The crystal structure of Li,Sb04 Janet M. S. Skakle," Maria A. Castellanos R.,b Sonia Trujillo Tovar,b Susan M. Fray' and Anthony R. West" "Departmentof Chemistry, University of Aberdeen, Meston Walk, Aberdeen, UK AB24 3UE bUniversidad Nacional Autonoma de Mexico, Facultad de Quimica, Mexico DF04510,Mexico 'School of Chemistry, St. Andrews University, St. Andrews, Fife, UK K Y16 9ST The crystal structure of Li3Sb04 has been determined by analogy with Na,BiO,, and refined by the RiFtveld method using X-ray powder diffraction data. It is monoclinic, space group P2/c, a= 5.1579(2),b= 6.0922(3), c=5.1397(2) A, B= 108.839(4)". The structure is an ordered rocksalt type, with distorted M06 octahedra. The crystal structure of Li,SbO, was first examined by Blasse using X-ray powder diffraction (XRD) d?ta.' The unit cell was given as tetragonal, a=6.12, c =8.50 A; the structure was suggested to be an ordered rocksalt structure, where a=$4 and c =2aR (aR =unit cell of rocksalt).The Sb atoms were esti- mated to be located in positions ($ 3 0), (+ $), (a + 3)and (31 $) within a cubic close-packed (ccp) oxide array, giving a cation order related to that of spinel. Based on the Sb-0 framework, the agreement between observed and calculated intensities was reasonably good, with an R factor of 12%. The powder pattern of Li,SbO, given in the powder diffrac- tion file (card no. 17-824)2 was found to match that of Li2Te04 (card no. 26-8$1),, which has a similar unit cell with a=6.045 and c =8.290 A. The structure of Li2Te04 was determined in the space group P4,22, and was found to be a distorted inverse spinel with Te in octahedral sites and Li in both tetrahedral and octahedral sites., In spite of the similarity between the XRD patterns of Li,SbO, and Li2Te04, it seemed unlikely, from its stoichiometry, that Li3Sb0, could have a spinel structure.Since, however, the XRD patterns of both Li3Sb04 and Li2Te0, would be dominated by the scattering powers of Sb and Te, respectively, the similarity between the two patterns may confirm Blase's observation of a spinel-like cation arrangement. Other Li,MO, structures with similar-sized M atoms to Sb are Li,NbO, and Li,TaO,; these belong to the rocksalt family, with ordered Na,Ta04 and Na,Nb04 also have rocksalt-related structures and can exhibit both order and disorder of the cations, in different polymorphs.lO,ll The XRD pattern of Li,SbO, was examined as part of an investigation into rocksalt-related phases in the system Li,SbO,-CuO. Although the XRD data were collected on a high-resolution instrument, the pattern could not be indexed adequately on the previously described tetragonal unit cell.The structure of Li3Sb04 has therefore been reinvestigated, and we report the results here. Experimental Li3Sb0, was prepared by reaction of stoichiometric quantities of Li2C0, and Sb205 in an Au foil boat, initially at 700 "C for a few hours to expel C02 and then at 1000 "C for 24 h. XRD data for Rietveld refinement were collected with a Stoe Stadi/P diffractometer in transmission mode using a small linear position-sensitive detector (PSD) and a germanium mono-chromator providing Cu-Ka, radiation (A= 1.540 56 A).A scan range of 10<28 <110 " in steps of 1 was used in the refine- ment; the detector resolution was 0.02 ",Refinement was carried out using a squared Lorentzian function to model peak shape. This has been found to give the best fit for the peak shape profile on the Stoe diffractometer. Analysis was carried out using the Stoe software packages: initial indexing of the room- temperature powder pattern was attempted using the auto- indexing program INDEX, unit-cell refinement was performed using LATREF, theoretical powder patterns were generated using THE0 and Rietveld refinement used the pattern fitting structure refinement (PFSR) program.Differential thermal analysis (DTA) was carried out on a Stanton Redcroft STA675 instrument, using A1203 as a refer- ence material. Results The structure postulated by Blasse gave moderately good agreement with the intensities of the observed powder pattern. Hence, the structure was essentially correct, in so far as the Sb ions, which give most of the scattering, are located in a spinel- like arrangement within a ccp oxide structure. However, there are serious deficiencies in the fit of the proposed tetragonal cell to the observed d spacings for Li,SbO,. Table 1 gives the the first ten lines of the observed data together with the calculated d spacings obtained by refinement of the proposed tetragonal unit cell.It can be seen that the fit is extremely poor, and a number of lines could not be indexed in the complete pattern. Indexing of the powder pattern of Li,SbO, was therefore attempted using the auto-indexing procedure. No solutions gave a reasonable figure of merit; an orthorhombic tell of approximate dimensions a=5.985, b =6.084, c =8.362 A was found to index many of the lines, but the fit was still poor. It was felt, therefore, that lower symmetry was required to fully index the powder pattern. A survey of other compounds of the type A&O, revealed the monoclinic phase Na,BiO,, which has an ordered rocksalt structure.12 Chemically, it seems reasonable that there would be a similarity between the two structures, since Na and Bi lie one period below Li and Sb, respectively, in the Periodic Table.Na3Bi04 is moneclinic, space group P2/c, with a=5.871, b=6.696, c=5.659 A, /?= 109.8'. A theoretical powder pattern was generated for Li3SbO4 using the atomic coordinates for Na3Bi04 and was found to be similar to the observed powder pattern. The unit cell for Li3Sb04 was therefore indexed by comparison, and the first ten lines of the fully indexed pattern are given in Table 1. Comparison of the two sets of refinements in Table 1 shows an order of magnitude improvement in the fit of the monoclinic unit cell; the original tetragonal unit cell is therefore clearly incorrect. To refine the structure of Li,SbO,, 14 profile parameters were refined initially, including four cell parameters, instrumen- tal zero-point and scale factor.The variation of the full width J. Muter. Chem., 1996, 6(12), 1939-1942 1939 Table 1 The observed d spacings for Li,SbO, together with calculated d spacings for the tetragonal‘ and monoclinicb unit cells (first ten lines) tetragonal cell monoclinic cell d(obs)/A 1001/1, d(calc)/A h k I d(obs)-d(calc)/A d(calc)/A h k I d(obs)-d(ca1c) 6 0924 63 4 6 0389 1 0 0 0 0535 6 0923 0 1 0 00001 4 8817 1000 4 9024 1 0 1 -0 0207 4 8814 1 0 0 0 0003 3 8045 98 6 3 8061 1 1 1 -0 0016 3 8094 1 1 0 -0 0049* 3 8013 0 1 1 0 0032 3 4509 54 8 3 4468 1 0 2 0 0041 3 4509 -1 1 1 OoooO 2 6882 21 2 2 7007 2 1 0 -0 0125 2 6883 1 1 1 -0 0001 2 5818 31 1 2 5709 2 1 1 0 0109 2 5843 1 2 0 -00025 2 5817 0 2 1 0 0001 2 5267 16 6 2 5391 1 0 3 -0 0124 2 5269 -1 0 2 -0 0002 2 4634 11 3 2 4512 2 0 2 0 0122 2 4633 -1 2 1 0 0001 2 4405 75 -0 0107 2 4407 2 0 0 -0 0002 2 3390 16 7 2 3406 1 1 3 -0 0016 2 3397 -2 1 1 -0 0007 *broadened peak, resolve3 8092 42 2 3 8061 d into two separ 1 ate refl 1 ections i 1 n Rietveld refinement -0 0016 3 8095 1 1 0 -0 0003 3 8016 63 6 3 8013 0 1 1 0 0003 ‘Tetragonal cell a=b=6039(2), c=8 395(6)A, figure of merit=5O Monoclinic cell a=5 1578(1), b=60923(2), c=5 1397(1)A, ,!?= 108 841 (3)”, figure of ment =58 6 Figure of ment gven by l/(av A20) x (no observed reflections)/(no possible reflections), where the number of possible reflections is generated by the lattice type and crystal symmetry Note space group absences are not included and so the number of possible reflections may be an overestimate Table 2 Structural parameters for Li,SbO, after Rietveld refinement atom position X Y Z u,,, /A2 Sb 2e 0 0 1421(6) 0 25 0 0057(4) L1( 1) 2e 0 0 604( 9) 0 25 0 027(7) L1(2) 2f 05 0 87(2) 0 25 0 027( 7) W3) 2f 05 0418(8) 0 25 0 027( 7) O(1) 4g 0 221(2) 0091(2) OOOl(2) 0 010( 3) O(2) 4g 0 240( 2) 0 367( 2) 0 472(2) 0 014(3) Space group P2/c, 2=2, u=5 1578(2), b=60923(3), c=5 1397(2) A,/?=lo8 841(4)”, R,=403%, R,,=566%, R,=3 53% Table 3 Selected bond lengths for Li,SbO, bond bond length/A 234(4) x2 200(4) x2 218(1) x2 208(8) x2 223(2) x2 2 16(8) x2 255(4) x2 205(1) x2 208(3) x2 199(1) x2 201(1) x2 195(1) x2 I I 1 I I 111111 111111111 11M111111II IIIIIINIIIIIIIIIIIIRI1II I1 25 50 75 100 with atomic coordinates and a selection of bond lengths and 2Bldegrees angles in Tables 2 and 3 t Na3Bi04 has been shown to have a high-temperature cubic Fig.1 Expenmental XRD pattern with difference profile after Rietveld rocksalt form, with Na and Bi disordered over the cation refinement positions This form was stabilised by annealing at low tem- perature in flowing oxygen l3 In an attempt to transform the at half maximum was described by three Tchebychev poly- monoclinic Li,Sb04 structure to such a disordered form, nomials The background was also modelled using a series of samples were annealed at temperatures between 300 and 400 “C shifted Tchebychev polynomials up to the 5th degree Starting in oxygen, and also quenched from high temperatures (between parameters for the structural refinement were taken from the 1000 and llOO°C) into mercury No changes in the XRD ~~~~~~~~~Na3Bi04 structure l2 For the structural refinement, all Li 7 Atomic coordinates, thermal parameters, and bond lengths and atoms were constrained to have the same thermal parameter angles have been deposited at the Cambridge Crystallographic Data In the final refinement, ten positional and four thermal param- Centre (CCDC) See Information for Authors, J Muter Chem , 1996,eters were refined giving final R factors of R, =4 03%, R,, = Issue 1 Any request to the CCDC for this matenal should quote the 5 66% and RI=3 53% The final profile fit is shown in Fig 1 full literature citation and the reference number 1145/16 1940 J Muter Chem, 1996, 6(12), 1939-1942 Fig. 2 Relationship of the monoclinic unit cell of Li,SbO, to the cubic rocksalt subcell pattern were observed.Differential thermal analysis also showed no evidence for a transition up to 1200°C. Discussion Description of the Li,SbO, structure Li,Sb04 has a crystal structure derived from that of rocksalt by cation ordering. The cation ordering gives rise to a mono- clinic supercell whose volume is twice that of the pseudo-cubic subcell. The unit cells are related, as shown in Fig.2, by the transformations: umono=+I:1 2 11cutic=(J6/2)~; bmono=Ci 0 1Icubic =J~u;c,,~, =+ 1 2 1lcubic=(J6/2)~;p =109.5 O. As well as doubling the unit cell volume, the cation ordering causes a distortion from the pseudo-cubic symmetry of the subcell (e.g. umono#cmono, Table 1). The SbO, octahedra show small distortions in bond lengths (Table 3) whereas the LiO, octahedra exhibit considerable variations. Bond angles for the SbO, octahedra range from 162.0( 5) to 174.2( 5) O for the three 'ideal' 180 O angles and from 78.5(5) to 96.9( 5) O for the twelve ideal 90 angles. The Li06 octahedra show variations in bond angles from 163(2) to 174(4)O and from 75(1) to 102(1)". Projections of the crystal structure down b and a are shown in Fig.3 and 4. In Fig. 3, ccp oxide layers (open spheres) run horizontally and are interleaved by the cations in octahedral sites, with site ordering and overall, full site occupancy. The SbO, oxtahedra are not isolated from each other but link at opposite edges to form zigzag chains running parallel to csinp (Fig. 4). The structure shows considerable departures from Fig. 3 Projection of the Li,SbO, structure onto the ac plane. Large white circles represent oxygen, small white circles Sb and black circles Li. Layers of LiO, octahedra alternate with layers of mixed LiO,, SbO, octahedra. "tL c sin p Fig.4 Projection of the Li,SbO, structure down a, showing SbO, octahedral chains parallel to csinp local electroneutrality, using the simple criteria enunciated in Pauling's rule.The two types of oxygen have, as cation nearest neighbours, 4Li + 2Sb and 5Li + lSb, respectively, giving electrostatic bond strengths (ebs) for each of: 0(1):ebs=(4x-+ 2x-=2.333 ( 3 O(2): ebs= (5x-+ lx-=1.673 ( 3 Several worker~~~-l~ have developed the concept of bond valences from the electroneutrality principle of Pauling; throughout the cation-anion network, the valence of an atom is assumed to be distributed between the bonds it forms. Each bond can be assigned a bond valence, S, such that the sum of the bond valences at each atom is equal to the atomic valence. The bond valence can be calculated from the bond lengths using the simple relationship. S= exp["", "1=($)-N~ where R,, N and B are tabulated parameters." Using these equations and the bond lengths from Table 3, the bond valences and hence atomic valences were calculated; the results are given in Table4.Had there been any serious errors in the structure solution, the bond valence sum around each atom would differ greatly from its atomic valence; however, the results show the sums to be in good agreement with the oxidation state of each atom. Table 4 Bond valence sums (BVS) for the Li,SbO, structure bond bond length/A bond valence BVS 2.34(4) x2 0.113 2.00(4) x2 0.229 1.oo 2.18(1) x2 0.157 2.08(8) x2 0.194 2.23(2) x 2 0.142 1.oo 2.16(8) x2 0.164 2.55(4) x2 0.073 2.05(1) x2 0.206 0.95 2.08(3) x2 0.194 1.99(1) x2 0.784 2.01(1) x2 0.739 4.82 1.95(1) x2 0.886 J.Muter. Chem., 1996, 6(12), 1939-1942 1941 Comparison with other Li3M04 structures The phases L13M04 fall into two general structural families, depending on the size of M For smaller M (P, As, V, Cr and Mn), tetrahedral coordination of both Li and M is preferred and there are two main structure types, the so-called p and y structures l9 The p structures are ordered wurtzite superstruc- tures, with cation ordenng over one set of tetrahedral sites within a hexagonal close packed (hcp) oxide array The y structures have the cations ordered over both sets of tetrahedral sites within an oxide ion array that is somewhat distorted from hcp and may alternatively be descnbed in terms of tetragonal packing 2o 21 Most phases are polymorphic with /3 as the low- temperature form and y as the high-temperature form With increasing slze of M, the structures change completely and both Li and M occupy octahedral sites within a ccp oxide array to give a family of structures based on rocksalt Several structural variants occur and full structural details for all of them are not yet available In spite of the fact that Nb, Ta and Sb often form crystallochemically smilar phases, their phases L13M04 are significantly different from each other, whilst st;ll belonging to the rocksalt family Li3Nb04 is cubic (a= 8 405 A, 2=8)5 6, but can also be preppred as a cation-disordered, metastable form with a=4 212 A, Z= 1 by low temperature synthesis ' Li3Ta04 forms three polymorphs,22 the low- and high-temperature foFs are both monoclinic C2/c, a = 8 500, b=8 500, c=9 34,4 A,8 p= 117 05" and P2/n, a=6 018, b= 5 995, c= 12 865 A, p= 103 53 O respectively The intermediate temperature form, occurring between 900 and ca 1400OC2, has not been characterised fully8 since it has proved difficult to obtain the phase at room temperature by quenching 2o 23 It has been suggested, ho?ever,* that it is the disordered cubic We would like to thank Dr R A Howie for his assistance with bond length calculations, and we acknowledge the use of the EPSRC funded Chemical Database Service at Daresbury M A C and S T T thank UNAM for support, project number IN 101893 PAPIIT References 1 G Blasse, Z Anorg Allg Chem , 1963,32644 2 Powder Diffraction File, PDF card no 17-824, International Centre for Diffraction Data, Newtown Square, Pennsylvania, USA 3 PDF card no 26-861, in ref 2 4 F Daniel, J Moret, E Philipott and M Maurin, J Solid State Chem ,1977,22,113 5 J C Grenier and G Bassi, Bull SOC Fr Miner Crist , 1965,88,345 6 K Ukei, H Suzuki, T Shishido and T Fukuda, Acta Crystllogr Sect C, 1994,50,655 7 J C Grenier, C Martin and A Dunf, Bull SOC Fr Miner Crist, 1964,87,3 16 8 M Zocchi, M Gatti, A Santoro and R S Roth, J Solid State Chem, 1983,48,420 9 M Zocchi, M Gatti, A Santoro and R S Roth, J Solid State Chem , 1984,55,277 10 J Danet and J Galy, Bull SOC Fr Miner Crist , 1974,97, 3 11 M G Barker and D J Wood, J Chem SOC Dalton Trans, 1972,19 12 B Schwedes and R Hoppe, Z Anorg Allg Chem , 1972,393,136 13 S M Fray, PhD Thesis, University of Aberdeen 1996 14 I D Brown, in Structure and Bonding in Crystals, vol 11, ed M O'Keeffe and A Navrotsky, Academic Press, London, 1981 15 G Donnay and J D H Donnay, Acta Crystallogr Sect B, 1973, 29,1417 16 I D Brown and R D Shannon, Acta Crystallogr Sect A 1973,rocksalt form (a = 4 203 A), reported by Lapicky and Siman~v~~ and by Pfeiffer 25 The structure of Li3Sb04 reported here differs from these various Li3Nb04 and Li3Ta04 polymorphs Other L13M04 structures have also been reported Both L13uo4 and L13BlO4 havf tetragonal, ordered rocksalt struc- tures% with a z4 5, c = 8 5 A for Li3U0423 26 27 and a = 8 75, c = 4 22 A for Ll3B104 23 Li3Re04 has two polymorphs,28 the low- temperature form is monoclinic (C2/c) and is isostructural with Li,Sn03, the Sn sites being occupied by statistically disordered Re and Li to give Li, [ Re, 75Lio 25]0323 The high-temperature form is a disordered cubic rocksalt structure, isostructural with Li3OsO4 27 and Li3Ta04 24 25 Recently, the structure of Li3Ru04 was determined by energy minimisation procedures 29 Li3Ru04is, essentially, isostructu- ral with Li3Sb04 and Na3Bi04, however, one of the oxygen positions in Li3Ru04, 0(2), is shifted across a mirror plane with respect to the corresponding oxygen [0(l)] in Li3Sb04 This difference has been confirmed by Rietveld refinements of the Li3Ru04 structure using powder X-ray diffraction data, based on both the proposed Li3Ru04 structure and the Li3Sb04 structure 30 29,266 17 R Allman, Monatsh Chem ,1975,106,779 18 I D Brown, Acta Crystollagr Sect B, 1977,33, 1305 19 A R West, Z Kristullogr , 1975,111,422 20 A R West and P G Bruce, Acta Crystollagr Sect B, 1982, 38, 1891 21 W H Baur, Muter Res Bull, 1981,16,339 22 L C Martel and R S Roth, Bull Am Ceram SOC, 1981,60,376 23 G Blasse, Z Anorg Allg Chem , 1964,331,44 24 A V Lapicky and J P Simanov, cited in Struct Rep, 1953, 17, 392 25 P P Pfeiffer, PhD Thesis, Technische Hochschule, Karlsruhe 1961 26 C Miyake, H Takeuchi, H Ohya-Nishiguchi and S Imoto, Phys Status Solrdi A, 1982,74, 173 27 H Glaser, PhD Thesis, Technische Hochschule, Karlsruhe, 1961 28 R Scholder and P P Pfeiffer, Angew Chem Int Ed Engl, 1963 2,265 29 T S Bush, C R A Catlow and P D Battle, J Muter Chem, 1995, 5,1269 30 J M S Skakle, R A Howie and A R West, unpublished results Paper 6/03984G, Received 6th June, 1996 1942 J Muter Chem, 1996, 6(12), 1939-1942