摘要:

J. MATER. CHEM., 1991, 1(6), 1061-1063 Synthesis and Structure of LiCaPO, by Combined X-Ray and Neutron Powder Diffraction Philip Lightfoot,"* Marian C. Pienkowski," Peter G. Brucea and Isaac Abrahamd a Centre for Materials and Electrochemical Sciences, Department of Chemistry, University of St Andrews, St Andrews, Fife KY76 9ST, UK Department of Chemistry, Heriot- Watt University, Riccarton, Edinburgh EH74 4AS, UK LiCaPO, has been synthesized and characterized by combined X-ray and neutron powder diffraction. The phase adopts the trigonal space group P3,c, a=7.5247(1) A, c=9.9657(2) A. The structure is composed of a three- dimensional framework of vertex-sharing LiO, and PO, tetrahedra enclosing five-sided channels running parallel to the c axis, which are occupied by the Ca2+ ions.This structure is isotypic with LiNaSO, but is believed to be novel amongst phosphates. The combined use of X-ray and neutron scattering is found to improve significantly the determination of both the light (Li) and heavy (Ca) atom positional and thermal parameters, and it is proposed that this strategy may be useful in the structural characterization of more complex lithium-containing solids, containing heavier elements of similar neutron scattering length such as La and Sr, where neutron diffraction alone is insufficient to determine the site ordering uniquely. Keywords: Lithium calcium phosphate ; Powder diffraction ; Neutron diffraction ; Solid electrolyte Phosphates of general formula ABPO,, where A is a mono- valent cation and B a divalent cation, are of interest for their optical' and ferroelectric2 properties.Many of these phases crystallize in one of three basic structure types,3 depending on the sizes of the cation, uiz. (i) the Na2S04 family, where both A and B are large enough to occupy eight- or nine-co- ordinated sites (e.g. NaCaPO,), (ii) the 'stuffed tridymite' family, for example NaZnPO,, where B is sufficiently small to occupy a tetrahedral site, and (iii) olivine-related materials, e.g. NaMnPO,, where both A and B are located in octahedral sites. While exploring such systems as potential solid-electrolyte materials, we have prepared LiCaPO,, and here we describe its structural characterization by a combination of X-ray and neutron powder diffraction.An apparently different form of LiCaPO, was originally reported by Thilo4 to belong to the olivine-related family. Subsequently, Wanmaker and Spier' published an X-ray powder pattern of that phase, but this was unindexed. Careful comparison of Wanmaker's powder pattern with that of our own sample revealed that the two phases are in fact identical. Wanmaker's powder pattern appears to have been incorrectly reported, and is subject to a constant 26' (zero-point) error of ca. 0.9 "C. When this is taken into account, it is readily apparent that this pattern consists of LiCaPO, together with an appreciable amount of c~-Ca~(P0,)~as an impurity. Here, we report the structural characterization of LiCaPO,, which is shown to be isostructu- ral with the sulphate LiNaSO,.' As far as we are aware, this is a novel structural type for a phosphate of this stiochiometry.Experimental Polycrystalline 'LiCaPO, was prepared by a solid-state reac- tion. Stiochiometric quantities of dry CaHPO, and iso- topically enriched 7Li2C03 were thoroughly ground and fired at 300 "C for 1 h, 650 "C for 2 h and 800 "C for 3 days, with two intermediate regrindings. The final product was quenched into liquid N2 from 800 "C. Time-of-flight neutron powder diffraction data were col- lected on the medium resolution diffractometer POLARIS at the ISIS facilty, Rutherford Appleton Laboratory. Ca. 10 g of sample were loaded into a 12 mm diameter vanadium can and placed in an evacuable chamber in the neutron beam. Data were collected for 6 h simultaneously on low-angle, 90 " and backscattered detector banks.Data from all three detector banks were employed in the initial indexing of the pattern, with subsequent Rietveld analysis being carried out only on the higher-resolution backscattering data in the d-spacing range 0.5-2.43 A. X-Ray powder diffraction data were col- lected on a Stoe STADI/P high-resolution diffractometer in symmetric transmission mode, using Ge-monochromatized Cu-Kcr, radiation. Data were collected in the 26' range 10-120" in steps of 0.02", using a small linear position-sensitive detector (PSD) covering an angular range of ca. 6"in 28. No absorption correction was applied. Rietveld analysis was carried out using the program GSAS,6 which allows simultaneous refinement of both X-ray and neutron diffraction data.The peak shapes used were a pseudo- Voigt function for the X-ray data, and a convolution of Gaussian and exponential components7 for the neutron data. The scattering lengths used in the neutron refinement were as follows: Li= -0.220, Ca=0.490, P=0.513 and O= 0.5805 x 10-l2 cm.8 Polyhedral plots were generated using STRUPL0.9 Structure Determination Initial indexing of the data was carried out on the neutron profile using the program ITO." A hexagonal unit cell of approximate dimensions a = 15.03 A, c =9.96 A was found by the program; however, careful scrutiny of this solution revealed that the true cell dimensions were ca.a=7.52 A, c=9.96 A, with the apparent supercell reflections being due to the presence of a very small amount of Ca2P207 impurity. Using the CDIF" database on the Chemical Data Service computer at the Daresbury Laboratory, no likely isostructural phos- phate was found; however the sulphate, LiNaS0,,5 was found to have hexagonal cell dimensions of a =7.6270(7)8, and c = 9.858(1) A. The structure of LiNaSO, were therefore used as a starting model in the Rietveld refinement with the Na and S atoms replaced by Ca and P, respectively. Refinement proceeded in the space group P3,c (no. 159). Both sets were first refined independently, and then a combined refinement was carried out, with the individual and combined refinement converging smoothly on the basis of the LiNaSO, model.In the final combined and neutron refinements, isotropic thermal para- meters were allowed to refine independently for all atoms. In the X-ray refinement it was found necessary to tie the thermal parameters of the same atom types, with Li and Ca thermal parameters also tied together. Refined parameters from the individual and combined refinements are given in Table 1 with selected interatomic distances and angles in Table 2. The final difference profiles for the combined refinement are given in Fig. l(a) and (b). Discussion The structure of LiCaPO, (Fig.2) is composed of a three- dimensional framework of vertex-sharing Li04 and PO4 tetrahedra, which enclose large five-sided channels parallel to the [OOOl] direction, in which the Ca2+ ions reside. The Ca2+ ions have six short contacts (2.31-2.54 A) to oxygen and two longer contacts (2.76 and 2.90 A), completing an irregular eight-co-ordinate geometry. The structure adopted by LiCaPO, appears to be quite novel, especially when compared with other phosphates of the type ABP04.The stoichiometric analogues LiMnP04,12 LiMgPO4I3 and LiFeP0414 are isostructural with each other and possess octahedral co-ordinations for both the mono- valent and divalent cations. In the case of LiCdP04," where Cd2+ is a similar sized ion to Ca2+, the structure is again Table 1 Final refined parameters for X-ray (top line), neutron (middle) and combined (bottom) Rietveld refinements, with e.s.d.s given in parenthesesa atom site xja Ylb z/c U(iso)/A2 6c 0.058(4) 0.036(3) 0.252(4) 0.237(3) 0.239(5) 0.250( 3) 0.0 18(1) 0.016(3) 0.035(2) 0.234(2) 0.2 56( 2) 0.019(2) 6c 0.0174(5) 0.019(1) 0.5362(5) 0.532( 1) 0.480( 1) 0.474( 1) 0.018(1) 0.025(2) 0.0179(5) 0.5353(5) 0.4775(8) 0.0199(8) 2a O.O(-) O.O(-) O.O(-) 0.012(1) O.O(-) O.O(-) O.O(-) 0.0 13(2) O.O(-) O.O(-) O.O(-) 0.015(1) 2b 0.3333(-) 0.6667(-) 0.183(2) 0.012(1) 0.3333(-) 0.3333(-) 0.6667(-) 0.6667(-) 0.188(2) OM( 1) 0.019(2) 0.0 18(1) 2b 0.6667(-) 0.3333(-) 0.258(2) 0.0 12(I) 0.6667(-) 0.6667(-) 0.3 3 3 3(-) 0.3333(-) 0.257( 1) 0.2575(9) 0.004( 1) 0.0047( 9) 2a 2b O.O(-) O.O(-) O.O(-) 0.3333(-) 0.3 3 3 3( -) 0.3333(-) O.O(-) O.O(-) O.O(-) 0.6667(-) 0.6 6 6 7( -) 0.6667(-) 0.160(2) 0.155(1) 0.1554(9) 0.347(5) 0.338(2) 0.342(2) 0.01 7(2) 0.01 l(2) 0.01 I( 1) 0.017(2) 0.019(2) 0.019(2) 2b 0.6667(-) 0.3333(-) 0.103(4) 0.01 7(2) 0.6667(-) 0.3333(-) 0.103(2) 0.023(2) 0.6667(-) 0.3333(-) 0.105( 1) 0.022(2) 6c 0.225(1) 0.2236(8) 0.1 16(2) 0.1 lO(1) -0.045(2) -0.053( 1) 0.017(2) 0.0 14(1) 6c 6c 0.2249(6) 0.220( 3) 0.2 19(2) 0.2 16( 1) 0.462( 2) 0.462(1) 0.1 137(9) 0.450(2) 0.453( 1) 0.45 18(7) 0.161 (2) 0.166(1) -0.0512(9) 0.130(2) 0.127(1) 0.1288(8) 0.31 l(2) 0.308( 2) 0.0 144(8) 0.017(2) 0.020(1) 0.0189(8) 0.017(2) 0.028(2) 0.4627(9) 0.165(1) 0.310(1) 0.030( 1) a Space group P31 c, a =7.5247( 1) A, c =9.9657(2) A (from combined refinement). X-Ray range 0-120", 252 reflections, R,, = 17.6%, Rex,= 6.67, x2=7.0.Neutron data range 0.5-2.43 A, 1467 reflections, R,, = 6.2%, Rex,= 1.58, x2 = 15.3. Combined R-factors: R,, =9.44%, Rex,= 3.17%, x2=8.87. J. MATER. CHEM., 1991, VOL. 1 Table 2 Selected distances and angles from the combined refinement of LiCaPO, bond bond length/A bonded atoms bond angle/ O Li-O(1) I .94( 1) O(1)-Li -0(4) 11 1.7(6) Li -0(4) 2.00(I) O(1)-Li-O(5) 98.3(7) Li -O(5) 1.96( 1) O(1)-Li-0(6) 127.9(7) Li-O(6) 1.87(1) 0(4)-Li-O( 5) 12446) 0(4)-Li -0(6) 90.1(7) O(5)-Li -0(6) 107.2( 7) Ca -O(3) 2.464( 7) Ca -O(3) 2.4 14( 6) Ca -O(4) 2.76 1(7) Ca-0(4) 2.396(6) Ca -O(5) 2.473(8) Ca -O(5) 2.540(8) Ca-O(6) 2.3 1O( 7) Ca-O(6) 2.907(7) P(1)-O(1) 1.55(1) O(1)-P( 1)-0(4) 109.2( 3) P(1)-0(4) 1.563(4) x 3 0(4)-P(1)-0(4) 109.7(3) P(2)- O(2) 1.57(1) 0(2)-P(2)-O(5) 111.9(3) P(2)-O(5) 1.513(5) x3 O(5)-P(2)-O(5) 106.9(4) P(3)-0(3) 1.52(1) O(3)-P(3)-0(6) 1 11.3(4) P(3)-O(6) 1.541(7) x 3 O(6)-P( 3)- 0(6) 107.6(4) 7-II I 1 1.0 2.0 3.0 4.0 5.0 6.0 7.0 26/10" I1 1 I I I I I I I I I II 1.o (b) tI I/ -1 0 0.51 ! 111111111111 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 time of flight110 ms Fig. 1 Final observed (points), calculated (solid line) and difference (below) profiles for the combined Rietveld refinement of LiCaPO,: (a) X-ray data and (b) neutron data.For clarity only a portion of the X-ray refinement is shown related to that of NaMnPO,. The five-sided channels observed in LiCaPO, are unusual, and contrast with the more com- monly observed six-sided channels in other tetrahedral frame- work structures, such as the SiOz polymorphs, quartz, cristobalite and tridymite.To our knowledge, this is the first characterized phosphate to adopt this structure type. Previous attempts to prepare LiCaPO, yielded materials contaminated with significant quantities of ~t-ca~(PO,)~, owing to the loss of Li under the conditions of the reaction (1 100 "C).Sub-sequent annealing of our own sample at 1100 "C produced an analogue decomposition, as confirmed by X-ray powder diffraction. In the combined refinement the number of observables is J. MATER. CHEM., 1991, VOL. 1 Fig. 2 A view of the LiCaPO, structure projected along the c axis. Li04 tetrahedra shaded, PO4 tetrahedra unshaded, Ca circles increased significantly with respect to the number of variables and this results in generally lower estimated standard devi- ations than in the individual refinements. Almost all structural parameters from the three refinements are self-consistent within three e.s.d.s.Another significant improvement lies in the refinement of thermal parameters, particularly when com- pared with the values obtained by X-ray, where it was necessary to tie thermal parameters together in order to obtain a stable refinement. The use of neutron diffraction in determining structures of lithium-containing materials is well established. However, these determinations often lead to a less accurate description of atomic parameters for heavier atoms whose scattering would dominate in an X-ray diffraction pattern.Therefore, by combining the two refinements the best features of these techniques are preserved. The joint refinement method has been employed recently in the field of high-T, superconduc- tors,I6 and fast-ion conductors based on hydrogen uranyl ph~sphate,'~for determining 0 and H atom positions, respect- ively. The corresponding benefits in determining the Li and Ca positions in the present case are clear. From the data in Table 1 it may be noted, that the Li position is relatively ill defined from the X-ray data, whereas, in contrast, the Ca position is relatively well defined, the e.s.d.s on the Ca x and y parameters being a factor of 2 better than the corresponding values for the neutron refinement.We wish to thank Dr. S. Hull at the Rutherford-Appleton Laboratory for his assistance during data collection and SERC for financial support. P.G.B. gratefully acknowledges the Royal Society for a Pickering Research Fellowship. References 1 W. L. Wanmaker and H. L. Spier, J. Electrochem: SOC., 1962, 109, 109. 2 D. Blum, J. C. Penzin and J. Y. Henry, Ferroelectrics, 1984, 61, 265. 3 G. Engel, Neues Jahrb. Mineral. Abh., 1976, 127, 197. 4 E. Thilo, Naturwissenschaften, 1941, 16, 239. 5 B. Morosin and D. L. Smith, Acta Crystallogr., 1967, 22, 906. 6 A. C. Larson and R. B. Von Dreele, Los Alamos National Laboratory Report No. LA-UR-86-748, 1987. 7 W. I. F. David, J. Appl. Crystallogr., 1986, 19, 63. 8 L. Koester and H. Rauch ZAEA Report, 2517/RB, 1981. 9 R. X. Fischer, J. Appl. Crystallogr., 1985, 18, 258. 10 J. W. Visser, J. Appl. Crystallogr., 1969, 6, 380. I1 CDIF, NIST Crystal Database, US Secretary of Commerce, 1991. 12 S. Geller and J. L. Durand, Acta Crystallogr., 1960, 13, 325. 13 F. Hanic, M. Handlovic, K. Burdova, J. Majling, J. Crystallogr. Spectrosc. Res., 1982, 12, 99. 14 0. V. Yakubovich, M. A. Simonov, N. V. Belov, Doklady Akade- mii Nauk SSSR, 1977, 235, 93. 15 L. Elammari, B. Elouadi, W. Depmier, Acta Crystallogr., 1988, 44,1357. 16 T. Sakurai, T. Yamashita, J. 0. Willis, H. Yamauchi, S. Tanaka and G. H. Kwei, Physica C, 1991, 174, 187. 17 A. N. Fitch and M. Cole, Muter. Res. Bull., 1991, 26, 407. Paper 1/03506A; Received 11th July, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101061

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1065-1070 Growth of Il-VI Compounds by Metal-organic Chemical Vapour Deposition Structural Character isation of Dimethylcadmium-N,N,N’,N’-Tetramethylethane-I,2-diamine by Gas-phase Electron Diffraction and its Use in the Growth of Epitaxial Layers of CdS and CdSe upon GaAs Matthew J. Almond,” Michael P. Beer,” Kolbjern Hagen,b David A. Rice*” and Peter J. Wright“ “Department of Chemistry, University of Reading, Whiteknights, Reading RG6 2AD, UK bDepartment of Chemistry, University of Trondheim, N7055, Trondheim, Norway “Royal Signals and Radar Establishment, St. Andrews Road, Great Malvern WR14 3PS, UK Aspects of the use of Lewis acid-base adducts of dimethylcadmium in the growth of CdY (Y =S or Se) by metal- organic chemical vapour deposition (MOCVD) have been investigated.The adduct dimethylcadmium-N,N,N’,N’-tetramethylethane-l,2-diamine, Me,Cd Me,NCH,CH,NMe, (l),was made and its structure determined by gas- phase electron diffraction. In 1 the cadmium atom is four-co-ordinate, being bound to two methyl groups [r,(Cd-C) =2.11(2) A] and two nitrogen atoms [ra(Cd-N)=2.47(5) A] with LC-Cd-C and LN-Cd-N being 132(11) and 84(3)”, respectively. The electron diffraction study provides the first unequivocal evidence for the existence of an adduct of dimethylcadmium being transported in the gas phase. Further evidence for the existence of 1 in the gas phase is provided by mass spectrometry and gas-phase infrared spectroscopy. Compound 1 was subsequently employed in the MOCVD process to grow visibly good-quality layers of CdY (Y = S or Se) on gallium arsenide substrates, but without overcoming the problem of pre-reaction experienced in the growth of CdY (Y =S or Se) by MOCVD.Keywords: Metal-organic chemical vapour deposition ; Electron diffraction ; Cadmium sulphide; Cadmium selenide The problem of pre-reaction,’ the premature combination of the reactants, experienced in the MOCVD of CdY (Y =S or Se) from the reaction of dimethylcadmium with H2Y (Y=S or Se), can be reduced to some extent by replacing dimethyl- cadmium with the dimethylcadmium-1,4-dioxane adduct.2 Simplistically, it was envisaged that some of the adduct was transported in the gas phase and only decomposed to its constituents when it reached the heated zone.It was further argued that co-ordination of the ligand to the cadmium centre prevented attack on the metal in the adduct by H2Y (Y =S or Se) at low temperature. Thus reaction to form CdY (Y= S or Se) only took place in the heated zone when free dimethylcadmium was formed. The mechanism by which the 1,4-dioxane adduct and some related species reduce pre- reaction has recently received further attention3 but an answer has proved elusive as so far no evidence to confirm the existence of the adducts of dimethylcadmium in the gas phase has been obtained. Similarly, a gas-phase infrared spectro- scopic study of dimethylcadmium- 1,4-dioxane, carried out in this laboratory, gave no evidence for the existence of the adduct in the gas phase.4 Some other adducts have also been used in attempts to reduce the pre-reaction in 11-VI systems.Among them are the NEt, adduct of dimethylzinc’ and the tetrahydrothio- phene adduct of dimethylcadmium6. From these and related studies a difference in behaviour between the zinc and cad- mium systems has been noted. In the zinc system, with some adducts, pre-reaction is eliminated,’ whereas elimination of pre-reaction by the use of adducts has never been achieved for the cadmium compounds. The metal atom in dimethylcad- mium is a weaker Lewis acid than that in dimethylzinc. Therefore it has been suggested, that for a given donor ligand, an adduct is more likely to exist in the gas phase with dimethylzinc than with dimethylcadmium.Furthermore, it is said that this difference in gas-phase stability accounts for the difference in behaviour between the zinc and cadmium MOCVD systems. With the aim of discovering the true role of adducts we decided to study a range of adducts of dimethyl- cadmium with the hope of discovering one adduct that is transported undissociated in the gas phase. If such an adduct were obtained it would be used to grow CdY (Y =S or Se) by MOCVD, thus providing some definitive evidence concern- ing the role of adducts in controlling pre-reaction. Previous work in this laboratory on the adduct formed between dimethylcadmium and 2,2’-bipyridy17 provided tenta- tive evidence for the transport of dimethylcadmium-2,2’-bipyridyl (2) in the gas phase, pointing to the strength of the bonding present between the two components, namely dimethylcadmium and a chelating nitrogen Lewis base.How- ever, 2 lacks sufficient volatility to be useful in MOCVD. Complexes of near-planar ligands such as 2,2’-bipyridyl usu- ally pack well in the solid state and thus it is not surprising that 2 is involatile. In attempts to obtain a volatile adduct of dimethylcadmium with a chelating nitrogen ligand it was de- cided to investigate the reaction of the bulky ligand N,N,N’,N’-tetramethylethane-l,2-diamine (Me2NCH2CH2NMe2) with dimethylcadmium and we now report an extensive study of the product of the reaction, namely compound 1. That an adduct is formed by the reaction of dimethylcadmium with Me2NCH2CH2NMe2 has been reported,8 but the nature of the adduct, its structure and reactivity have not be fully investigated. By using mass spectrometry, gas-phase Fourier transform infrared spectroscopy and gas-phase electron diffraction, the nature of 1 in the gas was determined. By using 1 in MOCVD growth studies, further information on the role of adducts in preventing pre-reaction was obtained.Experimental Preparation of 1 Dimethylcadmium was prepared by the reaction of anhydrous CdC12 (98.2 g, 0.54mol) with MeMgI, which was in turn preprared from Mg turnings (30.0 g, 1.23 mol) and Me1 (180.0 g, 1.23 mol), under argon following published pro-cedure~.~The resulting ethereal solution of dimethylcadmium was distilled onto 83.7 g (0.54 mol) of dry 2,2'-bipyridyl under a dynamic vacuum using a glass vacuum line.The bright- yellow adduct thus formed was isolated and heating to 70 "C under a dynamic vacuum in order to liberate pure dimethyl cadmium. Sodium-dried N,N,N',N'-tetramethylethane-1,2-diamine (7.8 g, 0.067 mol) was distilled onto dimethylcadmium (9.2 g, 0.065 mol) held at -196 "C on a vacuum line to produce 15.6 g of the colourless crystalline adduct 1. The excess of amine was removed under vacuum to leave the pure product, which was transferred to an oxygen-free, nitrogen-containing dry box and subsequently packed into a stainless steel bubbler suitable for use in the MOCVD process. Further samples of 1, suitable for use in the electron diffraction study, were prepared as before in evacuated ampoules fitted with Young's taps.In order to carry out the remaining spectroscopic examin- ations, a sample of 1 was prepared in a tapped ampoule. The ampoule was connected to a Vacuum Generator's SXP800 quadrupole mass spectrometer allowing the mass spectrum of the vapour above the sample to be recorded. Gas-phase infrared measurements were obtained by condensing a sample of 1 from the aforementioned ampoule into an evacuated gas cell fitted with KBr windows, thus allowing examination of the vapour above the sample. The gas cell was placed in an oven fitted with CsI windows and raised to a temperature of 50°C. The time taken for the cell to reach the desired temperature (ca. 30 min) is unfortunately long enough to allow some degree of decomposition of the adduct and hence a mixture of 1 and its component species is to be expected in the spectrum.A Perkin-Elmer 1720-X Fourier transform interferometer was used to record the infrared spectrum. Electron Diffraction Study Gas-phase electron diffraction data of the adduct 1 were obtained from the Balzers Eldigraph KDG-2 apparatus at the University of 0~10.'~The nozzle-to-plate distances were 496.71 and 247.01 mm and the data were obtained with the nozzle at room temperature. The electron wavelength (0.058690A) was calibrated against diffraction pictures of benzene. Six plates from the short camera distance and four from the long camera distance, traced on the microdensitometer of the Chemistry Department of The University of Oslo, were used in the final analysis.The data, covering the ranges 2.50 5s/A-514.00 and 6.00 5 s/A-529.00 at intervals of As = 0.25 A-were processed as previously described'* with scattering factors taken from ref. 13. The average curves produced for each camera distance are shown in Fig. 1 together with the theoretical curve for the favoured model (see below) and the difference curves. Analysis of the Structure of 1 The refinements of the structure were based upon the molecule illustrated in Fig. 2, in which is given the atom-numbering scheme. It was assumed that: (1) the molecule has a C2 axis that bisects the LC-Cd-C and LN-Cd-N angles; (2) all methyl groups are identical and have local C3"symmetry with the axis of symmetry being along the Cd-C or C-N bond; (3) all the C-H bonds are of the same length.Using these constraints a model based upon 13 parameters was constructed. These consisted of the bond distances r(Cd-C), r(Cd-N) r(N-C), r(C-C) and r(C-H), the angles LC-Cd-C, LN-Cd-N, LCd-C-H, LN-C-H, LC(lO)-N(3)-Cd and LC(8)-N(2)-C(9), plus two tor- J. MATER. CHEM., 1991, VOL. I 20lo SIP Fig. 1 Intensity curves s[Z,(s)] for 1. The experimental curves are averages of all plates for the two camera distances. The theoretical curve was calculated from the structural parameters shown in Table 1. The difference curves result from subtracting the relevant part of the theoretical curve from the experimental curves Fig. 2 The molecular picture of 1 with the atom numbering scheme sion angles: q51, which is the Cd-N(3)/N(2)-C(6) angle, and q52, which is the H-C(lO)/Cd-N(3) angle.Root-mean-square amplitudes (1) and perpendicular ampli- tude corrections (K) were calculated14 from an assumed force field using values for the force constants obtained for related molecules. Refinements of the structure were carried out by the least- squares procedure" based on the intensity curves, by adjusting the theoretical curve to the two averaged experimen- tal intensity curves using a unit weight matrix. Eleven indepen- dent parameters were refined together: the bond distances r(Cd-C), r(Cd-N), r(N-C), r(C-C) and r(C-H); the angles LC-Cd-C, LN-Cd-N, LC(IO)-N(3)-Cd and LC(8)-N(2)-C(9); and the torsion angles $1 and $2.It was found necessary to fix both LCd-C-H and LN-C-H at 112.0". The vibrational amplitudes were allowed to refine for J. MATER. CHEM., 1991, VOL. 1 r(Cd-N), r[Cd-C(4)], r[Cd-C(6)], and the amplitudes of the Cd-C(methy1 groups on nitrogen) distances e.g. r[Cd-C(8)], were refined as a group. All other amplitudes were held at calculated values. The radial distribution curves were calculated in the usual way, the experimental curve and the difference curve between the experimental and theoretical curves are shown in Fig. 3. Selected bond distances, angles and amplitudes are given in Table 1, and the correlation matrix for the final refinement is given in Table 2. MOCVD Crystal-growth Study Epitaxial layers of CdY (Y =S or Se) were grown on GaAs substrates which had been cut and polished on either the (100) or (1 11) faces.A horizontal reactorI6 was employed and operated at atmospheric pressure. Prior to growth the substrates were etched in a mixture of H2S04-H202-H20(5:1:1), heated to 600 "C, and maintained at this temperature for 10 minutes in a flow of hydrogen to remove surface moisture. The growth conditions are outlined in Table 3. Layers of CdY (Y =S or Se) were grown to a thickness of ca. 0.5 pm over periods of up to 150 min. The layers were found to be of high quality as shown by photolumi- nescence and visible microscopy examinations. Results and Discussion As stated in the introduction, the aim of the study was to identify an adduct of dimethylcadmium that is stable in the gas phase under the conditions used in MOCVD.Initially we present the results of a study of 1 in the gas phase using infrared spectroscopy and mass spectrometry. This study clearly reveals the usefulness and weaknesses of these methods in determining the nature of the compounds formed in the gas phase upon heating 1. Table 1" Final structural parameters for Me,Cd -Me,NCH,CH,NMe, Spectroscopic Study The gas-phase infrared spectrum of 1 is illustrated in Fig. 4. It was obtained by placing a sample of 1 in a gas cell, which, in the apparatus available, took ca. 30 min to reach 50 "C. This is the temperature at which 1 has sufficient volatility for the spectrum to be observable.The spectrum so obtained provides evidence for the presence in the gas phase of a mixture of dimethylcadmium, Me,NCH,CH,NMe,, and the adduct 1. The spectrum in the region associated with the ligand is extremely complex and provides little interpretable information. The most relevant region of the spectrum is in the wavenumber range 600-400cm-' in which there is evi- dence for the existence of free and co-ordinated C-Cd-C moieties. A striking feature of the spectrum is the pair of vibrations seen at 485 and 425 cm-I which is not present in the gas-phase infrared spectrum of either dimethylcadmium or Me2NCH2CH2NMe2. This pair of vibrations we assign to v,, (C-Cd-C) (485 cm-') and vsym (C-Cd-C) (425 cm-') vibrations of the non-linear C-Cd-C fragment of 1. In support of this assignment the corresponding values obtained from the vibrational spectra of solid 2,2'-bipyridyldimethyl- cadrnium(11)~ are 461 and 440cm-'.In addition to the C-Cd-C vibrations of 1 the P and R branches of the asymmetric stretch of the linear C-Cd-C fragment of free dimethylcadmium are observed at 544 and 529 cm- ',respect-ively. The ratio of the intensity of the C-Cd-C vibrations of free dimethylcadmium to those of 1 varied with the length of time the sample was held at 50 "C. Thus, with increasing time the intensities of the vibrations ascribable to free dimethyl- cadmium grew while those of 1fell. We are unable to state if this observation is due to the system slowly attaining an equi- librium in which dissociation to free ligand and metal alkyl is being attained or whether it arises from general decomposition.The mass spectrum of 1 is illustrated in Fig. 5. The parent ion with a relative molecular mass of 258 was not observed. (1) and comparable parameters for Me,Mg -Me2NCH2CH2NMezb selected bond lengths (A) and angles (") for 1calclA Me,Mg .Me,NCH,CH,NMe, (3) 0.058 2.257( 12), 2.227( 12) 0.054 2.166( 12) 1.487(14) 1.570(28) 130.0(8) 81.5(6) 110.7(8) r[ M -N(3)]' r [M -C( 4)]' r~~(3)--~(10)14c--c) 4C--HI LC-M -C' LN-M-N' L41 LC(8)-N(2)-C(9) LM -N(3)-C( 1O)E L 4* LC( 4) -M -N( 3 y LM-N(2)-C(8)' LC(8)-N(2)-C(6) LM -N(2)-C(6)E LN(2)-C(6)-C( 7) r[C(8)* --C(9)] r[C(6)--*C(8)] r[N(2)-* -C(4)] rCN(2).* .C(7)] r[Cd. * -C(6)] r[Cd* * *C(8)] r[Cd-**H(12)] r[Cd. --H(22)] r[Cd--H(23)] r[Cd-* .H(24)] rglA ra/A IrefinedIA 2.47(5) 2.47(5) 0.1Ol(48) 2.12( 2) 2.1 l(2) 0.062(21) 1.484(6) 1.469(6) 0.047' 1.53(4) I .53(4) 0.049' 1.14(1) 1.11(1) 0.079' 132(1 1) 84(3) 19(2) 96(6)120(4) 24(26) selected dependent parameters 108(4) 109.3(4) 120(4) 1 17(4) 113.8(10) 108.4( 10) 88(2)1lO(5) 2.2 1( 10) 0.105' 103.4( 10) 107.6( 18) 2.52(6) 3.70( 11) 0.089' 0.1 44' 2.46( 6) 2.83(7) 3.45(8) 2.7 5(2) 4.15(24) 0.075' 0.123(43) 0.2 59(90) 0.1 19* 0.181' 0.088 0.129 4.25(20) 3.32(12) 0.1 70' 0.236' "Uncertainties in parentheses are, for the electron diffraction study 20 plus estimates for the uncertainties in the electron wavelength etc.and correlation of the data, while for the X-ray study they are 2a. For numbering scheme see Fig. 1. bData taken from ref. 17. The numbering scheme is the one used here and not in the reference. 'M =Cd or Mg. 'Held at calculated value. Table 2 Correlation matrix (x 100) for Me,Cd Me2NCH2CH2NMe2 r[Cd -C( 4)] 0.76 100 21 17 -35 -12 -8 -26 3 -26 23 15 -43 -12 -5 -19 r[Cd -N( 3)] 1.85 100 4 -11 -3 12 -78 8 42 -14 -5 31 10 -1 -48 m3)-c( wi 0.20 100 -68 -4 -10 9 -47 13 23 1 0 2 -4 5 4c-C) 1.41 100 13 5 -6 61 -11 -15 3 11 18 10 -4 4C--HI 0.35 100 -4 -1 11 -8 -3 3 -3 1 1 1 LC-Cd-C 403 100 -9 9 7 -21 -26 2 -5 -1 38 LN-Cd-N 93.6 100 -53 9 25 -3 1 7 -6 63 L 41 83.1 100 -51 -37 14 -37 -24 11 -29 LC(8)-"(2)-C(9) 60 100 37 -1 1 77 30 21 13 LCd -N( 3)- C( 10) I40 100 14 32 34 39 23 L42 921 100 -11 0 4 -13 l[Cd -N( 3)] 1.7 100 54 39 2 1[Cd -C(4)] 0.7 100 19 4 l[Cd.--C(6)] 1.5 100 7 l[Cd* * *C(S)] 3.1 100 "Standard deviations (x 100)from least squares. c 0 r J. MATER. CHEM., 1991, VOL. 1 Table 3 MOCVD growth of CdS and CdSe from 1 growth temp./ "C 300-500 H, carrier gas flow/dm3 min-' 8 adduct flow/cm3 min- ' 50 hydride flow/cm3 min- ' 20 (5% vol mixture H2S or H,Se in H,)growth rate/pm h-' 0.7" 1I:VI mole ratio CU. 1:30-1500 (assuming adduct vapour pressure 0.5 Torrbat 20 "C) "This growth rate is 3 times smaller than that obtained using Me,Cd. 1,4-dioxane., bl Torr z133.322 Pa. C-NCd-C Cd-N C6*C8 CdGG Cd*CB Cd*H22 1 2 3 4 5 6 rlA Fig.3 Radial distribution curve for 1 showing the experimental radial distribution curve, and the difference curve obtained by subtractingthe theoretical radial distribution curve (calculated from the theoreti- cal curve given in Fig. 1) from the experimental radial distribution curve shown here. Both radial distribution curves were calculated from the curves in Fig. 1 after multiplication by [(&dzN/ fCdfN) exp(-0.0025s2)]and using theoretical data for the unobserved area s <2.50 A-'. The contribution of the most important distances are indicated on the radial distribution curves as bars The highest mass number peaks that occur around 243 a.m.u. are assignable to 1 minus a methyl group, giving further evidence for the existence of 1 in the gas phase.We assign the peak at 243 to the ion [MeCd.Me2NCH2CH2NMe2]+ and not [Me2Cd.Me,NCH2CH,NMe]+, as the ion +[MeCd*2,2'bipyridyl] was the ion with the highest mass detected in the mass spectrum of 2.7The peaks observed at 228 and 2 13 a.m.u. are the result of molecules of 1 losing two and three methyl groups, respectively, in the mass spectrometer. Electron Diffraction Study In the gas-phase electron diffraction study of 1 the compound was held at 25 "C and the vapour over the sample allowed to flow into the apparatus. A similar flow of material, from a sample held at 25 "C, was used in the MOCVD experiments. Thus, the results from the electron diffraction study are very likely to reveal the nature of the compound present in the gas phase in the growth experiments.The determination of the structure of 1 by electron diffrac- tion is not a simple procedure and indeed the complexity of the determination proved to be near the limits of the method as shown by the size of the uncertainties associated with some of the values for the bond lengths and angles (Table 1). The results of the electron diffraction study reveal that 1, under the conditions of the experiment, maintains its molecular 600 400 v/cm-' Fig. 4 Gas-phase infrared spectrum of the vapour obtained by heating a solid sample of 1 to 50°C (range 600-400cm-'). The important features give evidence for the presence of free dimethylcadmium [peaks A and B are the P(centred at 544cm-') and R(centred at 529cm-') branches of v,,(C-Cd-C) of a free linear C-Cd-C moiety] and for co-ordinated non-linear dimethylcadmiumfragments [Cv,(C-Cd-C)(425 and D are v,,(C-Cd-C)(485cm-') cm-'), respectively] in the gas phase and 1.o o.8/ 20.6 1 I? $ 0.41 .150 160 170 180 190 200 210 220 230 240 250 miz Fig.5 Mass spectrum of the vapour above a sample of 1 held at ambient temperature integrity in the gas phase. This is shown by a comparison of the experimental and theoretical radial distribution curves where the fit between the two curves is good (Fig. 3). The small discrepancy between the two curves may be attributable to uncertainty in a number of the torsion angles. Alterna- tively, the uncertainty may be due to the occurrence of a small amount (ca.2%) of free dimethylcadmium and Me,NCH,CH,NMe,. The model, which was refined to fit the experimental data, is based upon 13 parameters. The realistic nature of the model adopted is revealed by a comparison of the values obtained for a number of the parameters with the related ones found by X-ray diffraction studies on 2' and (Me2Mg.Me2NCH2CH2NMe2) 3.'' In 1, Me2NCH2CH,NMe2 acts as a chelate with an N-Cd-N angle of 84(3)" which is in agreement with that found in 3 [81.5(6)"].'7 This angle is larger than the related angle found in 2 [64.0(10)"]' reflecting the greater flexibility and bulk of the ligand in 1compared to that of 2,2'-bipyridyl. The increase in the N-Cd-N angle allows an increase in the N-N distance [r,(N--N) 3.30(5)A in 1; 2.670(26)A in 2.7.Not surprisingly, the C-Cd-C angle in 1 [132(11)"] is smaller than that previously found in 2 [148.4(16)"17 but is identical to the C-Mg-C angle in 3 [130.0(8)"].'' The C-Cd distance in 1 [r,(Cd-C)=2.1 l(2)A] is comparable to those in 2 [2.172(50) and 2.152(44) A].' There is also similarity between the Cd-N distances found in 1 [r,=2.47(5) A] and 2 [2.503(26) and 2.535(28) A].' The distances and angles found for the ligand in 1 are comparable to those found in 3 with the exception of M-N-C(methy1) angles (M =Cd or Mg; see Table 1). The differences found between comparable M-N-C angles in 1 and 3 may not be significant, since for 1 there are large uncertainties in the angles and in 3 the ligand is somewhat disordered.Thus, the evidence drawn from the comparison between 1, 2 and 3 supports the statement made above that the model used in the electron diffraction study is a realistic one and that it is reasonable to suppose that 1 is transported in the vapour state with no or very little decomposition. MOCVD Crystal-growth Study From 1 and H2Y (Y=S or Se) good-quality layers of CdY (Y =S or Se) were grown onto the (100) or (1 11) planes of crystalline GaAs supported on a heated susceptor. Pre-reac- tion was, however, observed on the reactor walls upstream from the susceptor. Thus, although the compound exists as an adduct in the gas phase, reaction of dimethylcadmium with the Group VI hydride appears to take place in the cold zone. Therefore adduct existence alone is not sufficient to suppress the unwanted pre-reaction.In the reactors commonly employed to grow 11-VI compounds, in order to achieve suitable flow conditions, the precursors pass through a mixer nozzle and over an inclined plane held at near room tempera- ture prior to reaching the heated GaAs substrated. It is known that the thermal decomposition of dimethylcadmium takes place as a mixture of gas-phase and surface reactions even when the walls of the reactor vessel have been carefully conditioned.'* It was decided therefore to investigate if the nature of the material from which the inclined plane and mixer nozzle were made influenced the occurrence of pre-reaction.Three mater- ials were investigated, namely boron nitride, sapphire and stainless steel and with each of them pre-reaction occurred. In addition experiments were conducted to investigate the influence of heating these materials in the reactor to 500 "C for ca. 0.5 h prior to conducting a growth experiment. It was found that the treatment did not reduce the amount of pre- reaction. It is known that dimethylcadmium undergoes surface reactions leading to the formation of deposits.18 It was found that the passage of pure dimethylcadmium through the nozzle and over the the inclined plane did not lead to any measurable amount of deposition. However, as soon as H2Y (Y=S or Se) was added to the dimethylcadmium, pre-reaction occurred. Thus, the pre-reaction truly is a reaction between dimethylcad- mium and H2Y.As stated earlier, the use of dimethylcad-J. MATER. CHEM., 1991, VOL. 1 mium- 1,4-dioxane,' an adduct that is fully dissociated in the gas phase, led to reduction in the amount of pre-reaction. One way in which the 1,4-dioxane could reduce pre-reaction is through its coating of the surface of the nozzle and inclined plane. It may be an advantage for the adduct to dissociate in the gas phase and allow the ligand to adhere to the surface. Using infrared spectroscopy we have found that 1,4-dioxane is readily adsorbed onto the surface of silica and that once adsorbed it is difficult to remove." Conclusions Using electron diffraction we have been able to show that 1 can be transported without dissociation in the gas phase.From mixtures of 1 and H2Y (Y=S or Se) we have been able to grow visibly good-quality layers of the semiconductor materials CdY (Y =S or Se). The problem of pre-reaction was not eliminated, however, and we can therefore conclude that the reduction of pre-reaction experienced when using some adducts of dimethylcadmium in the growth of CdS and CdSe is not a function of gas-phase adduct transport. It is suggested that the role of the ligands may be to coat surfaces and so prevent dimethylcadmium and H2Y (Y =S or Se) undergoing a series of surface reactions and we are currently investigating this possibility. We thank SERC and RSRE (Malvern) for a CASE award to M.P.B. SERC is also thanked for the provision of the mass spectrometer.Special thanks are due to Mr. Hans Volden and Ms. Snefrid Gundersen for their help in collection and primary manipulation of the electron diffraction data, and to the Norwegian Research Council for Science and the Humanities for their financial support of K.H. and the provision of the electron diffraction facilities at The University of Oslo. References 1 B. Cockayne, P. J. Wright, A. J. Armstrong, A. C. Jones and E. D. Orrell, J. Cryst. Growth, 1988, 91, 57. 2 P. J. Wright, B. Cockayne, A. C. Jones, E. D. Orrell, P. O'Brien and 0. F. Z. Khan, J. Cryst. Growth, 1989, 94, 97. 3 0. F. Z. Khan, P. O'Brien, P. A. Hamilton, J. R. Walsh and A. C. Jones, Chemtronics, 1989, 4, 244. 4 M. J. Almond, M.P. Beer, M. G. B. Drew and D. A. Rice, J. Organomet. Chem., in the press. 5 P. J. Wright, P. J. Parbrook, B. Cockayne, A. C. Jones, E. D. Orrell, K. P. O'Donnell and B. Henderson, J. Cryst. Growth, 1989, 94, 441. 6 A. C. Jones, S. A. Rushworth, P. J. Wright, B. Cockayne, P. O'Brien and J. R. Walsh, J. Cryst. Growth, 1989, 97, 537. 7 M. J. Almond, M. P. Beer, M. G. B. Drew And D. A. Rice, Organometallics, 199 1, 10, 2072. 8 P. R. Jacobs, E. D. Orrell, J. B. Mullin and D. J. Cole-Hamilton, Chemtronics, 1986, 1, 15. 9 E. Krause, Ber. Dtsch. Chem. Ges., 1917, 50, 1813. 10 W. Zeil, J. Hasse and L. Wegmann, 2. Znstrumentenkd., 1966,74, 84. 11 K. Tamagawa, T. Iijima and M. Kimura, J. Mol. Struct., 1976, 30, 243. 12 K. Hagen, R. J. Hobson, C. J. Holwill and D. A. Rice, Znorg. Chem., 1986, 25, 3659. 13 L. Schafer, A. C. Yates and R. A. Bonham, J. Chem. Phys., 1971, 56, 3056. 14 R. L. Hilderbrandt and J. D. Weiser, J. Chem. Phys., 1971, 55, 4648. 15 K. Hedberg and M. Iwasaki, Acta Crystallogr., 1964, 17, 529. 16 P. J. Wright and B. Cockayne, J. Cryst. Growth, 1982, 59, 148. 17 T. Greiser, J. Kopf, D. Thoennes and E. Weiss, J. Organomet. Chem., 1980, 191, 1. 18 C. M. Laurie and L. H. Long, Trans. Faraday SOC., 1957, 53, 1431. 19 M. J. Almond, M. P. Beer and D. A. Rice, unpublished. Paper 1/03576B; Received 15th July, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101065

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1071-1077 Alkali-metal Fluorides supported on y-Alumina Surface Reactions involving 18F- and 35S-labelled Sulphur Tetrafluoride and Thionyl Fluoride, 35S-labelled Sulphur Dioxide, l8F=and 14C-labelled Carbonyl Fluoride and 14C-labelled Carbon Dioxide Thomas Baird, Abdallah Bendada, Geoffrey Webb* and John M. Winfield* Department of Chemistry, University of Glasgow, Glasgow G12 8QQ, UK Reactions between the Lewis acids, SF,, F,SO, SO2, F2C0 and CO,, radiolabelled with 14C, 18F and 35Sas appropriate, and CsF or KF supported on y-alumina have been studied under heterogeneous conditions at room temperature. Fluorine-containing Lewis acids react in two ways, with surface F- anions to form Lewis acid- base complexes and to fluorinate surface hydroxyl groups.Maximum surface F-anion activity occurs at a metal fluoride loading of 5.5 mmol g-'. The reaction between CsF or KF and y-alumina to give hexafluoroalumin- ate salts, which occurs during the impregnation process, becomes more important as the metal fluoride loading increases over the range 0.6-20.0 mmol g-'. This reaction is accompanied by dehydroxylation of the surface of the support, but there is no evidence for the presence of free hydroxide anions. Keywords: Alkali-metal fluoride ; y-Alumina ; Radiotracer; Transmission electron microscopy; Surface species The widespread use of anhydrous ionic fluorides as bases in organic synthesis' has generated a large number of appli- cations in which ionic fluorides are supported on high-surface- area inorganic oxides in order to carry out reactions more efficiently under heterogeneous conditions.Several materials have been suggested as supports,2 but potassium fluoride supported on chromatographic alumina appears to be the reagent of choice in most situation^.^ Potassium fluoride supported on alumina is a complex material whose surface properties are composition dependent. Impregnation of alumina with KF in aqueous solution leads to the formation of K3AlF6 to some extent, possibly according to4 12KF+3H20+A1203 +6KOH +2K3AlFs (1) This has resulted in the suggestion that the basic properties of KF/alumina are due wholly' or in art^,^ to the presence of 'free' hydroxide anions on the surface. "F MAS NMR spectro~copy~~'and fast-atom bombardment mass spec-trometry (FABMS)' have provided good evidence for the presence of well dispersed F-ions on alumina at low loadings of KF, but the evidence for KOH is inconclusive.There was no evidence in the FABMS study for OH-except in very heavily loaded samples.' The carbonate anion has been ident- ified by FTIR spectroscopy in alkali-metal fluorides supported on alumina after exposure to the atmosphere;' this is indirect evidence for free OH- anion on the surface. In an attempt to clarify the surface properties of alkali- metal fluorides supported on alumina, particularly their vari- ation with the composition of the materials, we have studied the behaviour of CsF and KF supported on y-alumina towards volatile Lewis acids.The radiotracer approach used is similar to that employed to quantify the process of activating unsup- ported CsF for use as a heterogeneous base or catalyst." Our results are in agreement with many of those from previous physicochemical st~dies,~,',' notably the predominance of MF, M=Cs or K, at low loading and the formation of M3AlF6 as a result of the impregnation process. However, it is considered that eqn. (1) is unrealistic for describing the process, and an alternative explanation is offered based on a consideration of the y-alumina surface, an aspect that has been largely ignored in previous work. Some parts of this work have been reported in a preliminary communication.' Experimental Except where noted below, the vacuum, dry-box and radio- tracer techniques used in this work have been described previously.",' Instrumentation For the XRD measurements, a Philips diffractometer was used with Cu-Ka radiation and the powdered samples were mounted on adhesive tape in a glove box with the tape rolled into a cylinder; for TEM (JEOL JEM 1200 Ex) specimens were prepared by dipping an adhesive-coated carbon-filmed grid in the powder and were then inserted immediately into the microscope to minimise hydrolysis during transfer; for 27Al MAS NMR spectroscopy [Varian UXR-300/89 spec- trometer (IRL, University of Durham) at 76.1 52 MHz], samples were contained in a 0.3 cm3 zirconia tube; for vibrational spectroscopy (PE 983 and Spex Ramalog spec- trometers) solids were sampled in sealed Pyrex capillaries or as Nujol mulls, and gases were contained in a Pyrex cell with AgCl windows; B.E.T.areas were determined using a Pyrex system with N2 as adsorbate. The compositions of volatile product mixtures resulting from reactions of volatile Lewis acids with supported metal fluorides were determined by IR spectroscopy using a calibrated Pyrex manifold equipped with a constant volume manometer and IR gas cell, and experimen- tally determined pressure us. peak area calibration relationships. Materials The Lewis acids used were either commercial samples, C02 and SO2which were dried by multiple distillations over P205, or were prepared by standard methods, SF4,13 F2S0,14 F2C0,15 and stored in metal pressure vessels over activated NaF.Immediately before use, SF4 was purified further via its BF3 adduct.16 The labelled compounds SF3"F, 3'SF4, 18FFC0 and I4CO2 were prepared as previously described," F23'S0 and 3'S02 by hydrolysis of 3'SF4 over calcined y-alumina at 295 and 373 K, respectively, and "FFSO by hydrolysis of SF318F over calcined y-alumina at 295 K. 14C- J. MATER. CHEM., 1991, VOL. 1 Labelled F2C0 was prepared from 14C02 by the sequence The contents were shaken vigorously for 2 h, MeCN was removed in vacua at room temperature, and the vessel was 14C02 Zn CIF NaF F2 14C0 then heated under vacuum at 398 K for 16 h to decompose 14C0 C1F14C0 the Cs[OCF(CF,),]. Finally the material was calcined at (2) 523 K for 5 h and stored in the glove box.Bands in the IR The radiochemical purity of all 18F-labelled compounds was spectrum of the material due to the [(CF3),CFO]- anion checked by half-life and y-ray spectrum determinations and were absent. linear relationships between count rate and pressure were Preparations in which various quantities of dried metal verified experimentally for all 14C- and 35S-labelled com- fluorides and calcined y-alumina were ground together in the pounds. glove box, followed by calcination at 523 K, led to irreproduc- ible B.E.T. results from different preparations; this method of preparation was therefore discontinued. Preparation of Caesium and Potassium Fluorides supported on y-Alumina ResultsSupported metal fluorides, composition range 0.6-20.0 mmol g-', were prepared following the literature method4 from CsF y-Alumina from two sources (Degussa and Pural) was used or KF (B.D.H.Ltd. Optran grade) and y-alumina (Degussa in this work with essentially identical results. For consistency C or Pural Sb90). In a typical preparation, KF (44.0mmol) the data reported in Tables 1-4 refer to metal fluorides in distilled water (130 cm3) and y-alumina (10.0 g) were mag- supported on Degussa C y-alumina. netically stirred at room temperature for 1 h, the bulk of the water was removed by rotary evaporation at ca. 333 K, and the solid calcined under dynamic vacuum at 523 K for 5 h. Examination of y-Alumina-supported Caesium and Potassium All materials were stored under dry N2 in the glove box.Fluorides by Physical Methods Similar materials were prepared under non-aqueous con- B.E.T. areas of CsF and KF supported on y-alumina are ditions using alkali-metal heptafluoroisopropoxides,'7 dry given in Table 1. Impregnation of y-alumina by MF, M=Cs acetonitrile," and y-alumina previously calcined at 523 K. or K, under aqueous conditions resulted in marked reductions Typically, y-alumina (5.0g) was added in the glove box to a in B.E.T. area. At least seven samples, normally from different solution of CS[OCF(CF,)~] (22.0 mmol) in MeCN (20 cm3), preparations were examined for each metal fluoride loading. the mixture being contained in a Monel metal pressure vessel. Both preparative methods were satisfactory in the degree of Table 1 B.E.T.areas" of KF and CsF supported on y-alumina after calcination at 523 K (CsF/y-alumina)/m2 g- (KF/y-alumina)/m2g-' metal fluoride loading/ mmol g- ' prepared from H,O prepared from MeCN prepared from H20 prepared from MeCN 1.1 124-135 140-156 2.0 76-88 62-73 104-1 16 80-93 4.4 55-65 40-50 91-101 68-8 1 5.5 51-61 3 1-47 76-88 5 1-69 6.0 42-56 24-32 69-8 1 47-54 8.8 33-45 19-26 51-65 32-4 1 15.0 15-27 10-19 29-39 17-25 20.0 11-18 19-29 a 95% Confidence limits. B.E.T. area of y-alumina (Degussa C) calcined at 523 K = 155-165 m2 g-'. Table 2 Summary of examination of KF supported on y-alumina by XRD and TEM identified by XRD identified by TEM loading/mmol g- KF y-alumina K,AlF, KF particle size/A y-alumina 2.0 35 4.4 33 5.5 40 8.8 20.0 " An unidentified 'Al-F' phase was also present.Table 3 "A1 NMR chemical shifts" of metal fluoridely-alumina CsF/y -alumina KF/ y-alumina 4.4 54.3 5.4, -1.6 59.3 4.9, -0.7 8.8 61.6 5.2, -1.6 59.9 4.8, -1.8 10.0 60.5 4.8, -1.6 58.7 4.0, -1.3 20.0 55.2 -1.8 71.2 -1.3 =W.r.t. aluminium(III) chloride (aq). For y-alumina calcined at 523 K 6(27Al,,t)=73.1 and 6(27A10c,) 6.9. J. MATER. CHEM., 1991, VOL. 1 Table 4 18FExchange occurring on exposure of CsF or KF supported on y-alumina to OCF18F or SF318F at room temperature" ~~ volatile fluoride 18F specific count rate/ "F count rate from solid/ count min-' (mg atom F)-' count min- metal fluoride and loading/ mmol g- initialb finalb expt.' calc.' (a) Reactions with OCF18F CsF 4.4 14 598 12 624 23 889 24 096 CsF 4.4 14 028 12 134 24 045 24 682 KF 4.4 14 598 13 030 24 996 25 756 KF 4.4 14 028 12 521 23 499 23 373 CsF 8.8 13 408 12 234 15 375 15 732 CsF 8.8 14 904 13 531 18 417 18 723 ICF 8.8 13 408 12 431 19 161 19 623 KF 8.8 14 904 13 892 20 769 21 314 (b)Reactions with SF318F CsF 8.8 35 955 30 089 84 903 84 221 CsF 8.8 39 731 33 761 94 312 95 217 KF 8.8 35 955 32 161 90 41 1 91 007 KF 8.8 39 731 35 920 99 841 99 576 ~~~ Reaction conditions 1 h using 0.5 g supported fluoride and 1 mmol (300 Torr initial pressure) volatile fluoride.fraction of 18F exchange (%) 34 34 27 27 18 19 15 14 44 47 31 30 Error < f2%.'Determined as described in the text. Error 51%. reproducibility obtained. The data were broadly in agreement with a previous, more limited, e~amination.~ B.E.T. areas decreased as the metal fluoride loading increased and areas of KF/y-alumina were slightly greater than those of CsF/y- alumina. However, the effect of increasing the MF, M =Cs or K, loading was far more marked than the effect of a change in cation. Samples prepared by impregnation of y-alumina by M[OCF(CF,),] in MeCN, followed by thermal decompo- sition of the alkoxide and calcination of 523 K, resulted in slightly smaller B.E.T. areas for each loading and cation but in other respects showed identical behaviour.Samples that were calcined at 773 rather than 523 K had smaller surface areas. Representative results for samples prepared by the non- aqueous route were: loading 4.4mmol g-', 40-50 (Cs) and 68-81 (K) m2 g-'; loading 8.8 mmol g-', 19-26 (Cs) and 32-41 (K) m2 g-'. Structural studies of CsF/y-alumina and KF/y-alumina by powder XRD and TEM indicated, in agreement with previous XRD re~ults,~,'~F MAS NMR4,7,8 and FABMS9 studies, that metal fluoride particles were present at low loadings of metal fluoride. TEM examination of KF/y-alumina samples indi- cated that KF was well dispersed at loadings G4.4 mmol g-' (Table 2), however, the particles were mobile on the surface of the samples under the influence of the electron beam; consequently, quite large, well formed KF particles were detected (Fig.1). As expected y-alumina was present at all compositions examined (Table 2); the only other aluminium- containing species positively identified were M3A1F6, M =Cs or K, which were detected by XRD at loadings 28.8 mmol g-(Table 2). The latter finding is consistent with the previous XRD study of KFly-al~mina.~ Strong bands in the IR spectra of MF/y-alumina samples, loading >5.5 mmol g-I, at v,,, 580 and 395 cm-' indicated the presence of [A1F613-by comparison with the spectra of unsupported [A1F6I3-salts;'* these bands were absent in samples of lower MF loading whose spectra were almost identical to that of calcined y-alumina. Raman spectra con- tained a very broad band, vmaX 596cm-', at MF loadings >6.0 mmol g- ', which was not observed at lower loadings.The band could be due to v1 of [A1F6I3- although it occurred at a higher wavenumber than v1 in unsupported [A1F6I3- salts.l9 27Al MAS NMR spectrum of MF/y-alumina samples were similar at all MF loadings (4.4-20.0 mmol g-I) examined and Fig. 1 Electron micrograph and diffraction pattern for KF/y-alumina, loading 4.4 mmol g-consisted of two broad signals due to 27Al nuclei in tetrahedral and octahedral environments2' (Table 3). The signals at lower applied frequency resembled a combination of those from calcined y-alumina and K3A1F6, prepared by a literature method,21 and thus the component ca. Sppm was assigned to 27Aloc, in y-alumina and that at ca.-1.6 ppm to [A1F613-. The latter component appeared to grow in intensity with increasing MF loading and the higher-frequency component was observed only as a shoulder in the 20.0 mmol g -samples. Although the identification of the [A1FJ3-anion by 27Al MAS NMR spectroscopy must be regarded as tentative, the line shapes were distinctly different from those obtained from fluorinated y-alumina samples and there was no evidence for AlF3 nor for any other simple fluoroaluminate(Ir1) species. Interaction of y-Alumina-supported Caesium and Potassium Fluorides with Volatile Lewis Acids Exposure of y-alumina-supported metal fluorides, prepared from aqueous solution, to sulphur tetrafluoride at room temperature resulted in very exothermic reactions, particularly using samples in which the MF loading was low; for example, the temperature of the outer surface of the Pyrex reaction vessel rose to >400 K when the MF loading was 4.4 mmol g- l.Thionyl fluoride and sulphur dioxide were identified as products in all cases; unchanged SF, was identified only at MF loadings >5 mmol g- '. Similar behaviour was observed using F2S0 or carbonyl fluoride, although the reactions were less exothermic. The changes in composition of the volatile products with variation in the loading of CsF or KF (Fig. 2) indicated that although hydrolysis occurred in every case its extent was greater when M =K. In both materials the extent of hydrolysis decreased as the loading of MF increased.There was no evidence for SiF, in the vapour phase, suggesting that HF, which must have been the other product from the hydrolysis reactions, was retained completely by the surface. The IR spectra of the solids after removal of volatile material and pumping at room temperature, showed that the anions [SF,]-(ref. 22) and [FS02]-(ref.23), [FSOJ, and [F3CO] -(ref. 24) had been formed from reactions that involved SF,, F2S0 and F,CO, respectively. There was no evidence for the formation of the anions [HF2]-, [F3SO]- or [FC02]-. Admission of 35SF4, F235S0, or 35S02 to MF/y-alumina samples at room temperature led to rapid growth in 35S surface count rates indicating that adsorption and/or reaction of the Lewis acid had occurred on the surface.When material from the vapour phase was removed, the 35Ssurface count rates fell to <30% of their maximum values, with the magnitude of the decrease depending on the adsorbate used, the identity of MF, and its loading. Multiple treatment of CsF/y-alumina (4.4mmol g- ') with 35SF4 (initial pressure 3 10 Torrt) indicated (Fig. 3) that although small increases in 35S activity occurred on the surface, the 35S surface count rate due to permanently retained species, presumably [SF,] -and [FS02]-, remained constant. Similar behaviour was observed when F214C0 or 14C02 were admitted to MF/y-alumina samples except that the surface interactions were complete within the time of the first 14C surface count-rate determination. Removal of volatile material after F2I4CO treatment resulted in 14C surface count rates that were <45% of the original values depending on the MF loading. On removal of 14C02, however, surface count rates decreased to the background level irrespective of loading and identity of M.In all cases the maximum 35Sor 14C surface count rates were pressure-dependent, over the range 10-350 Torr, and in contrast to unsupported CsF," saturation coverages of the surface were not observed. More importantly, the surface count rates varied with the identity of MF and its loading in a characteristic way, as illustrated for F214C0 in Fig. 4. The maximum surface count rate occurred at a loading of 5.5 mmol g-'; for a given loading CsF had a slightly larger value, and at 20.0 mmol g-' the count rate decreased to background levels when the volatile material was removed.Admitting SF318F, 18FFS0, or 18FFC0 to MF/y-alumina samples at room temperature led to immediate detection of 18Factivity from the solids. Growth in the 18F count rate t 1 Torrzl33.322 Pa. Fig. 2 Variation in the volatile product composition (mol%) after exposure of (a)SF,, (b) OSF, and (c) F,CO (1 mmol in each case) to CsF/y-alumina and KF/y-alumina samples (0.5g), loadings in the range 1.1-15.0 mmol g-'. Reaction conditions, 1 h at room tempera- ture, initial pressure 300 Torr. ., OSF,; & SO,; *, F,CO;SF,; 0, 0,co2 84 -h s-E 72-Y !2 601 a 48 > 36-.-0 c..-rn g 24-E 8 l2 t 0 961 884 h s-8 72.Y rn c 60 L P Q) -48, 0 r-O 36 .-0 Y.-24 E 8 l2 I1 0 96-cc 1 84 -s-72-Y rn Y 02 60-2 P ," 48-c -0 5 36-C .-Y.-24-E8 12 -I 0 J. MATER. CHEM., 1991, VOL. 1 2.5 5.0 7.5 10.0 12.5 15.0 (MF/y-alumina loading)/rnmol g-' I I I I I I 2.5 5.0 7.5 10.0 12.5 15.0 (MF/y-alumina loading)/rnrnol g-' I I I I I I 2.5 5.0 7.5 10.0 12.5 15.0 (M Fly-al urn ina load ing)/rnmol g-' J. MATER. CHEM., 1991, VOL. 1 desorption time/min 60 50 40 30 20 10 0 0 10 20 30 40 50 60 adsorption tirne/min Fig. 3 Adsorption-desorption behaviour observed on exposure of CsF/y-alumina, loading 4.4 mmol g-', to "SF, at room temperature. Initial pressure of SF, =310 Torr in each case r .-C E 90r 1 1 I I I 1 zo 5 10 15 20 (MF/y-alumina loading)/rnmol g-' Fig. 4 Variation of 14C surface count rate with composition.Upper trace CsF/y-alumina, lower trace KFly-alumina. Initial pressure of F2C0=300 Torr was rapid over the first 0.5 h and slower thereafter. Removal of volatile material after 1 h led to small decreases in solid count rates but most of the activity was retained even after pumping. Specific ''F count-rate determinations for SF318F, '*FFSO, and "FFCO before and after reaction indicated that partial "F exchange had occurred between MF/y-alum- ina (8.8 mmol g-') and SF3"F or ''FFCO but not with I8FFSO. The extent of "F exchange was marginally greater when M=Cs, and in the "FFCO, MF/y-alumina systems, where measurements were possible for the loading 4.4 mmol g-', it was greater at the lower loading (Table 4).In separate experiments it was established that there was no observable "F exchange at room temperature between SF318F or "FFCO and M3AlF6, M =Cs or K, compounds under hetero- geneous conditions, therefore the ''F exchange reactions with supported metal fluorides were ascribed to the presence of labile C1'FF2C0]- or [SF418F] -anions as the intermediates. "F Exchange between unsupported CsF and "FFCO has been observed at room temperature but there was no observ- able exchange between CsF and SF3"F under identical conditions." The exchange between SF318F and supported CsF observed here may have been the result of local heating from the hydrolysis reaction leading to the reversible decomposition of the [SF,"F] -anion.The assumption that "F activity produced in MF/y-alum- ina was the result of hydrolysis of SF3"F or "FFCO on the surface, formation of [SF,"F-] and [18FS021-or ["FF2C0]-, and "F exchange was tested by calculating the solid count rates that would result from the combination of the three routes (Table4). The extent of hydrolysis was determined from vapour-phase IR and manometric measure- ments (Fig. 2) and anion formation from manometric studies and vapour phase 35Sor 14C data. Combining these with the "F data led to calculated "F solid count rates that were in satisfactory agreement with those determined experimentally (Table 4).The behaviour of the "F count rate from the solid, deter- mined after 1 h exposure to SF3"F, "FFSO, or "FFCO at room temperature, with change in the loading of MF was identical, irrespective of the Lewis acid or metal fluoride used and is illustrated for "FFCO vs. KF/y-alumina and CsF/y- alumina in Fig. 5. The largest count rates were observed at the lowest loading (0.6 mmol g-') used. They decreased sharply over the range 0.6-10.0 mmol- ', there being relatively little change thereafter. Count rates from KF/y-alumina were slightly larger than those from their CsF counterparts. Although the behaviour of MF/y-alumina samples, pre- pared from the MeCN route, towards volatile Lewis acids was not investigated in detail, it appeared to be similar in most respects to that described above.The most noticeable difference was that hydrolysis occurred to a smaller extent. Discussion The behaviour of the fluorine-containing, volatile Lewis acids towards CsF and KF supported on y-alumina indicates that the main reactions occurring at the surface are the hydrolyses SF4-+F2SO+S02 and F2CO+C02 and formation of Lewis acid-base complexes. Radiotracer experiments using the weak p-emitters 14C and 35Sprobe only the surface, since self-absorption prevents the detection of radiation from the bulk solid. Weakly adsorbed surface species are observed in all cases and perma- nently retained species in all cases except for C02. The results obtained are very similar to those of an earlier radiotracer m i 1 1 1 1 eo 5 10 15 20 (MF/y-alumina loading)/mmol g-' Fig.5 Variation of '*F solid count rate with composition. Upper trace KF/y-alumina, lower trace CsF/y-alumina. Initial pressure of F2C0=300 Torr; count rate determined after 1 h in each case investigation using unsupported CsF. lo This finding, together with the spectroscopic detection of the fluorine-containing anions that would be expected from the addition of F-ions to SF4, SO2 and F2C0, suggest strongly that F-ions are directly involved in the Lewis base adsorption sites. Although C02 and SO2 are both weakly adsorbed on the surface of calcined y-alumina, it does not appear that there is a major contribution from this source to the observed variation in 14C and 35S surface count rates with change in loading of MF/y-alumina, M =Cs or K (Fig.4). If this were the case a closer correlation with B.E.T. areas (Table 1) would have been expected. It is considered, therefore, that the surface count rate us. MF loading relationships reflect the variation in F-ion surface concentrations. The maximum surface concentration of F-ion is observed at 5.5mmol g-I for both supported fluorides (Fig.4). A loading of 5 mmol g-I of KF on alumina has been shown to correspond to the optimum reagent activity for methylation of phenol, while the greatest catalytic activity in Michael addition reactions was observed at a loading of 0.6mmol g-1.4 Our TEM study shows the existence of KF particles on the surface at loadings <5.5 mmol g-' (Table 2).Taken overall, the structural and spectroscopic evidence indicates, in agreement with previous w~rk,~~~~~ that <5.5 mmol g- MF is the major fluorine-containing species on the surface. There is no evidence, however, for monolayer coverage of the surface by MF, as has been assumed implicitly by some previous worker^.^,^ Increasing the loading of MF above 5.5 mmol g-' leads to a sharp decrease in 14C and 35Ssurface count rates (Fig.4), indicating that the conversion of MF to M3A1F6 during the impregnation process becomes more important as the loading of MF increases. The only fluoroaluminate positively ident- ified, in agreement with previous was [A1F6]3- but, rather surprisingly, TEM did not reveal its presence. Because of this, we suggest tentatively that M3AlF6 is located in the bulk material rather than on the surface.Information concerning the nature of the support can be inferred from the composition of the volatile products after exposure of SF4, F2S0, and F2C0 to the supported metal fluorides and from the variation of 18F count rates from the solids with change in MF loading. The rapid and exothermic hydrolysis reactions that occur, are consistent with the use of y-alumina to catalyse hydrolysis and methanolysis of acyl and phosphoryl fluorides under heterogeneous condition^.^' NMR spectroscopic investigation of alumina impregnated with aque- ous NH4F and subsequently calcined at high temperature, has been interpreted on the basis of the replacement of Al-OH surface groups by Al-F groups,26 and related behaviour is shown by sili~a.~~,~~ In the present work the hydrolysis reactions become less dominant as the MF loading, and thus the proportion of M3AlF6, increases (Fig.2). This is to be expected since the B.E.T. areas decrease (Table l), although if free MOH were to be formed according to eqn. (l), the reverse effect would be expected. A decrease in the 18Fsolid count rate is observed also as the MF loading increases, irrespective of the fluorine- containing Lewis acid used (Fig. 5). The reactions of '*FFSO with the supported metal fluorides do not involve 18F exchange nor formation of [F,SO]-, hence the 18F count rates for the solids must arise solely from 18Fdeposited on the surface as a result of hydrolysis.This source appears to be the major contribution in the other reactions, where 18F exchange and complexation with F-ion both occur (Table 4). Rather surprisingly, there is no evidence that HF, which is a product expected from the hydrolysis reactions, is released to the vapour phase. The nature of the surface-adsorbed HF is speculative but it is possible that it is dissociatively adsorbed J. MATER. CHEM., 1991, VOL. 1 in a manner similar to that suggested for anhydrous HCl on calcined y-alumina.28 It is noteworthy that 18Fcount rates from supported KF are generally greater at a given loading than those from supported CsF (Fig. 5 and Table 4).This is consistent with the volatile product compositions after hydrolysis (Fig. 2) and the greater B.E.T. areas of supported KF materials (Table 1). In contrast, 14C and 35S surface count rates for which the major contributions are from interactions that involve F-ions, are in the reverse order, supported CsF >supported KF (Fig. 4). The composition of the defect spinel29 y-alumina is [A12 .,(vacancy), in which the hydroxyl groups are exclusively on the ~urface.~'.~~ A recent TEM is in agreement with the earlier suggestion29 that the predominant surface plane is (1 10). Because of its amphoteric nature, a y-alumina surface can be modified by exposure to dilute solutions of aqueous electrolytes, leading to the specific adsorption of Group 1133or Group 134cations or F-anion.34c We suggest that under the impregnation conditions used to prepare supported alkali-metal fluorides, surface adsorption of F-ions is followed by further reaction leading to [A1F6I3-. Were metal hydroxides to be formed in this reaction, according to eqn. (l), the extent of hydrolysis observed on exposure of the supported metal fluorides to SF4, F2S0 or F2C0 would have been expected to increase with increased metal fluoride loading, which is contrary to the behaviour observed.In order to preserve charge neutrality, we suggest that the reaction of MF, M=Cs or K, with y-alumina to give M3A1F6 is accompanied by dehydroxylation of the y-alumina surface to give H20 and surface [Al-0-1 groups which act as specific sites for M ad~orption,~~.~~+ and which are also potentially basic sites.Calcination of y-alumina also leads to a partial dehydroxyl- ation of the surface, the extent of which depends on the calcination temperature, although complete dehydroxylation cannot be achieved without loss of the y-alumina struc-t~re.~'.~~Weakly bound surface water can be removed more easily, for example the results of a recent study of the chlorination of y-alumina, calcined at various temperatures, by anhydrous HC1 are consistent with the removal of weakly bound surface water below 373 K.28 Some of the surface water on the supported metal fluoride, however, is likely to be strongly bound, for example via hydrogen bonding to the MF particles, and will not be removed completely by calcination at 523 K.The importance of 'residual water' in determining the activity and the selectivity of oxide-supported metal fluorides3v4 and other supported reagents3' is well docu- mented. The behaviour of SF4, F2S0, and F2C0 towards MF/y-alumina materials of varying composition does not enable the effects due to the hydroxylated support surface and strongly bound water to be differentiated. Both are likely to be important. We thank SERC for support of this work, the staff of the SERC solid-state NMR service (University of Durham) for the 27Al NMR spectra, and a referee for helpful comments. References 1 J. H. Clark, Chem. Rev., 1980, 80, 429; G.G. Yakobson and N. E. Akhmetova, Synthesis, 1983, 169.2 e.g. J. Yamawaki and T. Ando, Chem. Lett., 1979, 755; J. H. Clark, A. J. Hyde and D. K. Smith, J. Chem. SOC., Chem. Com- mun., 1986,791; J. Ichihara, T. Matsuo, T. Hanafusa and T. Ando, J. Chem. SOC., Chem. Commun., 1986, 793; R.G. Sutherland, A. S. Abd. El Aziz, F. Piorko and C. C. Lee, Synth. Commun., 1987, 17, 393; L.M. Harwood, G.C. Loftus, A.Oxford and C. Thomson, Synth. Commun., 1990,30, 649. J. MATER. CHEM., 1991, VOL. 1 1077 3 e.g. T. Ando, J. Yamawaki, T. Kawate, S. Sumi and T. Hanafusa, Bull. Chem. SOC. Jpn., 1982, 55, 2504; J. H. Clark, D. G. Cork and M. S. Robertson, Chem. Lett., 1983, 1145; J. Yamawaki, T. Kawate, T. Ando and T. Hanafusa, Bull. Chem. SOC. Jpn., 1983, 56, 1885; J. H. Clark, D.G.Cork and H. W. Gibbs, J. Chem. SOC., Perkin Trans. I, 1983, 2253; D. Villemin and M. Ricard, Tetrahedron Lett., 1984, 25, 1059; F. Texier-Boullet, 19 20 21 1982,495,200; U. Bentrup, R. Stodolski and L. Kolditz, Z. Chem., 1986, 26, 187. K. Wieghardt and H. H. Eysel, 2. Naturforsch., Teil B, 1970, 25, 105; M. J. Reisfeld, Spectrochim. Acta, Part A, 1973, 29, 1923. D. Muller, W. Gessner, H-J. Behrens and G. Scheler, Chem. Phys. Lett., 1981, 79, 59. H. M. Haendler, F. A. Johnson and D. S. Crocket, J. Am. Chem. D. Villermin, M. Ricard, H. Moison and A. Foucaud, Tetra- SOC., 1958, 80, 2662. 4 5 hedron, 1985, 41, 1259; J. M. Melot, F. Texier-Boullet and A. Foucaud, Synthesis, 1987,364;D. E. Bergbreiter and J. J. Lalonde, J. Org. Chem., 1987, 52, 1601. T. Ando, S.J. Brown, J. H. Clark, D. G. Cork, T. Hanafusa, J. Ichihara, J. M. Miller and M. S. Robertson, J. Chem. SOC., Perkin Trans. 2, 1986, 1133. L. M. Weinstock, J. M. Stevenson, S.A. Tomellini, S. H. Pan, T. Utne, R. B. Jobson and D. F. Reinhold, Tetrahedron Lett., 1986, 27, 3845. 22 23 24 25 L. F. Drullinger and J. E. Griffiths, Spectrochim. Acta, Part A, 1971, 27, 1793; K. 0. Christe, E. C. Curtis, C. J. Schack and D. Pilipovich, Znorg. Chem., 1972, 11, 1679. E. A. Robinson, D. S. Lavery and S. Weller, Spectrochim. Acta, Part A, 1969,25, 151; D. F. Burow, Inorg. Chem., 1972, 11, 573; K. Garber and B. S. Ault, Znorg. Chem., 1983, 22, 2509. K. 0. Christe, E. C. Curtis and C. J. Schack, Spectrochim. Acta, Part A, 1975, 31, 1035. G. H. Posner, J. W. Ellis and J. Ponton, J.Fluorine Chem., 1981, 6 T. Ando, J. H. Clark, D. G. Cork, T. Hanafusa, J. Ichihara and 19, 191. 7 T. Kimura, Tetrahedron Lett., 1987, 28, 1421. J. H. Clark, E. M. Goodman, D. K. Smith, S. J. Brown and J. M. 26 27 J. R. Schlup and R. W. Vaughan, J. Catal., 1986, 99, 304. C. V. A. Duke, J. M. Miller, J. H. Clark and A. P. Kybett, Spectro- 8 9 10 Miller, J. Chem. SOC., Chem. Commun., 1986, 657. C. V. A. Duke, J. M. Miller, J. H. Clark and A. P. Kybett, J. Mol. Catal., 1990, 62, 233. S. J. Brown and J. M. Miller, Can. J. Spectrosc., 1989, 34, 33. K. W. Dixon and J. M. Winfield, J. Chem. SOC., Dalton Trans., 28 29 chim. Acta, Part A, 1990, 46, 1381. J. Thomson, G. Webb and J. M. Winfield, J. Mol. Catal., 1991, 67, 117. B. C. Lippens, Ph.D. Thesis, Delft University of Technology, 1961. 11 12 13 14 15 16 1989,937. T. Baird, A. Bendada, G. Webb and J. M. Winfield, J. Fluorine Chem., 1991,52,245. K. W. Dixon, M. F. Ghorab and J. M. Winfield, J. Fluorine Chem., 1987, 37, 357. F. S. Fawcett and C. W. Tullock, Znorg. Synth., 1963, 7, 119. C. W. Tullock and D. D. Coffman, J. Org. Chem., 1960,25,2016. F. S. Fawcett, C. W. Tullock and D. D. Coffman, J. Am. Chem. SOC., 1962, 84, 4275. D. D. Gibler, C. J. Adams, M. Fischer, A. Zalkin and N. Bartlett, 30 31 32 33 34 35 S. Soled, J. Catal., 1983, 81, 252. J. B. Peri, J. Phys. Chem., 1965, 69, 220. A. Reller and D. L. Cock, Catal. Lett., 1989, 2, 91. C-P. Huang and W. Stumm, J. Colloid Interface Sci., 1973, 43, 409. (a) L. Vordonis, P. G. Koutsoukos and A. Lycourghiotis, J. Catal., 1986, 98, 296; (b) Langmuir, 1986, 2, 281; (c) J. Catal., 1986, 101, 186. e.g. M. Onaka, K. Sugita and Y. Izumi, J. Org. Chem., 1989,54, 11 16. 17 18 Inorg. Chem., 1972, 11, 2325. M. E. Redwood and C. J. Willis, Can. J. Chem., 1967, 45, 389. M. Epple, W. Riidorff and W. Massa, Z. Anorg. Allg. Chem., Paper 1/03696C;Received 19th July, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101071

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1079-1080 MATERIALS CHEMISTRY COMMUNICATIONS Defect Calculations in Solids beyond the Dilute Limit Robert A. Jackson," James E. Huntingdon" and Richard G. J. Ball" a Department of Chemistry, University of Keele, Staffordshire ST5 5BG, UK Materials and Chemistry Division, AEA Technology, Harwell Laboratory, Oxfordshire OX11 ORA, UK Preliminary results are presented on the calculations of defect parameters beyond the dilute limit. The method employs lattice-energy minimisation with a defect supercell. Defect parameters for UO, are calculated as a function of concentration. Keywords: Defect; Lattice-energy minimisation Recent years have seen considerable progress in the calculation of defect properties of inorganic solids. A wide range of materials have been investigated, and quantitative agreement has been established in many cases.A number of general reviews are available (see for example Catlow and Mackrodt,' Catlow2) and specific applications include nuclear fuels3 and transition metal oxide~.~ In general, these calculations have employed the Mott-Littleton method' which embeds the defect in a perfect lattice, and they are therefore restricted to concentrations of defects in the dilute limit. In this communication we present the initial results of the application of a method which enables higher defect concen- trations to be studied. The method is based on a standard lattice-energy minimisation calculation, but employs a super- cell composed of a number of basic unit cells.Because this supercell is repeated throughout all space, a concentration of defects beyond the dilute limit can be established. The defect concentration can be controlled by varying the size of the supercell and/or the number of defects within it. A related approach has been used by Cormack6 to investigate shear plane ordering in transition metal oxides; the emphasis of the present work is in the calculation of defect energies and structures. Additionally, in contrast to the Mott-Littleton approach, calculations of lattice properties (elastic constants, relative permittivities etc.) are possible for the defective solid. As an example of the application of the method, U02 is chosen; this has been the subject of a series of Mott-Littleton calculations, both of its basic defects3-' and of the behaviour of fission gas within the However, during the operation of a nuclear reactor, high concentrations of defects and fission products may be generated within the U02 fuel, so there is clearly a need for calculations beyond the dilute limit.The method involves (i) generation of the supercell, (ii) incorporation of defects and (iii) lattice-energy minimisation. Step (i) is a straightforward process involving repetition of the basic unit cell by lattice translation. Step (ii) involves addition, removal or substitution of atoms within the supercell to create the appropriate defect. The method necessarily involves an ordering of the defect distribution over the super- cells, It is, however, possible to investigate a range of ordering schemes by varying the supercell size and shape and the distribution of defects within the supercell itself.The calcu- lations reported here relate to one particular ordering scheme, that of a central distribution where defects are placed at the centre of the supercell, which has a 2 x 2 x 2 index consisting of 96 atoms. The defect distribution and the variation of shape of the supercell at this relatively small size have minimal effect on the results obtained; the effect of alternative distributions on larger supercells will be the subject of a future paper. Step (iii), lattice-energy minimisation, employs the THBREL code." This program has been used extensively in the calcu- lation of structural and energetic properties of a wide range of materials.In carrying out this step for a defective supercell care must be taken to avoid minimisation problems. Diffi- culties can be overcome by performing the minimisation in stages, i.e. by selectively 'freezing' parts of the cell while others are allowed to relax. In the final calculation, however, the whole structure is allowed to relax to a minimum-energy configuration. As with any atomistic simulation study, the specification of interatomic potentials is of paramount importance. In this study, potentials are taken from the earlier Mott-Littleton study of Jackson et d3It should be noted that for higher defect concentrations, cation-cation potentials could become important.In the plutonium substitution calculations described below, it was found that the inclusion of U4+-Pu4+ and Pu4+-Pu4+ potentials could improve the speed of -0.3484 22 -0.3486 .-CBI + P2 -0.3488 L aQ $ -0.3490 a a -0.3492 .-c 3 c..-c;-0.3494 v) -0.3496 IIII111111 102030405060108090100 pu4+ ions (Yo) Fig. 1 U0,-PO, substitution energy for a supercell of index 2 x 2 x 2 (96 atoms). 0,Central distribution; *side-ordered distribution; 0, random distribution Table 1 Comparison calculations for basic defect energies defect U, supercell/eV U, Mott-Littleton/eV anion vacancy anion interstitial 17.33 -12.20 17.16 -12.29 cation vacancy cation interstitial 80.11 -61.22 80.27 -61.43 cation Frenkel 18.88 18.71 anion Frenkel 5.12 4.87 Schottky trio (unbound) 11.27 10.04 convergence of the minimisation.These potentials were obtained by electron-gas methods.I2 Results of supercell calculations on U02 are presented below. Table 1 is a comparison of basic defect formation energies calculated by the present method at low concen-trations with those calculated using the Mott-Littleton meth-od~logy.~The variation of substitutional defect formation energies with concentration is shown in Fig. 1, illustrating the minimal effect of various distribution schemes on the results obtained. Fig. 2 shows the variation of lattice parameter with 5.446 0 5.444 0 $ 5.442 .I-0 a g 5.438 .-.I- s 5.436 0 1.434 1 5.4305-432 L----2 10 20 30 40 50 60 70 80 90 100 Pu4+ ions (%) Fig. 2 Lattice parameter us.Pu4+ concentration for a 2 x 2 x 2 (96 ion) supercell J. MATER. CHEM., 1991, VOL. 1 increasing Pu4+ concentration,which is approximately linear, in agreement with the predictions of Vegard's Law. In this short communication we have presented preliminary results of a programme of work to investigate the consequences of defect concentrations beyond the dilute limit. The validity of the method is borne out by the close agreement between the low concentration and dilute limit results. This method has many potential applications in solid-state chemistry for the study of properties associated with non-stoichiometry, fission product incorporation etc. that have hitherto not been poss-ible using Mott-Littleton methods.Further calculations are in progress in these areas. The authors are grateful for the provision of computer time from the SERC and ULCC. Some of this work was funded as part of the corporate research programme of AEA Tech-nology. References 1 C. R. A. Catlow and W. C. Mackrodt, Computer Simulation of Solids, Lecture Notes in Physics no 166, Springer-Verlag, Berlin, 1982. 2 C. R. A. Catlow in Defects in Solids: Modern Techniques, ed. A. V. Chadwick and M. Terenzi, NATO AS1 Series B: Physics vol. 147, Plenum, Oxford, 1987. 3 R. A. Jackson, A. D. Murray, J. H. Harding and C. R. A. Catlow, Philos. Mag., 1986, A53, 27. 4 S. M. Tomlinson, C. R. A. Catlow, and J. H. Harding, J. Phys. Chem. Solids, 1990, 51, 477. 5 N. F. Mott and M. J. Littleton, Trans. Faraday Soc., 1938, 34, 485. 6 A. N. Cormack, Solid State lonics, 1983, 8, 187. 7 C. R. A. Catlow, Proc. R. SOC. London, Ser. A, 1977,353, 533. 8 R. A. Jackson and C. R. A. Catlow, J. Nucl. Muter., 1985, 127, 161. 9 R. A. Jackson and C. R. A. Catlow, J. Nucl. Muter., 1985, 127, 167. 10 R. G. J. Ball and R. W. Grimes, J. Chem. SOC.,Faraday Trans., 1990, 86, 1257. 11 R. A. Jackson and C. R. A. Catlow, Molecular Simulation, 1988, 1, 207. 12 J. H. Harding and A. H. Harker, UKAEA Harwell Report, AERE R-10425. 1982. Communication 1/0452I K; Received 29th August, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101079

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1081-1082 1081 Crosslinked Layered Materials formed by Intercalation of Octameric Siloxanes in Metal(iv) Hydrogen Phosphates Jacques Roziere,* Deborah J. Jones and Thierry Cassagneau Laboratoire des Agregats Moleculaires et Materiaux Inorganiques, URA CNRS 79, Universite Montpellier 2, 34095 Montpellier Cedex 5,France The intercalation of octa(aminopropylsi1asesquioxane) into metal( tv) hydrogen phosphates [MI" =Zr, Ti, Sn] leads to silica crosslinked materials. The layered structure and the silica framework are retained after thermal processing. Bilayer formation is obtained with tin phosphate. Keywords: Siloxane; Phosphate ; Pillaring ; Crosslinking It is now well established that layered solids other than smectite clays may be pillared to form three-dimensional crosslinked materials, the porosity of which is ultimately tunable by the nature of the host substrate and of the chemical species acting as pillar.' The possibility of forming porous derivatives using inorganic pillaring agents and layered metal(1v) hydrogen phosphates of the a form was initially questioned on the basis of charge-density considerations and alternative routes, including pillaring with organic molecules or organic derivatization of the phosphate, were preferred.' Early results obtained on the crosslinking of a-zirconium phosphate by polyhydroxometallic ions would seem to vindi- cate this reasoning by giving only materials of low surface area;3 however, more recently, the synthesis of highly porous pillared tin phosphate has been rep~rted,~.~ although the origin of the porosity has been discussed.' An alternative synthetic pathway, designed to overcome the charge-density limitations associated with the insertion of inorganic pillars, lies in the use of organometallic guest molecules with expendable and bulky organic groups, remov- able by subsequent thermal processing.Oligosilasesquioxane routes to pillared a-zirconium phosphate were first suggested by Lewis et d6Here we report the formation of silica crosslinked zirconium, titanium and tin phosphates (ZrP, TiP and SnP, respectively) from aminopropyltriethoxysilane (APTEOS), hydrolytically polymerised ex situ to the octameric form [ZSiO, .5]8, where Z=(CH2)3NH3+.Preparation of octa(3-aminopropylsilasesquioxane) was undertaken following the synthetic criteria described by Voronkov and La~rent'yev.~ Thus APTEOS was diluted in ethanol-water (v/v = 14:1) to give a solution of concentration 0.45 mol dmP3, with stirring. "Si NMR spectroscopy of the solutions confirmed the nature of the constituent oligomer.? The outcome of intercalation reactions was found to depend on the nature of the alcohol, and methanol, isopropyl alcohol and isopentyl alcohol were also used. Expanded metal(1v) hydrogen phosphates were prepared by contacting 1 g of the solid, prepared according to published methodsg and sus-pended in deionised water (20 cm3), with aliquots of the above siloxane solution such that the molar ratio R =[Si ]/[M'"] lay in the range 0.2-3.0.After contact for 15 h to 5 days, the materials were separated by high-speed centrifugation, washed, and air-dried. Selected expanded phases were calcined in air, or under vacuum, at temperatures between 350 and 550 "C. The uptake of [H2N(CH2)3Si01.5]8 by ZrP, (Fig. 1; gravi-t Recorded at 49.6 kHz on a Bruker AC 250 spectrometer. A signal at -68 ppm, corresponding to the cyclic octamer [H2N(CH2)3SiOlJ8 is observed. A weak line discernible at -60 ppm probably arises from the presence of small amounts of trimeric species. /~~""~~~~~"~~" 0 2.5 5 7.5 10 12.5 Si added/mrnol g-'(ZrP) Fig. 1 Uptake of [H2N(CH2)3Si0,.,]8, by ZrP from aqueous etha- nolic solution metric determination of Si in the supernatant liquid as quino- line silicomolybdate) as a function of R shows that maximal insertion occurs above R =2.0, when 5.2 mmol Si per gram of ZrP are taken up, corresponding to an [Si] :[PI molar ratio of 0.79.Preliminary X-ray diffraction (XRD) patterns (Cu-Ka radiation) indicated a biphasic system subsisting almost to the plateau region, when a single-phase, layered material of inter- layer spacing, dOo2,17.698, (dOo4=8.86 A) is observed (Fig. 2). The use of isopentyl alcohol for hydrolytic polycondensation of APTEOS, leads to an increase in the interlayer distance: dOo2= 18.34(dOo49.19 A). This expansion is in complete agree- I""""""""""""""""""" 4 13 22 31 40 2e/o Fig. 2 Variation of the do,, diffraction in (a) ZrH,,,(PO,), [H2N(CHJ3SiO1J8 (b)as (a),after thermal processing at 500 "C ment with that expected for the insertion of a cubic octamer of calculated dimension, including the organic groups, of 11.2A.' Furthermore, the nature of the organic side-arms is such that, for each of the two cube faces (each of surface area CQ.125 A2)lying closest or parallel to the [Zr(HPO,),], layers, proton transfer will occur from four active -POH sites (of total surface area ca. 96A2), thus rationalising the maximal [Si] :[PI ratio observed (0.79),and corroborating the intercal- ation of siloxane as octameric units. Characteristic fingerprints of the organic groups were observed in IR.1 The layered structure of these materials was retained after calcination, to remove the organic functional groups, at various temperatures.XRD indicated a progressive decrease in interlayer distance: doo2(360"C)= 14.9 A; doo2(500"C)= 12.5 A (Fig. 2), and IR the absence of organic matter, con- firmed also by C, H, N analysis. Washing the calcined phases with a solution 1 mol dm-3 in HCl did not alter the XRD. An identical experimental protocol was observed for TIP and SnP and, for the former, similar results to those described above for ZrP were obtained. However, intercalation of [H2N(CH2)3SiOl.5]8 into TIP could apparently only be achieved when isopentyl alcohol was used in the preparation of the siloxane octamer for [Si]/[Ti] >2. XRD patterns reflected a generally lower degree of crystallinity than that observed for ZrP derivatives, and indicated a basal spacing of 18.04 A (dOo4=9.91A), falling to 12.06 A after calcination in air at 440 "C.In contrast, the results obtained for SnP show some marked differences, in particular, dependence (i) of the extent of intercalation on the contact time, and (ii) on the [Si]:[Sn] ratio, even above R=2, not observed for ZrP or Tip. Thus, after 2 days stirring, SnP derivatives are characterised by an increasingly well defined diffraction line progressively dis- placed to lower angles at higher ratios (R=0.82, dOo2= 18.03; R = 1.14,dOo2= 18.57;R =1.63,dOo2= 19.88 A), and an increase in the extent of the reaction, as estimated from the area of the SnP fingerprint, doo2=7.8 A. When the ratio is further increased, different intercalation behaviour is observed, which may be related to the different surface acidity characteristics of ZrP and SnP, or to the more marked propensity of the latter to hydrolysis. Thus when R= 2, XRD shows the coexistence of two expanded phases, the minor component of which corresponds to that observed previously for R= 1.6 and less, and the major having an interlayer distance of 26.90A (dOo4= 13.42,dOo6=8.98 A).This latter phase is the only one present at R =3. The coexistence of two phases when R=2 would seem to indicate that the evolution from a single to a double layer of [H2N(CH2)3SiOlJ8, within the interlayer region, is probably a discontinuous phase transition. Bilayer formation in pre- pillar materials was also reported in the +-SnP[A104A1,2(0H)2,(OH2)12]7 system., Elimination of organic matter from the fully expanded intercalate at 460 "C leads to materials having a well defined dOo2diffraction line, and an interlayer distance of 17.95 A (Fig.3) clearly indicating that the double layer is conserved on calcination. Although the precursor phases to silica crosslinked phos- phate materials are characterised by a maximum occupation of the active sites of ca. 80%, the specific surface area of ZrP-derived compounds after calcination is only slightly increased with respect to the starting metal phosphate,§ and these 1Recorded on a BOMEM DA8 spectrometer p(CH,) 710, 6(CH,) 1472,6,(NHi) 1542, Gas(NHl)1600 cm-'. v(Si-0) and v(Si-C) are masked by stretching vibrations of the PO,.group (950-1250 cm-') as are the corresponding deformation vibrations.6 Flowsorb 11 2300 instrument. N, adsorption-desorption at 77 K, after initial degassing of the samples at 200 "C, gave specific surface areas of M'"(HPO4),-H20, 5-10 m2 g-'; M1VHo.4(P04)2(Si01 .6,J1 MIV=Ti, Zr maximum 30 m2 g-'. J. MATER. CHEM., 1991, VOL. 1 v ulfill~~~~~1(L~1~~I~~I~.I~1I ~.~~ 3 12 21 30 39 201" Fig. 3 Effect of temperature on the interlayer distance in bilayered [H,N(CH,),SiO, J,-SnP (a) 25 (b) 380 (c)460 "C materials cannot, therefore, be considered as porous solids. In a recent report, however, intercalation from an aqueous solution containing a mixture of NH2(CH2)3Si(OH)3 mono-mers, and the corresponding dimeric and trimeric units, into ZrP was reported (using a significantly higher [Si] :[Zr] ratio), and porosity, based on the adsorption of hexane, was claimed." In contrast, the 'double-pillared' SnP-derived phases, obtained after calcination at 540 "C under vacuum, are characterised by a specific surface area of ca.230 m2 g-'. This observation can be rationalised in terms of partial hydrolysis of SnP during the intercalation reaction, which randomly deactivates certain -POH sites, so preventing their interaction with [H2N(CH2)$i01 .&. The almost identical uptake of the latter by ZrP as by SnP then requires an alternative arrangement of octamers in the interlayer region (the observed double-layer) which produces a porous struc- ture, unlike that of the crosslinked ZrP material.References 1 Pillared Layered Structures: Current Trends and Applications, ed. I. V. Mitchell, Elsevier Applied Science, London, 1990. 2 G. Alberti, U. Costantino, F. Marmottini, R. Vivani and P. Zappelli, in Pillared Layered Structures: Current Trends and Applications, ed. I. V. Mitchell, Elsevier Applied Science, London, 1990, p. 119. 3 A. Clearfield and B. D. Roberts, Inorg. Chem., 1988, 27, 3237; D. J. MacLachlan and D. M. Bibby, J. Chem. SOC., Dalton Trans., 1989, 895. 4 P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, A. Jimenez-Lopez, L. Alagna and A. A. G. Tomlinson, J. Mater. Chem., 1991, 1, 319. 5 P. Maireles-Torres, P. Olivera-Pastor, E. Rodriguez-Castellon, A. Jimenez-Lopez and A. A. G. Tomlinson, J. Mater. Chem., 1991, 1, 739. 6 R. M. Lewis, R. A. van Santen and K. C. Ott, Eur. Pat., 0159756 BI, 1985. 7 M. G. Voronkov, V. I. Lavrent'yev, Top. Curr. Chem., 1982, 102, 199. 8 W. E. Rudzinski, T. L. Montgomery, J. S. Frye, B. L. Hawkins and G. E. Maciel, J. Chromatogr., 1985, 323,281. 9 G. Alberti and E. Torraca, J. Znorg. Nucl. Chem., 1968, 30, 317; E. Kobayashi, Bull. Chem. SOC. Jpn., 1975,48,3114; E. Rodriguez- Castellon, A. Rodriguez-Garcia and S. Bruque, Znorg. Chem., 1985, 24, 1187. 10 L. Li, X. Liu, Y. Ge, L. Li and J. Klinowski, J. Phys. Chem., 1991,95, 5910. Communication 1/04196G; Received 12th August, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101081

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991,1(6), 1083-1085 Particle Beam Microanalysis Fundamentals, Methods and Applications. By E. Fuchs, H. Oppolzer and H. Rehme. VCH, Weinheim, 1990. Pp xviii+507. Price €79, DM 215. This is a well produced book with a disciplined style and objectives, written by three experienced and authoritative workers. The book is definitive in its approach; clearly stating fundamentals, methods and applications at every stage. The principal aim is to support the analyst in his practical work. Particle-beam methods are eminently suitable for appli- cation in microtechnology where, with high spatial resolution, one can identify unknown substances in small volumes and determine their composition with suitable accuracy. There is strong emphasis in this work on solid-state physics, particu- larly on semiconductor theory and applications.This book is an in-depth ‘state-of-the-art’ assessment of the topic. The authors, in the preface, state that microtechnologies, and above all semiconductor technology, have made such rapid advances that no end can currently be discerned for this development. In view of this statement one wonders whether continued progress in this active field may necessitate the updating of certain topics in this book. The authors have disciplined themselves to avoid mention of historical development of the various aspects of particle- beam microanalysis but counteract this with a good set of references at the end of each chapter. The book is intended for the mature worker and not for the beginner in the field of particle-beam microanalysis, and its clinical-like approach may not appeal to every reader.The book has been produced with care and the occasional minor discrepancy is not dis- tracting. The first chapter is a definitive overview of microanalytical particle-beam methods. It reviews analytical errors and detec- tion limits and is a good brief introduction to the subject. The second chapter is entitled ‘Fundamentals’ and presents some refresher information on a number of concepts of solid- state physics which are needed to understand the methods of analysis described throughout the book. This chapter is adequate and well referenced. Scanning electron microscopes (SEM) are used as routinely as optical microscopes.There are a large range of models from the simple SEM to complex automatic linewidth measurement systems. All these instruments are based essen- tially on the same design principles. Chapter 3 covers the basic SEM principles, instrumentation, detectors, signal and image processing. It also includes specimen preparation, imag- ing with secondary electrons and with back-scattered elec- trons, cathodoluminescence and concludes with a brief description of the principles of thermal wave microscopy. Transmission electron microscopy (TEM) is outlined in Chapter 4.Initially it is compared with SEM. The principles of TEM systems, are suitably covered. Analytical electron microscopy is introduced at this point to prepare the reader for the in-depth treatment that is given in Chapter 5.Particular attention is given to specimen preparation as this often accounts for the major part of the work involved to ensure successful TEM application. The specific requirements of scanning transmission electron microscopy (STEM) are com- pared in detail with those of conventional TEM (CTEM) highlighting image contrast illumination and detection. Chapter 5, dealing with electron-beam X-ray microanalysis should attract the practising analyst who requires qualitative analysis and quantitative data on micro regions on solid surfaces. The text offers background information to assist an analyst who has received training and has instrument operating manuals. Whilst the theoretical comparison between EDS and WDS spectrometers is adequate a clearer interpret- ation of real and virtual standards for EDS, spectrum ‘fit index’ and dot-mapping procedures would be of value to the practising analyst.Looking at the vista of material and life sciences in academic institutions and industry there are prob- ably slightly more TEM instruments than SEM instruments in use and about half of these electron microscopes have an associated X-ray analytical facility. With this in mind, the main user/reader interest in this book will be Chapters 3, 4 and 5. Auger electron microanalysis (Chapter 6), secondary ion mass spectrometry (Chapter 7) and electron-beam testing (Chapter 8) are comprehensively described in the systematic style noted for SEM and TEM.Secondary ion mass spec- trometry (SIMS) is the mass spectrometry of atomic or molecular particles which are emitted when a solid surface is bombarded by energetic primary particles (sputtering). SIMS is surface destructive and surface specific, and over the last decade it has developed into a powerful technique for studying the chemical state of surfaces. Dynamic SIMS, static SIMS and imaging SIMS are described. The systematic style of description reviews the mode of operation of the instruments, sample preparation, evaluation of the measured signals as well as detection limits. This book demonstrates that no one technique can solve all problems and many analytical approaches are necessary. The book concludes (Chapter 9) with a selection of practical examples drawn mainly from the field of semiconductor technology demonstrating the range and limitations of the various particle-beam methodologies described in the text.The authors are to be congratulated for this valuable contri- bution to microanalytical literature. R. G. Blezard Received 1st August, 1991 Solid State lonics. By T. Kudo and K. Fueki. VCH, Weinheim, 1990. Pp. ix+241. Price €67. Readers of Journal of Materials Chemistry need hardly be reminded that solid state ionics as a subject has developed from concept to maturity in the space of about 30 years. In fact, the authors of this monograph attribute the first use of the term to Professor Takehiko Takahashi in 1960. Several specialized reviews of the area are available, such as Solid Electrolytes, edited by Hagenmuller and Van Gool in 1978, and (too recent for reference in the present volume) Superionic Solids and Solid Electrolytes: Recent Trends, edited by Laskar and Chandra in 1989.The need for an intermediate-level monograph that approaches the subject from the point of view of a graduate student or practising scientist who is entering the field is a real one, and this book has the potential of satisfying this niche for the next few years at least in this rapidly-changing field. Much well established information is already available; in fact, the contents of this book follow closely that of Hagenmuller and Van Gool. The book is divided into three parts: Fundamentals, Mater- ials and Applications.It is not always clear what audience is addressed in the Fundamentals section; if a beginning worker in the area, then the chapters The Concept of Solid State Ionics, Electronic Conduction in Metals and Semiconductors, Point Defects in Ionic Crystals, Diffusion in Ionic Crystals, and Electronic Conduction in Mixed Conductors will provide a useful introduction at an elementary level, although occasionally the reader seems to be expected to be familiar with concepts such as the Pierls (sic) transition and solitons. The worker new to the area would also benefit from a somewhat more extensive discussion of point defects and the associated nomenclature, which are used extensively in later sections on applications.On the other hand, if the reader is experienced in solid state chemistry and physics, these intro- ductory chapters are elementary. The Materials section comprises a long chapter on Solid Electrolytes, and a shorter chapter on Mixed Conductors. Sufficient detail is given to enable the reader to obtain a reasonable clear picture of some of the structural and defect properties that determine both electronic and ionic conduc- tivity. The final section of the book on Applications covers a wide variety of topics, as might be expected from the many devices which have been constructed using solid electrolytes. Thus there are chapters on Physico-chemical Measurements Using Solid State Electrochemical Cells, Batteries, Sensing Devices, Electrochemical Devices, and Photoelectrochemical Devices and Lithography.These chapters contain extensive references from the primary literature. Finally, there is an almost obliga- tory chapter on Future Prospects of Solid State Ionics, which suggests that the principal use may lie in chemical-to-electrical transducers rather than as energy storage devices. There is a good subject index; an author index would have been useful. The book thus covers in some detail the important, and many less important, areas in solid state ionics, and is a useful addition to any library despite two shortcomings. The first is a lack of specific references to standard texts and monographs in solid state chemistry and physics, which would serve as valuable aids for further reading, especially in the Funda- mentals section.The second is in the use (or misuse) of the English language. In a recent speech to a British Council training seminar on teaching English as a foreign language, the Prince of Wales observed that, as English becomes more and more the preferred language of international communi- cation, there is a danger of spoken English degenerating into a series of variants which may eventually be mutually incom- prehensible. The same could be said of written English, except that one might expect publishers to exercise particular care with manuscripts submitted by those whose native language is other than English. There is evidence that some care has been taken in this book, but it has been insufficient; this reviewer counted an average of about three errors in grammar, punctuation or spelling per page.Fortunately, most of these do not interfere with the meaning, and there are few really obscure passages: ‘Selective and reliable sensors are needed for the lockup system to monitor drunk driving’ is an atypical example. However, the accumulation of errors does reflect badly on the publisher and the authors, and on the overall quality of the book. J. W. Lorimer Receioed 8th August, 1991 Advances in the Synthesis and Reactivity of Solids, Volume 1. Ed. T. E. Mallouk. JAI Press, London, 1991. Pp. 300. Price €57.50. This is the opening volume of a new series that doubtless owes its inception to the present, extremely healthy state of materials chemistry research.Appropriately, the volume launches the series with a strong hand of contributions by distinguished authors, and the subjects chosen for review have, for the most part, been topics in which there have been substantial advances in recent years. The first chapter is devoted to the chemistry of high- J. MATER. CHEM., 1991, VOL. 1 temperature superconductors. The literature relating to this subject is so vast and in so chaotic a state that the preparation of a sound, well structured review of synthesis methods, characterisation and structure-property relationships rep-resents a valuable service to all those with an interest in the subject. The material is structured in a helpful fashion and the layout is very stylish so that the review is very readable, but it is a pity that here, as in other parts of the volume, a significant number of printing errors have survived proof reading.The theme of control over carrier density uia oxygen stoichiometry is presented clearly and it is refreshing to see related phenomena honestly acknowledged as ‘amazing’. The treatment of the modification of optical properties of organic molecules by incorporation into inclusion complexes is also a well up-to-date review and laid out so as to be enjoyable to read, but again, typographical errors are a distraction and the misplacing of a pair of figures to the wrong captions is potentially confusing. On the whole, how- ever, the review successfully illustrates this branch of photo science with an infectious enthusiasm.The study of conducting transition-metal oxides has a longer history than the other topics visited in this volume but the treatment here provides a clear insight into the crystal and electronic structures of the materials and the effects of doping. Ferroelectric liquid crystals have been known for only some 15 years and the story of their development represents a fascinating account of organic chemists tailoring molecules to achieve particular physical properties. The success in synthesis and understanding that has been achieved already is quite remarkable and this is likely to be a fertile area of development for years to come. Intercalation reactions have been known since the first half of the last century when early work focused on insertion into graphite. However, in the last 20 years or so, there has been an accelerating interest in this type of chemistry, in which solids with new physical properties can be prepared at rela- tively low temperatures.Much of this work has been driven by an interest in materials for electrical applications and a large part of the review here deals with the important area of lithium intercalation into cathode hosts in lithium battery systems. A second area in which intercalation processes can be exploited to exercise crucial control over electrical properties, is in the production of superconducting formulations. The intercalation of oxygen into complex oxides and intercalation of various species into transition-metal chalcogenides to pro- duce superconductors are both reviewed.In summary, this collection of reviews will be an asset in any library. As with all such series, however, the true test of its lasting success will be whether or not the high standard set by the first volume can be sustained. P.T. Moseley Receioed 27th August, 1991 Introduction to Nonlinear Optical Effects in Molecules and Polymers. P. N. Prasad and D. J. Williams. Wiley, New York, 1991. Pp x+305. The explosion of interest in the field of photonics in general, and in nonlinear optics (NLO) in particular, seems to be following a similar pattern to other recent high-technology fields such as conducting polymers and high-temperature superconductors.After an initial meteoric rise in popularity which attracts a great deal of interdisciplinary interest, and J. MATER. CHEM., 1991, VOL. 1 much superficial and hasty publication, a sobering realization sets in that much hard work remains to be done before the new field can be deemed mature. As in most interdisciplinary fields, the primary literature sources are scattered across many fields, and it is quite difficult for neophytes to learn what has been done, and more importantly, what needs to be done. This needed introduction to the field of nonlinear optics has now finally been provided by Paras Prasad and David Williams. The need for this book, I believe, has been particularly felt by synthetic chemists and materials scientists, who in general are not well versed in the arcane details of laser spectroscopy or applied optics.This book takes great pains to introduce the physics fundamentals underlying a detailed understanding of the origin of optical nonlinearity in the first several chapters, and provides clear descriptions of both the similarities and differences between second- and third-order processes. Chap- ters 6 and 9 review the common methods by which micro- scopic hyperpolarizabilities and macroscopic susceptibilities may be determined experimentally, and how the measurement technique chosen will often dictate the exact nature of the nonlinearity probed. Chapters 7 and 10 provide up-to-date (1990) surveys of second- and third-order materials, and provide a critical review of the advantages and disadvantages of single crystals, polymer films and Langmuir-Blodgett films for second- and third-order NLO applications.Prasad and Williams have tried to emphasize the import- ance of understanding how three- and four-wave mixing gives rise to optical nonlinearity. This is exemplified by very clear and precise descriptions of electric-field-induced second har- monic generation (EFISH) and degenerate four-wave mixing (DFWM) techniques. In addition, a great deal of time is spent in explaining how resonant us. non-resonant behaviour arises, and the importance of pulse duration on the observed mechan- ism of nonlinearity. In general, none of this is understood by newcomers to the field (the reviewer included), nor is this information usually obtainable in convenient format.Also useful are brief descriptions of current theoretical models, as well as a critical comparison between the capabilities of ab initio and semi-empirical approaches to structure-property relationships. The latter portion of the text is devoted to brief descriptions of how useful devices can be designed around NLO materials capabilities, and a final chapter defining future directions for research. For each type of device or application, the authors provide a synopsis of where current state-of-the-art resides uis d uis available materials. Useful comparisons to inorganic materials such as lithium niobate are also discussed for organic crystals and polymers with respect to the various types of device. A number of typographical and chemical formula errors have managed to go undetected, but most are obvious and should not cause any serious confusion to the astute reader. All in all, this book is an excellent introduction to the field of nonlinear optics and should encourage scientists from various synthetic or materials backgrounds to enter the new and fascinating world of photonics. In particular, this text should prove to be most useful for graduate students in the field, and it could easily serve as a text for a special topics course. C. Spangler Received 2nd September, 1991

ISSN:0959-9428    

DOI:10.1039/JM9910101083

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(6), 1087 CORRIGENDUM Corrigendum to Electrochromic Nb,O, and Nb,O,/Silicone Composite Thin Films prepared by Sol-Gel Processing G. Rob Lee and Joe A. Crayston, Department of Chemistry, Purdie Building, University of St Andrews, Fife KY16 9ST, UK J. Mater. Chem, 1991, 1, 381. The caption to Fig. 7 should read: Fig.7 Typical i-t-* plots for the decaying portion of the colouration current transients of pure Nb205(0)and com- posite electrodes (m)

ISSN:0959-9428    

DOI:10.1039/JM9910101087

出版商:RSC    

年代:1991

数据来源: RSC