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