [the analytical functionJs of the form V(r)=Ar l2 -Br with a short-range cut-off of 100 A] species AIeV B/eV A6 O-H 1557 522 5 574 O(1)-H 1557 522 5 5740-c 15118 161 22 579 O(1)-c 15118 161 22 579 Table 4 Three-body potential parameters of the analytical form V(r)= 0 5K[O -O0l2 species KIeV Bo/degrees cut-off/A C-C-H 14657 121 2 16 13 29 Table 5 Four-body potential parameters of the analytical form V(r)= K[ 1+ Scos(Phase -Phi)] species KIeV S phase cut-off/A H-C-C-H 0 7068 -1 2 13 16 13 Table 6 Ionic charges and shell model parameters of the component ions [the analytical function is of the form V(r)= 0 5Kr2+ Klv4] species charge/e K/eVA K,/eV A ~~ 0 shell -2 717 54 952 00 O(1) shell V C -2 717 50 -02 54 952 rigid ion rigid ion 00 H 01 rigid ion Crystal Morphology For a particular face of a crystal to be important in catalysis, it must be not only active, but also exposed The degree of exposure of the various crystallographic faces in V205 can be found by calculating the crystal morphology Note in this context that surfaces which are unstable with respect to alternative crystallographic planes, although possibly active, will not be observed in the crystal morphology However, these surfaces may be selectively exposed when supported, e g TiO,, or when stabilised by surface defects, as will be investigated in future work Crystal morphologies can be calculated using either attach- ment energies or surface energies, leading to growth and equilibrium morphology predictions, respectively ’The growth morphology will be determined by the relative deposition rates of particles on the various faces of the crystal, with the slowest growing faces of greatest morphological importance These growth rates have commonly been assumed to be dependent on the binding energy of a growth slice to a particular face which crudely approximates the particle binding energy The MARVIN code allows the calculation of such attachment energies, although we should emphasise the simplicity of the approach which ignores factors such as nucleation sites (steps and edge effects), temperature, supersaturation, solvent and impurity concentration The alternative approach is to assume thermodynamic control, I e to calculate the surface energies of the various faces with the most stable surfaces dominating the morphology In order to minimise the surface energy of the crystal Indeed, Gibbs first proposed18 that the equilibrium form of a crystal should, for a given volume, possess a minimum surface free energy, z e the crystal morphology will correspond to the case where y =Z,y,A,is a minimum for constant volume y, and A,are the surface free energy (which we will approximate as the surface energy) and the surface area of the zth crystallo- graphic face Equilibrium crystal morphologies, based on surface energies, also have limitations, as crystal growth is often not an equilib- rium process In this respect we therefore present crystal morphologies based on both methods of calculation Using either approach, the shape of a crystal may be predicted by ensuring that a vector h, normal to the face is proportional to either the surface energy or the attachment energies of the crystallographic face l9 Both methods are discussed in more detail by Gay and Rohl ’ Results V, O5surfaces The first problem in calculating the growth and equilibrium crystal morphologies of V2 0, concerns the crystallographic surfaces which must be considered In practice, the attachment energies of high-index faces are very highly negative and therefore such surfaces will not feature in the crystal mor- phology Unfortunately, this is not the case for equilibrium morphologies For example, if we consider MgO(20 10), we find that the surface energy is very similar to that of MgO( 100) and must be given equal weighting in the morphological prediction However, (20 1 0) is better described as a step [with 654 J Muter Chem, 1996, 6(4),653-660 Miller index (1 lo)] on the (100) surface and therefore most of the surface will be (100) with a small proportion of the (110) surface.We therefore introduce an approximation which assumes that the surface configurations of high-index surfaces are represented, in part, by surfaces of lower Miller index. For many simple materials, the important low-Miller-index crystallographic surfaces, which are expected to feature in the morphology, may be established by inspection. However, for V205the situation is more complicated and therefore the low- index surfaces are obtained by using procedures available in the BIOSYM INSIGHT11 program,16 which reveal all the low- index crystallographic surfaces with interplanar spacing greater than a pre-determined value. It also eliminates the possibility of minimising symmetry-equivalent surfaces which, for complex materials, may not be obvious from inspection of the Miller indices.In this study, the !urfaces of V205 with interplanar spacings greater than 1.7 A were considered. Surface Miller indices quoted in this work represent the irreducible growth slice of the surface, i.e. the smallest repeat unit to facilitate the complete addition of further planes. For example, the V205(100) surface is designated the V205(200) surface, as a (100) plane is reducible to two (200) planes. Care must be exercised when comparing the quoted Miller indices in this work with other works where Bachmann's notation2' may have been used (in which b and c are permuted). For any particular surface, there may be several termination planes resulting in bulk dipole free surfaces, in which case only the termination which results in the surface with the lowest energy is considered.Of particular importance to the V205 system is the vanadyl (V=O) species which is considered to play a significant role in the catalytic behaviour of vanadium oxide compound^.^ Moreover, our results suggest that cleavage of these strong bonds results in severe destabilisation of the surface. Vanadyl bonds are not therefore cleaved on any of the surfaces considered. Table 7 gives the calculated surface energies, attachment energies and interplanar spacings for all the surfaces studied. The relaxed structures of selected low-energy surfaces are displayed diagrammatically in Plate 1 (u)-(h).A common structural feature of the surface is the tendency for the vanadyl oxygens to relax outwards which enables the V=O species to be closer to the surface normal (Table8). This feature may enhance the accessibility of the vanadyl oxygens to any reacting molecule. Furthermore, such relaxations will change the dis- tances between neighbouring vanadyl oxygens, which may be of importance to the activity and specificity of the catalyst. We note that this detailed information on surface geometry will be of great utility in future quantum mechanical studies of reaction mechanisms. The equilibrium crystal morphology, based on calculated surface energies, is given in Fig. 2, while Fig. 3 shows the growth crystal morphology which is based on the calculated attachment energies.It is clear that the (001) face dominates the crystal morphology. The observed V205crystal habit3 is a rectangular prism which exposes predominantly the (001) surface. Oshio et ~1.~'obtained atomic resolution images of a V, O,( 001) single crystal using scanning tunnelling microscopy (STM). The STM image suggests that cleavage of the V205(OOl) surface is along the weak van der Waals V-0 bonds (which hold the layers together) and therefore the surface is terminated by the vanadyl oxygen species. The surface structure is therefore identical to our proposed model of the surface structure [see Plate 1(a)]. These gratifying results sug- gest that we can have confidence in our potential model and simulation methodologies.Furthermore, the surface structure and morphology can only Table 7 Calculated surface energies (E,) and attachment energes (EA)of low-index faces of V,O, with interplanar spacing greater than 1.7 A miller plane E,, unrelaxed/J m-' E,, relaxed/J m-' EA, relaxed/J mol-' x lo6 interplanar spacing/A ~ 3 2.2 -1.3 5.69 3 2.3 -1.4 4.37 1 0.7 -0.1 33 2 26 3.3 1.2 6.5 - -6.1 -0.9 -3.9 -16.4 4.08 3.46 3 1.6 -2.0 3.40 8 1.7 -4.8 2.86 36 2.0 -6.7 26 3 5.4 1.6 -6.4 -2.6 2.76 6 11 2.2 3.2 -1.3 -6.9 2.68 8 7 5 4 2.8 2.1 2.1 1.8 -8.3 -5.8 -1.2 -6.7 2.60 2.48 1.79 Table 8 Surface vanadyl oxygen concentrations and angles (after relaxation) of the low-index faces of V,05 (an angle of 90" represents a V=O bond normal to the surface plane) miller plane V=O surface concentration/pmol m-, V= 0 surfaceldegrees V=O bulkldegrees A8"ldegrees 4.1 67 90 +23 8.2 23 0 -23 3.8 10,20 21 +lo, -1 6.5 6.4 5.4 2.6 15, 44 65, 74 20, 33 56 37 90 49 51 +22, -7 +25, +16 +28, +15 -5 2.5 43 135 +92 9.3 6.7 17, 50, 64, 72 64 90 +26 ~______ ~ "A0=angle through which the surface V=O species have relaxed.J. Muter. Chem., 1996, 6(4),653-660 655 Plate 1 Diagrammatic representations of the various surfaces of V20, after relaxation (u)V20,(OOl) surface, (b)V20,( 200) surface, (c) V20,(020) surface, (d)V,O,( 110) surface, (e) V20,( 101) surface, (f)V,O,(Oll) surface, (8)V205( 111) surface, (h)v20,(301) surface be reproduced after the appropriate ‘manipulation’ (detailed Sorption of ethene on the V, 0, surfaces in the Appendix) of surface ions A simple cleavage of the surface will not give the experimentally observed surface A study of the adsorption of molecules on the relaxed V,O, structure and crystal morphology In this respect, we have surfaces is a first step in understanding the oxidative reaction direct evidence (we believe for the first time) that such a of vanadium oxide catalysts For example, the location of manipulation of the surface structure, to remove the surface such adsorption sites can, as noted, be used as starting dipole, has a physical basis and is not purely a requirement of configurations for quantum mechanical studies of the mechan- our calculations isms in partial oxidative reactions Location of low-energy 656 J Muter Chem, 1996, 6(4), 653-660 Fig.2 Predicted crystal morphology of V205 using the relaxed surface energies (equilibrium morphology). Three projections of the crystal habit are shown. Fig. 3 Predicted crystal morphology of V205 using the calculated attachment energies (growth morphology). Three projections of the crystal habit are shown. sorption sites presents a formidable undertaking for quantum mechanical techniques. The atomistic simulation approach is much less computationally expensive, and is therefore the natural starting point for any study of sorption and reaction. Considering the limitations of this present model (primarily resulting from the formal charges assigned to the individual atoms of V205) the resulting ‘absolute’ adsorption energies calculated in this work must be treated with some caution.However, we are primarily interested in the relative sorption energies of ethene on various V205 surfaces. In this instance the limitations of the potential model will largely cancel when a comparison between different surfaces is made, and the sorption behaviour of the ethene molecule will reliably reflect the essential differences between the potential field environment of each individual V205 surface. To calculate the adsorption sites of ethene on all the surfaces of the V,O, surface would be prohibitive. We therefore have to be selective in the surfaces chosen.Our main criterion must be the surface exposure of the various crystallographic planes and therefore, as the (001) and (200) surfaces feature in both the growth and equilibrium morphologies these surfaces are considered. We also examined the (301) surface, which has a high concentration of V=O species which have been linked to the catalytic activity1*’ of the material. Furthermore, these V=O species appear to be easily accessible to reactant mol- ecules [Plate l (h)]. To establish a low-energy adsorption site, the ethene mol- ecule was placed at various positions on each of the three surfaces and the system relaxed until zero force acted on each of the component ions. Several starting configurations were considered to ensure that the lowest adsorption-energy mini- mum has been located, from which we calculated the adsorp- tion energy (defined as the energy difference between the ethene molecule docked on the V205 surface compared with the perfect V205 surface and the ethene molecule at infinity).The adsorption energies, together with the ethene-V, O5 bond distances for the energy minima, are given in Table 9. Plates 2, 3 and 4 show the relaxed configurations of the ethene adsorbed on the surfaces of the (Ool), (301) and (200) surfaces, respectively. Table9 shows that both the adsorption energies and ethene-V,O, bond distances for ethene adsorbed on the (001) and (200) surfaces are very similar, suggesting that the ethene molecule is in a very similar environment or ‘sorption site’, which is perhaps surprising when one considers the structural differences between the two surfaces.Table9 also shows that the ethene molecule, when adsorbed on the (301) surface, is significantly closer to the surface than for configurations OF the (001) or (200) surfaces: in particu!ar, hydrogen is 0.13 A closer to the vanadyl oxygen and 0.70 A closer to the bridging oxygen for ethene on the (301) surface compared with the nearest distances for ethene on the (001) surface. That the molecule is able to approach closer to the V205 surface is reflected in the adsorption energies which are Plate 2 Ball-and-stick representation of ethene on the V205(OOl ) surface. Three views are displayed; the top left is a view looking down on the V205 surface.Only one ‘layer’ of the V205 surface is included for clarity. Vanadium atoms are yellow, vanadyl oxygens are red, oxygens are orange, carbons are green and hydrogens are white. Table 9 Adsorption energies and hydrogen-oxygen bond distances of ethene on the (200), (001) and (301) surfaces of V205 H-0 bond lengths/A V, 0,(200) surface V205(OOl) surface V2O5(3O1) surface v=o v-0-v v=o v-0-v v=o v-0-v ~ ~ ~ ~ ~~ ~ ~~ ~ ~ ~ ~ ~ ~ ~ ~ 2.65 3.40 2.65 3.40 2.68 3.25 2.68 3.25 2.58 3.14, 3.50 2.60 3.30, 3.34 2.60 3.30, 3.34 2.57 3.15, 3.50 2.44 2.50 2.44 2.50 -2.44 -2.44 Adsorption energy/kJ mol-’ 23 33 77 J. Muter. Chem., 1996, 6(4), 653-660 657 Plate 3 (a)Ball-and-stick representation of ethene on the V205(301) surface.(b) Space-filling representation of ethene on the V20,( 301) surface. (Colours as for Plate 2.) Plate4 Ball-and-stick representation of ethene on the V205(200) surface (colours as for Plate 2) ca. 50 kJ mol-I higher on the (301) compared with the (001) or (200)surfaces. Conclusions We have shown the crystal morphology of V,O, to be com- pletely dominated by the (001) surface. The predicted crystal morphology and surface structure are consistent with exper- imental observations. We have also found that the adsorption behaviour of ethene on the (001) and (200)surfaces are similar with respect to hydrogen-oxygen bond lengths and adsorption energy. However, for ethene adsorbed on the (301) surface substantial differences exist; the ethene molecule is observed to approach closer to the (301) surface compared with the (001)or (200) surface ions, which is reflected in higher binding energies to the former surface.Subsequent studies will explore the supported V, 0,-TiO, catalyst systems and will apply quantum mechanical techniques 658 J. Muter. Chem., 1996, 6(4), 653-660 to investigate the reaction mechanisms of the sorbed configur- ations identified in this study. We thank Rh6ne-Poulenc for financing this project and BIOSYM Technology Inc. for the provision of the INSIGHT11 software used in the molecular graphics representations of the surfaces and crystal morphologies. We would also like to thank Dr. D. J. Ilett for help in constructing the datasets used in this study.Appendix Here we describe the calculation of surface energies for dipolar surfaces. It is well known that the Ewald summation is only conditionally convergent. In standard applications the bound- ary conditions are automatically chosen so that this is not apparent. However, in the two-dimensional summations required for surfaces, the problem is unavoidable. A notorious example occurs when the repeating unit of a stack of planes has a finite dipole moment perpendicular to the surface. In such a case the Ewald summation diverges and the energy of the surface is infinite. This was first pointed out by Kummer and Yao22 for the particular case of the (111) surface of the alkali-metal halides.More extended discussions are given by Tasker13 and Berta~t.,~ The problem arises because, since each addition of a repeat- ing unit to the stack adds to the dipole moment, the moment of the stack increases without limit. The point may be illus- trated by approximating the crystal planes as charged sheets. The electrostatic potential of a plane with charge density 0= QIA at a distance Z is given by VZ) =2nQz/A. If the spacing between the planes is a, the potential of a pair of planes of opposite charge is V(z)= 2naQ/A; this is independent of z. It is still possible to calculate a surface energy for a polar stack provided that the dipole moment is quenched. We take first the simple case considered by Kummer and Yao.,, We consider a stack of planes where the interplanar spacings are all equal and the charge densities are fa on alternative planes.We construct a new stack where the top and bottom planes have charge density a/2 (i.e. we remove half the charge density from the top and put it on the bottom). This gives us a stack as shown in Fig. 4. This new construction has a zero dipole moment. It is, however, necessary to add an extra term to the energy. This is an interaction between the (now charged) outer regions: nQ2uE2, = -2A where is the distance between the innermost edges of the outer blocks. This is the case for the V205(OOl) surface; in order to quench the dipole, half the vanadyl oxygen species are removed from the V205(OOl) surface to the bottom of region 11.The weak van der Waals interactions between the vanadium ions and vanadyl oxygen species that hold the V205 layers together are broken, rather than the strong vanadyl (V=O) bonds. The interactions between ions on the outer regions (region 11) are automatically included in the MARVIN code. The result is the same if the two blocks have different Q/2 -Q Q Q -Q QQ Fig. 4 Schematic representation of the alkali-metal halide (111)surface where the dipole has been quenched by removing half the charge density from the top and placing it on the bottom. 0’ -Q Q -Q Q Q -Q Q -Q Q“ Fig. 5 Schematic representation of the wurtzite surface where the dipole has been quenched via charge manipulation. -V--400-+4 -V-4.0--4 -V--4001 4 -v--4-01-4 -V-400-+4 -V-4-0--4 -V-4001 +44-V--0--2 --v-400-+1 -V--4 -V----ooo-+4-0--v-49-4 -V--000-+4 -V--09-4 -V-400-+4 -V--2 Fig.6 Representation of the V,O,(OOl) surface before (a)and after (b) the dipole has been quenched. To the right of each figure a schematic of the ionic species and net charge of each plane is shown. The filled small circles represent vanadium and the hollow circles represent oxygen. The hatched circles are the vanadyl species which are removed from the top of the V,O,(OOl) surface and replaced at the bottom to quench the surface dipole. interplanar spacings [e.g. if we considered the ( 111) surface of NaCl joined to the (111) surface of KCl]. Matters are more complicated if all the interplanar spacings in a block are not the same.This is a little more unusual than one might think; many common crystal structures correspond to the simple case. An example that does not is the wurtzite structure. This case is shown in Fig. 5. In this case, as shown by Duffy and Ta~ker,~~ the charges on the two ends of the stack are not equal. They are given by Q’ = a2Q/(al + a2) and Q” = Q -Q’ = a, Q/(al + a2).The interaction between the outer blocks is now: 2nQ2alu2E-22 -A(u, + LI~)~ (2) The most complex case one is likely to encounter in practice is the case where two different materials, each with two interplanar spacings, are joined together. Here the charges on each end are: Q’ = rQ and Q” = Q -Q’, where and the interaction energy between the blocks is It is obviously possible to think of more complex cases, the most obvious being where charges on the planes for each block are different.These present no new questions of principle, merely increasing algebraic complexity. We now illustrate this methodology by considering the V20,(001) surface, which is shown graphically in Fig. 6, which also shows a schematic of the ionic species and net charge of each plane. This surface has equal interplanar spacings and charge density +o on alternate planes (if we consider the vanadium and bridging oxygens to comprise one plane and the vanadyl oxygens, the other). Clearly, the surface repeat unit will add to the dipole moment, resulting in an undefined surface energy.This surface is therefore classified as type I11 and must be modified, by quenching the dipole via charge manipulation, to calculate the surface energy. In practice this is achieved by removing half the vanadyl oxygens from the top and replacing them at the bottom (Fig. 6). The new charge density at the top and bottom is now -o/2 and the system has no dipole normal to the surface. As the charge density was modified by the manipulation of ionic species, the extra term in the energy [eqn. (1)] is implicitly calculated in the MARVIN code. This type of procedure is repeated for all the type I11 surfaces considered in this work to quench the dipole. References 1 G. Centi, S. Perathoner and F. Trifiro, Research on Chemical Intermediates, 1991, 15,49.2 A. Vejux and P. Courtine, J. Solid State Chem., 1978,23,93. 3 A. Vejux and P. Courtine, J. Solid State Chem., 1986,63, 179. 4 G. Centi, F. Trifiro, J. R. Ebner and V. M. Franchetti, Chern. Reu., 1988,88, 55. 5 A. Satsuma, A. Furuta, T. Hattori and Y. Murakami, J. Phys. Chem., 1991,95,3248. 6 P. W. Tasker, Harwell Report, AERE-R9130,1978. 7 D. H. Gay and A. L. Rohl, J. Chem. Soc., Faraday Trans, 1995, 91,925. 8 D. M. Duffy, J. Phys. C: Solid State Phys., 1986,17,4393. 9 J. H. Harding, Rep. Prog. Phys., 1990,53, 1403. 10 D. C. Sayle, T. X. T. Sayle, S. C. Parker, C. R. A. Catlow and J. H. Harding, Phys. Rev., 1994,50, 14498. 11 A. Bystrom, K-A. Wilhelmi and 0. Brotzen, Acta Chem. Scand., 1950,4, 1119. 12 R. Enjalbert and J. Galy, Acta Crystallogr., Sect. C, 1986,42, 1467. 13 P. W. Tasker, J. Phys. C: Solid State Phys., 1979,12,4977. 14 B. G. Dick and A. W. Overhauser, Phys. Rev., 1958,112,90. J. Mater. Chem., 1996, 6(4), 653-660 659 15 A Dietrich, C R A Catlow and B Maigret, Molecular Simulation, 1993,11,251 21 1993,7,33 T Oshio, Y Sakai, T Moriya and S Ehara, Scanning Microscopy, 16 17 Biosym/MSI Technologes, 9685 Scranton Road, San Diego, USA R Vetrivel, C R A Catlow and E A Colbourn, J Chem SOC, 23 22 F Bertaut, Compt Rend, 1958,246,3447 J T Kummer and Y Y Yao, Can J Chem , 1967,45421 18 Faraday Trans 2,1989,85,497 J W Gibbs, Collected Works, Longman, New York, 1928 24 D M Duffy and P W Tasker, J Appl Phys, 1984,56,971 19 20 G Wulff, 2,Krzstallogr ,1901,34,449 H G Bachmann, F R Ahmed and W H Barnes, Z Kristallogr , Paper 5/04556H Received 11 th July, 1995 1961,115,110 660 J Muter Chern, 1996, 6(4), 653-660

ISSN:0959-9428    

DOI:10.1039/JM9960600653

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Ordering and manipulation of MoS, platelets on differently charged micas by atomic force microscopy Suzanne Mulley," Angelo Sironi," Adriana De Stefanisb and Anthony A. G. Tomlinson*b "Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universitci degli Studi di Milano, Via G. Venezian, 21, 20133 Milan, Italy bIstituto di Chimica dei Materiali, Area della Ricerca di Roma del C.N.R., C.P. 10 Monterotondo Staz., 00016 Rome, Italy Atomic force microscopy (AFM) has been used to investigate how dispersed MoS, platelets (from colloidal suspensions) deposit onto three natural (Muscovite) mica (001) surfaces. Different platelet arrangements are observed, which are attributed to defects and charging effects of the mica (as well as the concentration of starting colloid).The provenance of the mica influences the self- organisation of the platelets into long tape-like assemblies (South Dakota mica) or individual flakes (Alps mica). Atomic scale imaging of the tapes reveals a distorted octahedral (Oh)-based local structure, different from the trigonal prismatic structure found in the 2H-polytype of annealed MoS,, in agreement with previous structural results on water-dispersed MoS, platelets. The buckling and susceptibility to stripping of the tapes is ascribed to the presence of a water layer between the substrate mica and MoS,, and after stripping by the tip, the platelets ultimately form small clusters. The ordering of these clusters depends not only on the defect and charge structure of the mica, but also on complex hydration reactions between the H,O layer associated with the MoS, and K+ ions of the mica.Relatively symmetrical squares may be lifted out of the tapes, supporting the presence of weak bonding between tape and mica. Conversely, Alps and Bihar micas give rise to separate platelets, which are resistent to tip manipulation, which is attributed to hydrophobic interaction between MoS, platelets and the mica surface. Much recent work on the interaction between AFM tips and surfaces has shown that it is possible not only to probe assembly, topology and atomic structure' but also to manipu- late a surface nanometrically (dig holes in it, remove specific areas, and even cut a line into a substrate to form a nano- gate)., During work on the assembly of MoS, platelets from colloidal suspensions on quartz, semiconductor and (001 ) mica surface^,^ it was found that they adopted tape-like assemblies on the mica, presumably directed by the charged surface of the substrate mica.We have now found that such tapes are sensitive to manipulation by the AFM tips (a phenomenon not previously observed) and also to the type of mica. In the interim, AFM investigation has revealed the atomic structure variations on mica surfaces with differing cation exchange and ~alcination.~We here describe the strong influence that charges on mica have on the ordering and morphology of small MoS, platelets and the resultant surface topographies produced by their interactions with AFM tips. Experimental As reported previ~usly,~ the platelets were obtained by simple casting of colloidal MoS,, prepared by 'blowing apart' Li,MoS, via forced hydration under argon.' A drop of colloid solution was placed on freshly cleaved mica and allowed to dry, and film purity (i.e.absence of Li') was assessed by optical spectro~copy.~ The samples of mica used as substrates originated from three different localities: South Dakota (ident- ical to that used previously3) the Alps, and a Ruby mica from Bihar, India. AFM measurements were carried out in air using a commer- cial microscope (Digital Instruments, Nanoscope 111).Images were made by scanning the samples with Si3N, cantilevers having integrated oxide sharpened pyramidal tips with spring constants of 0.12 and 0.58 N m-l.The microscope was oper- ated in contact mode and the contact forces between the tip and the surface were calculated to be no greater that 30nN (in many cases much less). A scanner with maximum oper- ational area 15 x 15 pm2 was used for both topological and zoomed scans. Samples were investigated both immediately after preparation and a few days later. Energy-dispersive analysis by X-rays (EDAX) studies were carried out on a Hitachi S2400 scanning electron microscope (SEM) fitted with a Kevex EDAX analyser. All three mica substrates were found to have identical chemical compositions, and no Mg2+ or other cations were detected. Results and Discussion South Dakota mica Of the three mica substrates investigated, only that from South Dakota produced the distinct tape-like features shown in Plate 1.The tapes, evidently formed from ordered MoS, platelets, were found to have a very uniform thickness (ca. 3.5-4.0 nm) with widths typically around 500-600 nm (although occasionally up to 2 or 3 pm) and lengths ranging from 3 to 10 pm. Some structures (reminiscent of the ribbon- like features recently observed for linear chalcogenides6) on the top of these tapes provided evidence that they are composite in nature. This was further verified by the existence of one or two particularly flat ribbons, such as that shown in Plate l(b), in which holes corresponding to 'missing' platelets and bumps corresponding to stacked platelets, are clearly visible.The average thickness of these flat tapes was found to be ca. 0.65 nm which is in keeping with the thickness of a single layer of MoS, (0.615 nm). Despite zooming in on a large number of wide-scale images, joins between platelets were not observed, although smaller groups of platelets, such as those shown in Plate 2(a), did give a hint of their agglomerating nature. When the same colloidal suspension was diluted tenfold, a different dispersion on this mica was found; platelets grouped together to form flat discs up to 1.0 pm across [see Plate 2(b)]. We originally attributed this type of dispersion to individual drops of colloid solution breaking up into many tiny micro- droplets during sample preparation caused by the charged surface of the mica ~ubstrate.~ Another, perhaps more plausible, J.Mater. Chem., 1996, 6(4), 661-666 661 Plate 1 (a) Large scale micrograph of MoS, colloidal platelets on (001) mica (image 5.0 x 5.0 pm). Note the tape-like development. (b)Thin ribbon (average height ca. 0.65nm, i.e. corresponding to single platelet thickness) running diagonally from top left to bottom right of image showing holes and bumps corresponding to missing and extra platelets, respectively (image 2.5 x 2.5 pm) Average height of surrounding ribbons is 3.5-4.0 nm. Plate 2 (a) Small scan of MoS, tape showing joins between agglomerated platelets (image 820 x 820 nm). (b)Dilute MoS, platelet dispersion aggregating at point defects on the mica surface (image 7.0 x 7.0 pm).The roughly circular features have uniform thickness of ca. 1.3 nm and are surrounded by a thinner halo; the centres of the circular features rise into small points ca. 10 nm in height. (c) Profile showing the ‘flatness’ and height of a typical flat disc of MoS, platelets. 662 J. Muter. Chem., 1996, 6(4), 661-666 Plate 3 Atomic scale images of MoS, tape. (a) Raw data (10 x 10 nm); (b)close-up (2.56 x 2.56 nm) of the same, filtered image. The image was filtered in two different ways; one by choosing appropriate masks to cover all the points in the power spectrum, the second by choosing two vectors in reciprocal space and fitting them via an iterative method to all the points in the power spectrum (36 significant values). The two methods produced virtually identical images, the apparent extra structure was therefore deemed to be real.explanation could be that the mica surface contains many point defects which act as seeding centres for agglomerating platelets. Nevertheless, the platelet thickness was again found to be very uniform even in this dispersion: ca. 1.3-1.5 nm in general, although when small groups of only two or three platelets were observed, they have a minimum thickness of ca. 0.6-0.7 nm. These changes imply directing of the platelets by the mica su~face,~and evidence to support this suggestion comes from atomic scale images of very thin parts of the tape (taken at low contact forces before tape break-up occurred). Plate 3 shows a Fourier filtered image of the surface, which appears to be corrugated with an almost 2a0 x 2a0 unit cell (ao is the lattice spacing of the 2H-MoS2 structure found on annealed plat$ets3).Analysis gives S-S bond lengths of ca. 2.5 and 3.7 A, in keeping with a distorted pseudo-octahedral based local structure and in good agreement with those found in XRPD and EXAFS studies for the MoS, platelets dispersed in water.’ Such a distorted structure is thought to arise from the presence of water bilayers associated with the MoS, layer, although the corrugation found previously is smaller than that found here, and a 2u, x 1 unit cell was deduced for the metastable MoS, layer.6 We suggest that both effects are caused by coordination of the layer-associated water to K + ions exchanged on the mica surface.The MoS, tapes on this mica were found to be exceedingly sensitive to the AFM tip, persistent scanning of a particular area generally resulting in the complete break-up of the material. Plate 4 shows a consecutive sequence of images of a single tape repeatedly swept by the tip (at an applied force of ca. 30 nN). The tape structure breaks up, initially into strips, most of the stripped material being swept to the sides of the area scanned [see Plate4(f)]. Close inspection of the final swept area shows that small, almost circular, ‘nanohenges’ are present Plate 4(d),(e)].The diameter of a henge is typically ca. 15 nm with each component ‘stone’ having an average height of 0.7 nm and comparable width. Although atomic-level analy- sis was not feasible, each ‘stone’ feature is suggestive of the presence of a cluster (There are no recorded instances of tips giving rise to such features on mica itself and removal of MoS, from a 2H-MoS2 crystal surface via STM rastering gives initially triangular features and then removal of a layer.)8 We speculate that the ‘stones’ may be Mo-S clusters (which are legion)’ and, significantly their heights are close to those of the most ubiquitous and stable Mo-S building block, [Mo~S,,]~-,which in bulk, complex inorganic chemistry form readily in the presence of K+ ions in aqueous solution.” Similar topologies were found after repeated ribbon removal over the sample surface, and in no case were other types of features (curled up, nanotubules, etc.) observed.This suggests that some platelet-surface interactions remain throughout the process. Also visible in Plate 4(e)is what appears to be a step with a height of 0.25-0.3 nm. This is much less than that of a single mica layer (ca. 1.0 nm) so it cannot be attributed to a growth plane of the substrate material. It also cannot be attributed to cations other than K+ (e.g. Mg2+) which would give rise to a step generated by the difference in cationic radii. The step appears exactly where one of the ribbon edges was, indicating that it plays an important role in the original tape growth in that particular area, which also provides clues about the underlying surface chemistry in operation. We recall that: (i) Hartmann et a!.” could not locate interlayer cations in 2 :1 clay minerals (e.g.montmorillonite, illite, having a basal struc- ture analogous to micas); (ii) according to Nishimura et ~l.,~ a similar observation for K+-exchanged Ruby mica under water can be ascribed to hydration, ‘keying-out’ of the K+ from the hexagonal hole and removal by tip (unlike Lif and Mg2+, K’ does not irreversibly ‘fix’ into the hexagonal 0-cavity of mica’,); (iii) in water, K+ ions readily react with the S atoms of MoS, (giving MoS2).l3 This suggests a simple model for the interaction between MoS, platelets and South Dakota mica. Hydrated platelets dock with hydration of the surface mica K +,the tip then ‘keys out’ (now larger) hydrated Kf ions, at the same time breaking J.Muter. Chem., 1996, 6(4),661-666 663 Plate 4 (a),(b),(c): Consecutive scans over a section of tape illustrating progressive destruction (image 2.7 x 2.7 pm). Note the area in the top right hand corner of the images; initially this appears to be one of the thin ribbons, whilst progressive sweeping of the area leaves a number of clusters (‘henges’). (d) Area left after sweeping clean with AFM tip (image 520 x 520 nm). The clusters appear to be sitting in a shallow dip in the mica (step height measured as ca. 0.3 nm). (e) Close-up showing step in mica surface and henges (image 200 x 220 nm). (f)Large scale scan after sweeping operation (image 15.0x 15.0 pm). The central part of the image clearly shows tape material has been deposited at the sides of the smaller scan area during the sweeping.-b Scheme 1 Suggested mechanism of removal of material by the tip, to leave an area of keyed-out hydrated K+ ions on the mica basal plane. weak axial Mo...S bonds of the metastable Oh-MoS2, as shown in Scheme 1. Reaction then occurs between hydrated K+ and [ MoS,]” fragments. Although speculative (AFM provides no chemical analytical information), the model fits the results (and is also in agreement with the completely flat 2H-MoS2 observed after ~alcination,~ which does not undergo any of these processes). In addition, and more convincingly, this tip/platelet/mica reaction can be used to systematically lift out pieces, as shown in Plate 5. This operation was accomplished by zooming in to 20 nm scale from 200 nm scale, applying a force to the tip and then zooming back.The square left, of dimensions ca. 50 x 50 nm, is presumably owing to a platelet having been removed, facilitated by the only weak attachment to the mica. There is a pronounced ‘tail’ to the hole left by platelet removal shown in Plate 5. This is suggestive of the tip point of attachment pressing down the lower platelet, making it adhere more strongly to the surface, and in turn being left behind after lifting*Note that an AFM tip taking Plate 5 Small Scan ca. 50 x 50 nm produces a hole in the tape (image Part in a chemical reaction, (hydrogen-bond breakage) has 150 x 150 nm). Note that the hole is not cubic, indicating that the hole recently been suggested.14 is formed uia removal of more than one platelet.664 J. Mater. Chem., 1996, 6(4), 661-666 Alps and Bihar micas Although substrate-directing effects were observed on the other two mica samples, compact tapes were not generated. On Alps mica, the platelets agglomerated to form flakes up to several microns in diameter (Plate 6) a morphology comparable to that described previ~usly,~ except that here the single platelets combining to form each flake are clearly distinguishable. Minimum platelet thicknesses measured lie in the range 1.2-1.5 nm. Platelets in this type of assembly were much more stable to tip manipulation than the tapes, indeed, repeated scanning over small areas and increasing the loading on the tip caused no visible damage to the material.In contrast, Bihar mica yielded a platelet distribution some- where between the other two micas. Here the platelets aggre- gated on the surface of the substrate in long bands similar to the tape dispersion, but the joins between single platelets could still be distinguished (Plate 7). Moreover, distinct layers of platelets (minimum thickness ca. 0.7 nm) are easily discernible, confirming our previous suggestion that MoS, platelets deposit onto a mica surface via a side-on parking mechanism. Again, the platelet dispersion was much less sensitive to tip manipulations. Plate6 Flake morphology of MoS, platelets on Alps mica (image 90Ox900nm). Each single plate is easily identified. Brighter areas indicate at least two platelets stacked one uuon the other.Plate7 MoSz platelets deposited in layers on Bihar mica (image 2.0 x 2.0 pm). The difference between platelet distributions on the different micas is not obvious; -all three show the same chemical composition (from EDAX measurements) and have the authi- genic 1M polytype structure.14 Hence, invoking differences in the ordering of K+ ions between the top and bottom surfaces of the cleave owing to inherent non-commensurability is unconvincing. Further, although having a directional effect on platelet deposition [Plates 2(b) and 61, surface defect phen- omena alone do not appear to be responsible. Also, the relative closeness of platelet approach seems to be determined by the particular mica substrate rather than the colloid preparation; platelets from the same MoS, preparation aggregate closer on Bihar mica than on Alps mica.(Preliminary results on a saponite clay, having platelets of dimensions similar to those of MoS,, show that on South Dakota mica they form the same compact ribbon-type films as found with MoS,.”). Extreme tip sensitivity of the tapes on South Dakota mica implies less strongly attached platelets (both to each other and to mica). In turn, reduced tip sensitivity to damage in Alps and Bihar mica implies that MoS, platelets dock without an associated water layer. Presumably, differences in water organ- isation (mono-, bilayer, etc.), which in turn depend on changes in interstratification immediately before MoS, docking, are responsible for the more ‘hydrophobic’ tip response.This would explain the different minimum measured platelet heights of ca. 0.7 and 1.4 nm on these two micas. It is notoriously difficult to follow the water involved in interstrati-fication ordering,16 and further work is underway on modified platelets. Conclusions We have shown that the provenance of mica used as a ‘flat support’ for AFM may, in fact, condition a complex micro aqueous chemistry, which for MoS, may be utilised to cut and manipulate platelet pieces at the nm-scale. However, the platelet ordering immediately before deposition has such a large influence on surface docking that it is unlikely that tip manipulation on water-containing species can lead to rational nano-engineering.we thank Dr. Moret for his s* is grateful to the HCM programme for a Fellowship and A.A.G.T. the BRITE-Euram programme of the E. C. (contract No. BRE2-CT93-0450) for their continuing financial support. References 1 G. Binnig, Ultramicroscopy, 1992, 42-44, 7; S. N. Magonov, Appl. Spectrosc. Rev., 1993,28, 1. 2 T. Schimmel, B. Winzer, R. Kemnitzer, T. Koch, J. Kuppers, M. Schoerer, C. M. Lieber and Y. Kim, Adv. Muter., 1993, 5, 392; E. Gardner, Science, 1994,206,543. 3 S. Foglia, A. A. G. Tomlinson, A. Sironi and S. Mulley, J. Muter. Chem., 1995,5,1191. 4 S. Nishimura, S. Biggs, P. J. Scales, T. W. Healy, K. Tsuematsu and T. Tateyama, Lungmuir, 1994, 10,4554. 5 P. Joensen, R. F. Frindt and S. R. Morrison, Muter.Res. Bull., 1986, 13, 487. 6 W. Liang, M. H. Whangbo, M. Evain, L. Monconduit, R. Brec, H. Bengel, H-J. Cantow and S. N. Magonov, Chem. Muter., 1994, 6, 678. 7 L. D. Yang, S. Jimenez-Sandoval, W. M. R. Divigalpitaya, J. C. Irwin and R. F. Frindt, Phys. Reu. B, 1991,43,1200. 8 B. Parkinson, in Suprumoleculur Architecture, ed. T. M. Bein, ACS Symp. Ser., vol. 499, Washington, 1992, p. 76; B. Parkinson, J. Am. Chem. SOC., 1990,112,7498. J. Muter. Chem., 1996, 6(4), 661-666 665 9 Eg D Coucouvanis, A Tompakis, S-M Koo and 13 J Rouxel, in Supramolecular Architecture, ed T M Bein, ACS A Hadjikyriacou, Polyhedron, 1989, 8, 1705 and refs therein, G-X Lin, A Muller and M Penk, 2 Naturforsch, B Chem Sci , 14 Symp Ser ,vol 499, Washington, 1992, p 88 G Bar, B Scott, S R Johnson, B I Swanson, J Ren and 1991,46,25 and refs therein M-H Whangbo, Chem Muter, 1995,7,391 10 W Clegg, N Mohan, A Muller, W Rittner and G M Sheldrick, 15 A De Stefanis, S Mulley and A A G Tomlinson, Clays Clay 11 Inorg Chem , 1980,19,2066 H Hartmann, G Sposito, A Young, S Manne, S A C Gould and P K Hansma, Clay Clay Miner, 1990,38,337 16 Miner, 1996, submitted Eg P H Nadeau, M J Wilson, W J Hardy and J M Tait, Science, 1984,225,923 12 G W Brindley and G M Brown, Crystal Structures of Clay Minerals and their X-ray IdentlJication, The Mineralogical Society, London, 1980, N Guven, 2 Kristallogr ,1971, 134, 196 Paper 5/04313A, Received 4th July, 1995 666 J Muter Chem, 1996, 6(4),661-666

ISSN:0959-9428    

DOI:10.1039/JM9960600661

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATIONS Supermolecular alignment in a liquid crystal-polymer gel as studied optically and by dielectric relaxation spectroscopy Monica M. Marugan, Sara Shinton and Graham Williams* Department of Chemistry, University of Wales, Swansea, Singleton Park, Swansea, UK SA2 8PP Macroscopically aligned liquid crystal (LC) gels have been prepared by photopolymerization in visible light of a non- mesogenic bifunctional monomer in a conventional LC mixture that was prealigned in an electric field. A high degree of molecular orientation is achieved in the gel, and it is stable with time, as shown by polarized optical microscopy and broad-band dielectric relaxation spectroscopy. In recent years it has been shown that macroscopically aligned liquid crystal/polymer networks may be formed by radical- initiated photopolymerization of mono-and bi-functional monomers dissolved in a liquid crystal.Polymerization leads to the formation of a network in which the branched chains or their aggregates form a continuous network dispersed in the smectic, nematic or chiral nematic LC medium. The polymerization process in an LC medium is guided spatially by the long range ordering of molecules of the mesophase. Once formed the network has a set three-dimensional topogra- phy but this may, in principle, be distorted by application of external fields as a result of the orientation and flow of the LC phase or by application of a shearing force. Since an LC-monomer medium may be aligned with an external field or by surface forces it is thus possible to obtain macroscopically aligned LC-polymer networks, also known as anisotropic LC gels. Hikmet and co-worker~’-~ have used bifunctional mon- omers that are themselves liquid crystals in their bulk phase and have reported the preparation of anisotropic LC gels over the entire composition range, and have made extensive investi- gations of their optical and electrooptical properties.They have shown that these novel hybrid materials show promise as new media for optical data storage and for active and passive optical elements for displays and optical processing (see refs. 1-5 and refs. therein). It has been shown recently that non-mesogenic monomers may be polymerized in an aligned nematic6 or smectic7 LC-monomer phase, to produce macro- scopically aligned LC gels.In the present work we describe the preparation of highly aligned LC gels formed by the polymerization of a non-mesogenic dimethacrylate monomer 1 in a conventional LC mixture E7 (Licryllite, Mixture E7 from Merck Ltd.). 1 Monomer 1 was obtained from Akzo Ltd. It is a feature of the present work that monomer 1 is non-mesogenic, in contrast with the pioneering work of Hikmet who used specially synthesized mesogenic monomers in conjunction with conven- tional LC mixtures to prepare aligned gels. We demonstrate that such a non-mesogenic monomer forms a strain-free gel that conforms with the LC phase, thus when a directing E-field is removed the gel retains its macroscopic alignment since this is held by the overall gel structure.A second and important practical feature of our work is that the photopolymerization of 1 at a concentration of 5% (w/w) in E7 was carried out in visible blue light (470 nm) in contrast to earlier works which used UV light. Thus LC-monomer mixtures contained in indium tin oxide (ITO) glass cells mounted in a microscope hot-stage with glass windows could be readily photopolymer- ized in situ, which is not possible using UV light. Since LC-polymer gels show considerable promise as materials for optical data storage and for optical elements, the practical advantages of using visible light for photopolymerization should be recognised (e.g. ordinary glass is used here in place of quartz, we use an inexpensive commercial visible light source used for dental cements, and we do not have the hazards associated with the use of UV light). Samples were prepared by placing the LC-monomer mix-ture (95:5 w/w) in an ITO-coated glass sandwich cell (1cm x 1cm x 20 pm) kept apart using Kapton spacers.The IT0 cell was mounted in a programmable hot-stage (Linkam TMS600) attached to an Olympus BH2 polarizing microscope. Prealignment of the LC-monomer mixture was achieved using ac or dc voltages applied to the IT0 cell. The optical properties of the LC-monomer, and of the LC-polymer gel subsequently made, were monitored using the polarizing microscope. A further feature of the work reported here is our use of dielectric relaxation spectroscopy (DRS) for the in situ monitor-ing of the nature and extent of macroscopic alignment in the unpolymerized and polymerized materials.A GenRad 1689-9620 DigiBridge ( 10-105 Hz), with associated computer control and software developed in-house, was used to obtain the dielectric permittivity and loss data. While optical microscopy provides us with qualitative information on the alignment of LC-monomer and LC-polymer gel samples, the dielectric data presented here provide direct unambiguous quantitative information of the extent of alignment of such samples (1cm2 x 20 pm) in the IT0 cells. Note that such information is not easy to obtain by other methods. For example, FTIR measurements are inappropriate owing to the opacity of the glass electrodes to IR light.NMR spectroscopy normally requires sophisticated instrumentation and a larger sample than that under investigation here and, importantly, uses high magnetic fields that will induce further director orientation in an LC sample so that it is not studied in its field-free state, unlike the DRS studies reported here. For an LC-monomer sample that was not deliberately prealigned using external voltages, the material appeared trans- parent in the microscope, but on illumination as the gel formed a turbid strongly scattering texture was immediately observed. The origins of the scattering are a combination of two factors: (i) a nanophase scale separation of network polymer may occur, as discussed by Braun et a1.* leading to scattering from J.Muter. Chem., 1996, 6(4), 667-669 667 the polymer network itself, and (ii) the network acts as a large internal surface at which the LC molecules interact and may be leading to strong scattering from the anisotropic LC polydomain itself. A further LC-monomer sample was aligned homeotropically using 30 V (1 kHz) and illumination applied for 10 min [using an Elipar-2 Dental Lamp (A,,, = 470 nm)] and maintained for a further 15 min after illumination to stabilize the alignment of the LC-polymer film. Plate l(a) shows a microscope picture of the resultant gel sample which includes the edge between the region to which the field was applied during gel formation (right-hand side) and that where no field was applied (left-hand side).The scattering texture of the nominally unaligned region is seen while the excellent transparency of the aligned region is apparent (appears black between crossed polarizers). Plate 1(b) shows the conoscopic image obtained for the transparent, homeotropically aligned region of this sample. A Maltese cross is seen, indicating a high degree of homeotropic alignment (nllz), against a scat- tering background. The optical transparency of the aligned gel is not of the quality of the pre-aligned LC-monomer mixture, showing that scattering of light occurs from the network in the aligned gel. The transparency of the aligned gel is, neverthe- less, very high and is sufficient to allow this hybrid material to act as an information storage medium.Fig. 1 shows the dielectric loss spectrum as a function of frequency for the liquid-crystal mixture E7 at -20 "C.In the off condition no directing voltage is applied to the sample and only a small loss is observed in this frequency range. Application of a dc biasing voltage (from an internal source in the DigiBridge) leads to a development of the loss peak centred on 20 kHz at this sample temperature. Under these conditions the sample is homeotropically (H) aligned. The data clearly show that the 'voltage-untreated' sample in Fig. 1 is planarly aligned and becomes H-aligned upon application Plate 1 (a) Nominally unaligned (left) and homeotropically aligned (right) regons of the LC gel viewed through crossed polarizers (magnification 100 x).(b)Conoscopic image of the homeotropically aligned LC gel. OD 00 L -0 w4 0 0 0 0 1 2 3 4 5 log (frequency/Hz) Fig. 1 Dielectric loss factor E" vs. log (frequency/Hz) for nominally unaligned (0)and homeotropically aligned (0)liquid-crystal mixture E7 at -20 "C 20 . 15 0 0 OO 0 w 10 0 0 "1 2 3 4 5 log (frequency/Hz) Fig. 2 Dielectric permittivity E' and loss factor E" us. log (frequency/Hz) at -20 "C for LC gels prepared in the absence (0)and presence (0) of a directing E field. The enhancement of the loss peak on alignment (0 to 0) demonstrates the alignment for a gel prepared from the aligned LC-monomer mixture. of 20V. The data of Fig. 1 act as a reference for the LC gels described below.Fig. 2 shows permittivity (E') and loss (E") data for gel samples made in the absence and presence of a directing voltage. The sample prepared in the absence of the voltage has a small permittivity (E'E5.8) and small loss factor in this range at -20 "C, and is planarly aligned. The aligned gel made from an LC-monomer mixture in the presence of 30 V at 1 kHz gives the well defined loss peak whose frequency location is essentially the same as that for E7 in its H-aligned state (see Fig. 1). The relaxation strength AE of the loss peak for both samples is readily estimated from the product of peak height (E",~,) and full-width at half-height (A+).The loss peak for the aligned gel is ca. 6% broader than that for E7, indicating a greater complexity for this process in the gel.The ratio of relaxation strengths, : for the aligned samples is ca. 0.84. Since 5% of the gel material is the polymer network, which has a much smaller contribution per repeat unit than that from the highly polar molecules comprising E7, this ratio would be ca. 0.95 if the LC gel has full homeotropic alignment. The dielectric properties of low molar mass and polymeric liquid crystals in the absence and presence of electric and magnetic fields and surface forces are well documented (see ref. 9 for a recent review). For a partially aligned LC sample having axial symmetry with respect to the measuring field direction (z axis) the measured dielectric permittivity &',(a)and loss factor E",(o) may be written as linear functions of the macroscopic director order parameter, &, of the material.2(l+2Sd)&'r,,(0)+j(1-Sd)E"(U)) (1)&"z(0)=-3 668 J. Muter. Chem., 1996, 6(4), 667-669 where 11 and I refer to measurements made for fully homeotropic (nllE)and fully planar (nlE)samples respectively. Thus, as Sd is changed from -3 (planar) through 0 (unaligned) to 1 for the H-aligned material, then E”,(u)) is changed system- atically. E”II (0)and E”~(U)) are functions of (i) the local order parameter, S, which is unchanged on changing Sd, (ii) of p1 and pt where p1 and pt are the longitudinal and transverse components of the effective dipole moment of the mesogenic groups, and (iii) Fourier transforms of four orthogonal relax- ation functions (see ref.11 for details). For the E7 mixture the 00 mode (also known as a 6 process) dominates the dielectric loss peak in the aligned samples in Fig. 1 and 2. Using eqn. (l), remembering the first term on the right- hand side refers to the 6 process of Fig. 1 and 2 and assuming that the E7 sample was fully aligned homeotropically, then we calculate Sd=0.83 for the aligned gel sample. This value falls below that for full H-alignment and suggests that a part of the LC material is bound to the internal surfaces of the LC-gel network, removing its contribution to the 6 process in the dielectric spectrum. We have made measurements of the low frequency permittivity, E,, for the aligned LC gel at room temperature over a period of time.We find no decrease (<0.1% change) in E, over a period of several weeks, indicating that the alignment is preserved quantitatively and that the network is in a strain-free condition in the aligned liquid crystal. Finally, note that Stannarius et aL6 used NMR spectroscopy, with a 4.7 T magnet, to study the director distribution in LC gels formed from 4,4-bis-acryloxylbiphenyl, as a non-mesogenic monomer, and a 50: 50 mixture of n-pentyl and n-pentyloxy cyanobiphenyls. In this case the strong magnetic field used to study a material will perturb the director orientation when the field direction makes an angle with the C, symmetry axis for the sample, thus such measurements determine director distri- butions in the presence of the B-field.6 In contrast, the weak measuring E-fields used in our dielectric relaxation experiments do not influence the director distribution in the gel.In a future publication” we shall show that strong directing E-fields may be applied to gels having different initial macroscopic align- ments and the resultant changes in director orientation may be measured simultaneously using our dielectric method. We shall demonstrate12 that dielectric studies provide a means of determining Sd in LC gels subjected to additional E-fields and that saturation of orientation can be achieved for gels that were initially only partially aligned or were planarly aligned, but only at fields far higher than that for an unpolymerized LC mixture.The authors gratefully acknowledge grant support from the EPSRC to S.S. and from the EC Human Capital Mobility Programme to M.M., and equipment support from the EPSRC. References 1 D. J. Broer, R. A. M. Hikmet and G. Challa, Makromol. Chem., 1989,190,3202. R. A. M. Hikmet, J. Lub and D. J. Broer, Adu. Mater., 1991,3,392. R. A. M. Hikmet, Liq. Cryst., 1991,9,405. R. A. M. Hikmet, Adu. Mater., 1992,4679. R. A. M. Hikmet and B. H. Zwerver, Liq. Cryst., 1993,13, 561. R. Stannarius, G. P. Crawford, L. C. Chien and J. W. Doane, J. Appl. Phys., 1991,79, 135. 7 A. Jakli, L. Rosta and L. Noirez, Liq. Cryst., 1995, 18,601. 8 D. Braun, G. Frick, M. Grell, M. Klimes and J. H. Wendorff, Liq. Cryst., 1992,11,929, and references therein. 9 G. Williams, in The Molecular Dynamics of Liquid Crystals, ed. G. R. Luckhurst and C. S. Veracini, Kluwer, Dordrecht, 1994, p. 431. 10 G. S. Attard, K. Araki and G. Williams, Brit. Polym. J., 1987, 19, 119. 11 K. Araki, G. S. Attard, A. Kozak, G. Williams, G. W. Gray, D. Lacey and G. Nestor, J. Chem. SOC., Faraday Trans. 2, 1988, 84, 1067. 12 G. Williams, M. Marugan and S. Shinton, manuscript in preparation. Communication 5/06571B; Received 5th October, 1995 J. Mater. Chem., 1996, 6(4), 667-669 669

ISSN:0959-9428    

DOI:10.1039/JM9960600667

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

~_________________~~ ~ Thresholdless antiferroelectricity in liquid crystals and its application to displays Shiroh Inui,"9t Noriko Iimura; Tsuyoshi Suzuki," Hiroshi Iwane," Kouichi Miyachi,b Yoichi Takanishib and Atsuo Fukudab* "Mitsubishi Chemical Corporation, Tsukuba Research Center, Ami Chu-ou, Inashiki, Ibaraki 300-03, Japan bTokyo Institute of Technology, Department of Organic and Polymeric Materials, 0-okayama, Meguro-ku, Tokyo 152, Japan By diminishing the energy barrier between SC; and SC*, antiferroelectricity has become thresholdless in a three- component mixture. It shows V-shaped switching, realizing attractive display characteristics: extremely wide viewing angle with very large contrast ratio, high speed response and ideal analogue grey scale with no hysteresis.A simplified model of the phase with this property is presented. Ordinary antiferroelectricity in liquid crystals (LCs) shows tristable switching, which is the electric-field-induced transition between antiferroelectric (AF) smectic CA*(SCA*) and ferroelec- tric (F) smectic C* (SC*) phases and has characteristic dc threshold and hysteresis.' By applying a positive or negative bias field, the tristable switching in AFLCs can be used in a way similar to the bistable switching in FLCS;~,~ two F states give the same transmittance, so that we can alternately use them symmetrically. This fact assures an extremely wide view- ing angle with a relatively large contrast ratio of 30. Since AFLCs have some additional characteristics which make them superior to FLCs, they have received much attention and two prototype AFLC displays using passive matrix (PM) address- ing have been developed and e~hibited.~*~What prevents AFLC displays from achieving much larger contrast ratios is the pretransitional effect in the electric-field-induced AF-F phase transition, which appears as a slight increase in transmit- tance below the threshold.During the development of materials to suppress the pretran- sitional effect, we encountered materials which show a large pretransitional effect and, at the same time, a remarkable decrease in the threshold field strength. Hence we propose that pretransitional-effect enhancement in combination with the use of active matrix (AM) or thin-film transistor (TFT) address- ing is another way to endow LCs with attractive display characteristics.The materials here investigated are compounds and their mixtures possessing trifluoroalkoxyalkyl carboxylate groups and partially fluoro-substituted phenyl rings. In a series of compounds with the general molecular structure 1 1 an ether linkage and its position in the chiral end chain were reported to have a subtle influence on the threshold field, Present Address: Mitsubishi Chemical Corporation, Yokkaichi Research Center, Chemicals Laboratory, Toho-cho, Yokkaichi, Mie 5 10, Japan. EthH.6,7When the ether linkage is located at the chain terminal, EthH is higher than that of the reference substance of equal chain length but with no ether linkage.As the linkage moves towards the core, EthHbecomes lower and finally F appears instead of AF. Blending further reduces EthHas illustrated in Fig. 1. Furthermore, the pretransitional effect can be enhanced by suitably substituting the phenyl rings with fluorine. To evaluate the pretransitional effect, we measured the contrast ratio, TF/TAF,just below EthH;the smaller the contrast ratio, the larger the pretransitional effect. Table 1 summarizes the results, indicating that Y1 substitution causes the most marked decrease in the contrast ratio, and hence the greatest increase in the pretransitional effect. In this way, we can enhance the pretransitional effect and reduce the threshold, But can we prepare a system where the threshold becomes zero, so that the pretransitional effect prevails and the AF-F phase transition occurs continuously? Surprisingly, it was not difficult to realize this property, which 0' I I I I I 't 1 I I I 1 I 0 20 40 60 80 100 mixing Tatio (%) 2 F 3 Fig.1 Phase diagram of (a) the binary mixture system of compounds 2 and 3, and (b)the threshold field us. mixing ratio plot J. Mater. Chem., 1996, 6(4),671-673 671 $-6-4-20 2 4 6 c electric fielwprn-' €= 0 +E ti3 0 Langevin-type Random Langevin-type alignment from boundary alignment to boundary F, F\ C02-6H(CF3)C4H80CH3 F4 6 Fig. 2(a) V-shaped switching observed in a three-component mixture of compounds 4,5 and 6, with the mixing ratio of 4:5 :6 =40 : 40 :20 (mass%). (b) Simplified model of the phase with thresholdless antiferroelectricity, SC,*.(c) Simulated light transmittance as a function of the normalized electric field. Table 1Effects of fluoro-substitution at positions Y,-Y3 of compound 7 on the contrast ratio 7 y, y2 y3 contrast ratio H H H 36 H F H 27 F H H 12 H H F 31 H F F 28 F H F 10 we designate as 'thresholdless antiferroelectricity'$ in a three-component mixture, and the field-induced F-AF-F phase transition now appears as V-shaped switching as shown in Fig. 2(u).When we observed the switching using a polarizing $ This term is self-contradictory, because going from AF to F requires a symmetry change. However, it is profitable to slightly extend the definition of antiferroelectricity because of the present novel and potentially very interesting observation in liquid crystals.The mechanism by which the net spontaneous polarization is cancelled is randomization, but not the antiparallel arrangement. If we include the randomization mechanism, the AF-F transition may become thresholdless. optical microscope, the visual field varied uniformly and continuously without showing any irregularities indicating the boundary movement characteristic of the tristable switching or the disclination lines caused by the helicoidal unwinding of FLCs. The V-shaped switching is totally different from the tristable switching and the helicoidal unwinding. Although we have to optimize the V-shaped switching by further developing suitable materials, the properties at 25 "C of the three-component mixture has already realized potentially attractive display characteristics: (i) a tilt angle >35" assuring efficient light transmission in its bright state; (ii) <2 V pm-l for completing F-AF-F switching which makes low-voltage driving possible; (iii) light transmission almost linear to the applied field and free from hysteresis, which produces an ideal analogue grey scale; (iv) <50 ps AF-F switching time; (v) a contrast ratio up to 100; and (vi) an extremely wide viewing angle of >60°.None of the currently available FLC and AFLC materials realized displays that possessed all of these characteristics; an analogue grey scale is difficult to achieve with surface-stabilized FLCs with PM addre~sing,~deformed-helix FLCs with AM or TFT addressing is disrupted by hy~teresis,'.~and AFLCs with PM addressing could not attain very large contrast ratio^.^.^ Only thresholdless antiferro-electric LCs with AM or TFT addressing, as described herein, are considered to possess all of those attractive display characteristics.Let us consider the thresholdless antiferroelectricity in con-nection with the molecular rotational motion which plays an essential role for the emergence of in-layer spontaneouspolariz-ations in liquid crystals." Recently, we experimentally con-firmed in a prototype AFLC 8 672 J. Mater. Chem., 1996,6(4),671-673 a that the hindered rotational motion of the carbonyl group in the chiral end chain is described by the distribution function where $ is the angle of rotation of the carbonyl about the long molecular axis and a the degree of hindrance; the hindered direction $o in SCA* is substantially different from that in SC* The pretransitional effect may be considered as a slight change in tj0, from $OAF towards Thus we speculate that in the phase with thresholdless antiferroelec- tricity, the hindered rotational motions (more generally, the states of the chiral end chain as a whole) characterized by $OAF and $OF in SCA*and SC* have nearly the same energies and furthermore the barrier between them diminishes, so that the states characterized by any t,b0 between $OAF and are equally thermally excited at zero electric field; this arbitrary nature of $o makes the molecular tilting direction non-corre- lated between adjacent layers.The director tilting is uniform and has constant polar and azimuthal angles in a smectic layer, but its azimuthal angle varies randomly from layer to layer. In-layer spontaneous polarizations exist at smectic-layer boundaries, with random orientations and variable magnitudes from boundary to boundary; hence there is no net spontaneous polarization. We designate the phase with thresholdless anti- ferroelectricity as SCR*with (R =random), § a simplified model of which is shown in Fig. 2(b). An electric field applied perpendicular to the substrate plates along the smectic layer induces a net spontaneous polarization according to the Langevin-type eq~ati0n.l~ However, it differs from the ordinary Langevin equation in that (1)we are dealing with very large effective dipole moments of variable magnitude owing to the cooperative interaction of many molecules which produce in-layer spontaneous polarizations, and (2) the Q Since the macroscopic symmetry of SCR*is the same as that of SA, the V-shaped switching could be regarded as an electroclinic effect.12 In an actual cell, the substrate surfaces impose some restrictions and the azimuthal angle distribution may not be cylindrically symmetrical around the smectic layer normal.rotation of the effective dipole moments is restricted in the two-dimensional space.The light transmittance, which is V-shaped as shown in Fig. 2(c), was obtained by assuming that the effective dipole moments have the same magnitude, peff, and follow the same distribution as the c-director which can be approximated by an apparent angle, and that the index of ellipsoid is uniaxial with eigenvalues, nll=1.7 and n, =1.5, which are independent of the field applied. Other parameters used were wavelength 1 =500 nm, cell thickness d= 1.7 pm and molecular tilt angle 8=35". This research was partly supported by a Grant-in-Aid for Scientific Research (Specially Promoted Research No. 06102005) from the Ministry of Education, Science, Sports and Culture, Japan. References 1 A. Fukuda, Y. Takanishi, T. Isozaki, K. Ishikawa and H.Takezoe, J. Mater. Chem., 1994,4,997. 2 N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 1980,36,899. 3 M. Terada, S. Yamada, K. Katagiri, S. Yoshihara and J. Kanbe, Ferroelectrics, 1993,149,283. 4 N. Yamamoto, N. Koshoubu, K. Mori, K. Nakamura and Y. Yamada, Ferroelectrics, 1993, 149,295. 5 E. Tajima, S. Kondoh and Y. Suzuki, Ferroelectrics, 1993,149,255. 6 S. Inui, T. Suzuki, N. Iimura, H. Iwane and H. Nohira, Ferroelectrics, 1993,148, 79. 7 M. Johno, T. Matsumoto, H. Mineta and T. Yui, Abstracts II Chem. Soc. Jpn. 67th Spring Meeting (Tokyo, 1994), 1 B3 12, p. 637. 8 J. Funfschilling and M. Schadt, J. Appl. Phys., 1989,66,3877. 9 T. Tanaka, K. Sakamoto, K. Tada and J. Ogura, SID'94 Digest, 1994,p. 430. 10 R. B. Meyer, Mol. Cryst. Liq. Cryst., 1977,40,33. 11 K. Miyachi, J. Matsushima, Y. Takanishi, K. Ishikawa, H. Takezoe and A. Fukuda, Phys. Rev. E, 1995,52, R2153. 12 S. Garoff and R. B. Meyer, Phys. Rev. A, 1979,19,338. 13 G. M. Barrow, Physical Chemistry, McGraw-Hill, New York, 1988, 5th edn., p. 671. Communication 5/07560B; Received 20th November, 1995 J. Mater. Chern., 1996, 6(4),671-673 673

ISSN:0959-9428    

DOI:10.1039/JM9960600671

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Striking effects of halogen substituents on the glass-forming properties, glass- transition temperatures and stabilities of the glassy state of a new family of amorphous molecular materials, 1,3,5-tris (4-halogenophenylphenylamino)benzenes Hiroshi Kageyama, Koji Itano, Wataru Ishikawa and Yasuhiko Shirota" Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka, Suita, Osaka 565, Japan A new class of n-electron starburst molecules, 1,3,5-tris(4- halogenophenylphenylamino) benzenes, are synthesized for use as amorphous molecular materials. It was found that they readily form amorphous glasses, whereas the parent compound 1,3,5-tris(diphenylamino)benzene instantly crystallizes and that the ease of glass formation, glass- transition temperature, and stability of the glassy state are greatly affected by the type of halogen substituent.Amorphous molecular materials that readily form amorphous glasses with relatively high glass-transition temperatures are expected to constitute a novel class of organic functional materials with excellent processability, transparency, isotropic properties and homogeneous properties owing to the absence of grain boundaries. In addition, developing such amorphous molecular materials, which exhibit a glass transition usually associated with amorphous polymers, is of interest and signifi- cance from an academic viewpoint, opening up a new field of organic solid-state science that deals with molecular glasses. There are still few known examples.'-13 We have synthesized several novel families of organic x-electron systems, which we refer to as 'starburst' molecules, for use as amorphous molecu- lar materials.They include 1,3,5-tris(diphenylamino)triphenyl-amine and its derivative^,^*^,'*.^' 1,3,5-tris(diphenyl-amino) benzene and its derivative^,^-^*'^ and 1,3,5-tris(4-diphenylaminophenyl) benzene and its derivatives.' For the development of amorphous molecular materials, it is of impor- tance to establish guidelines for molecular design through the study of the correlation between molecular structure and glass- forming properties. We report here striking effects of halogen substituents on the glass-forming properties, glass-transition temperatures (Tg), and stabilities of the glassy state for 1,3,5-tris(4-halogenophen-ylphenylamino) benzenes, e.g.1,3,5-tris( 4-fluorophenylphenyl- amino) benzene (p-FTDAB), 1,3,5-tris( 4-chlorophenyl- phenylamino) benzene (p-ClTDAB), and 1,3,5-tris( 4-bromo- phenylphenylamino) benzene (p-BrTDAB). F CI Br p-FTDAB p-ClTDAB P-BrTDAB The new compounds, p-FTDAB, p-ClTDAB and p-BrTDAB, were synthesized by the Ullmann reaction of 1,3,5-tris(phenyl- amino)benzene (1.8 g, 5 mmol), prepared from phloroglucinol and aniline according to the method in the literat~re,'~ with p-fluoro-, p-chloro-, and p-bromo-iodobenzenes (5.0 g, 22.5 mmol), respectively, at 170°C for 30 h in the presence of copper powder (0.2 g, 3 mmol) and potassium hydroxide (1.1 g, 20 mmol) under a nitrogen atmosphere.The products were purified by column chromatography using silica gel, followed by recrystallization from benzene-hexane, and identified by various spectroscopies, mass spectrometry and elemental analysis. 1,3,5 -Tris(4- halogenophenylphenylamino) benzenes (p-XTDAB, X =F, C1, Br) were found to form readily amorphous glasses when the melt samples were cooled, whereas the parent compound 1,3,5-tris(diphenylamino)benzene (TDAB) instantly crystallized. Fig. 1 shows differential scanning calorimetry (DSC) curves of p-ClTDAB as an example. When a crystalline sample obtained by recrystallization from benzene-hexane was heated, an endothermic peak due to melting was observed at 181"C. When the resulting isotropic liquid was cooled on standing in air, it formed spontaneously an amorphous glass via a supercooled liquid.When the amorphous glass sample was again heated, glass transition took place at around 64"C, followed by crystallization at around 112 "C to give the same crystal as obtained by recrystallization from benzene-hexane, which melted at 181 "C. Similar DSC curves were obtained for p-FTDAB and p-BrTDAB; however, no crystallization was noticed for p-BrTDAB even when heated above the Tg. The formation of the amorphous glasses of p-FTDAB, p-ClTDAB and p-BrTDAB was also evidenced by X-ray diffraction and polarizing microscopy. Thus, p-XTDABs (X =F, C1, Br) consti- tute a new class of amorphous molecular materials. It is of note that the halogen substituent in p-XTDABs (X= F, C1, Br) strikingly affects the ease of glass formation, q,and stability of the glassy state. That is, while p-FTDAB requires rapid cooling of the melt sample with liquid nitrogen to form a glass, p-ClTDAB and p-BrTDAB readily produce amorphous glasses even on slow cooling of the melt samples at a cooling rate as slow as 1"C min-'.The glassy state of p-FTDAB is much less stable than those of p-ClTDAB and p-BrTDAB; 7 Tm 181 "C r, 64°C 112°C Tm 181 "C TI"C Fig. 1 DSC curves of p-ClTDAB; heating rate: 5 "C min-'. (a)-Crystalline sample obtained by recrystallization from benzene- hexane; (b)glass sample obtained by cooling the melt. J. Muter. Chem., 1996, 6(4), 675-676 675 Table 1 Glass-transition, crystallization and melting temperatures ($ ,T, and Tm)and thermodynamic parameters [specific heat jumps at $(AC,), enthalpy and entropy changes (AH and AS)] for p-XTDABs" ACp/ AHCI ' AHml ASmlql"c JK 'mol T,rc kJ mol Tm/"C kJ mol JK 'mol ' p-FTDAB 54 170 65 -16 228 51 102 p-CITDAB 64 220 112 -26 181 37 81 p-BrTDAB 72 220 -b -b 165' 41' 94' 'Glass samples were heated at a heating rate of 5 "C mm ' bNo crystallization 'Values for the crystalline sample obtained by recrystallization from benzene-hexane while the p-FTDAB glass tends to crystallize when allowed to stand for a few days at room temperature, the glasses of p-ClTDAB and p-BrTDAB do not The p-BrTDAB glass in particular exhibits no crystallization behaviour even on heating above the Tg, whereas the p-FTDAB and p-ClTDAB glasses crystallize on heating above the Tgs Thus, the ease of glass formation and stability of the resulting glass were found to increase in the order p-FTDAB <<p-ClTDAB <p-BrTDAB The G is also greatly affected by the type of halogen atom, increasing in the order p-FTDAB <p-ClTDAB <p-BrTDAB as shown in Table 1 It should be noted that the melting temperature (T,) decreases in this order In contrast to TDAB, which readily crystallizes even when the melt sample is rapidly cooled with liquid nitrogen, the facile glass formation for p-XTDABs as well as for the methyl- substituted TDABs' may be ascribed to the increase in the number of conformers due to the incorporation of the halogen substituent in TDAB Thus, increasing the number of confor- mers of nonplanar molecules could be a concept for molecular design of amorphous molecular materials The marked effects of the halogen substituent on the ease of glass formation and stability of the glassy state observed for p-XTDABs (X =F, C1, Br), 1 e easier glass formation and increasing stability of the resulting glass in the order F<<Cl<Br, can be explained in terms of the bulkiness and weight of the halogen atom That is, the more bulky and heavier halogen atom will prevent easy packing of molecules and easy diffusion of molecules in the process of crystal growth, and hence prevent crystallization Intermolecular interactions operative in p-XTDABs, e g n-n interactions, hydrogen bonding interactions between the hal- ogen substituent and aromatic C- H," and halogen-halogen T,and 7'' will increase in both attractions," cause an F <C1< Br may be attributed to poorer packing of molecules in the crystal due to the more bulky halogen substituent in this order, leading to smaller crystal lattice energy On the other hand, the increasing in the order F<Cl<Br is also ascribed mainly to the difference in the weight of the halogen atom The incorporation of a heavier halogen atom will tend to hinder translational, rotational, and vibrational motions of the molecule, leading to an increase in Tg Further work is necessary to clarify the correlation between Tg and T, The present study shows the striking effects of halogen substituents on the ease of glass formation, Tg and stability of the molecular glass, presenting an important guide for future development of amorphous molecular materials References 1 B Rosenberg, J Chem Phys, 1959,31,238 2 D J Plazek, J H Magill, J Chem Phys ,1966,45,3038 3 Y Shirota, T Kobata and N Noma, Chem Lett, 1989, 1145 4 A Higuchi, H Inada, T Kobata and Y Shirota, Adu Muter, 1991, 3,549 5 W Ishikawa, H Inada, H Nakano and Y Shirota, Chem Lett, 1991,1731 6 W Ishikawa, H Inada, H Nakano and Y Shirota, Mol Cryst Lzq Cryst, 1992,211,431 7 W Ishikawa, H Inada, H Nakano and Y Shirota, J Phys D Appl Phys ,1993,26, B94 8 H Inada and Y Shirota, J Muter Chem, 1993,3,319 9 K Naito and A Miura, J Phys Chem ,1993,97,6240 10 Y Kuwabara, H Ogawa, H Inada, N Noma and Y Shirota, Adu Muter, 1994,6,677 11 A Higuchi and Y Shirota, Mol Cryst Lzq Cryst, 1994,242,127 12 E Ueta, H Nakano and Y Shirota, Chem Lett, 1994,2397 13 Y H Kim and R Beckerbauer, Macromolecules, 1994,27,1968 14 Ng Ph Buu-Hois, J Chem Soc ,1952,4346However, the opposite trends in Tp and T, observed for p-15 J A R P Sarma and G R Desiraju, Acc Chem Res 1986,XTDABs, namely, increasing Tp but decreasing T, in the order 19.222 F<Cl<Br, may be ascribed to other factors operating more strongly in p-XTDABs That is, the decrease in T, in the order Paper 6/01 1lD, Received 5th January 1996 676 J Muter Chem, 1996,6(4), 675-676

ISSN:0959-9428    

DOI:10.1039/JM9960600675

出版商:RSC    

年代:1996

数据来源: RSC