J. MATER. CHEM., 1994, 4(8), 1303-1305 1303

Li,Ni,TaO,: A Novel Rock Salt Superstructure Phase with Partial Cation Order

James G. Fletcher," Glenn C. Mather," Anthony R. West," Maria Castellanosb and Maria Pilar Gutierre? a University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen, UK AB9 2UE

Universidad Nacional Autonoma de Mexico, Facultad de Quimica, Mexico DF 04510, Mexico

The new phase Li,Ni2Ta06 has a novel rock salt superstructure, a = 8.4259(3) A, b = 5.9073(3) A, c = 17.7329 A, Fddd. Within the cubic-close-packed oxide array, Ta occupies one set of octahedral sites giving isolated TaO, octahedra which edge-share with Li/NiO, octahedra. Li and Ni are distributed non-randomly over three other sets of octahedral sites with Li : Ni occupancy ratios of 0.73 : 0.27, 0.59 : 0.41 and 0.55 : 0.45, respectively. The higher Li : Ni ratio of the first set, 0.73:0.27 may be associated with cation-cation repulsion effects since this site is closest to Ta. Li,Ni,Ta06 is a very modest semiconductor with conductivity of ca. 4 x l op6 R-' cm-' at 300 "C and activation energy 0.77 eV; the disorder in the Li' site occupancy is, therefore, static and does not yield significant levels of Li+ ion conductivity.

Several examples of complex oxides with rock salt superstruc- tures are known. They usually have fully ordered cation arrangements and different ordering sequences are possible, depending on the cation ratios, e.g. Li,TiO,, Li3Nb04, LiFeO, and LiCoO,. Recently, a family of phases Li,MXO,: M = Mg, Mn, Fe, Co, Ni, Cu, Zn; X=Zr, Hfl,, was discovered with partial cation disorder. The pairs of ions MX are disordered over one set of octahedral sites, giving a crystal structure which is essentially the same as that of ~r-LiFeo,.~ Here we report a second kind of rock salt superstructure with partial cation order but in this case, the disordered cations, Li+ and Ni2', exhibit a most unusual non-random distribution over three sets of octahedral sites.

L13Ni2Ta06 was synthesized by solid-state reaction of Li,C03, NiO and Ta,O, in P t crucibles. Samples were fired over the range 600-11OO0C, initially to expel CO, at 600-700 "C and then at higher temperature to complete reaction. After an intermediate regrinding, final firing was at 1100°C for 2 days. The green product was characterised by X-ray powder diffraction (Siemens D500, Cu-Kq radiation) and found to be very similar to the previously reported Li,Mg,XO,: X =Nb, Ta, Sb,4 whose patterns were indexed but whose structures were unknown. X-Ray data suitable for accurate lattice-parameter determination and Rietveld refinement were collected on a Stoe Stadi P powder diffractometer with a linear position-sensitive detector, Cu-Kcr, radiation, Ge monochromator; for cell-parameter determination, Si internal standard was added, collection time 3 h; for Rietveld refinement, Si was not added and the collection time was 14h over the range 1O"C<2~<11O"C. Various programs in the Stoe software package were used for data handling, processing and refinement.

The powder diffraction data were indexed by analogy with those of the phases Li,Mg,X0,4 on an orthorhombic cell, space group Fddd [no. 70, second setting]. By assuming a rock salt superstructure, structure models were generated using the software program THEO, taking into account the most likely cation ordering sequences and differences in intensities of certain reflections for different members of the Li,M,XO, family.

Before the calculated pattern was allowed to refine, the profile of the observed pattern was fitted to a squared Lorentzian function to describe the shape of the Bragg reflections. Thirteen parameters describing the calculated pat- tern were initially refined, including halfwidths, 28 zero-point, unit-cell dimensions, scale factor and background coefficients. Structural parameters were then refined. Comparison of calcu-

lated and observed patterns had indicated the probable cor- rectness of a rock salt superstructure model in which Ta sites are fully occupied but Li and Ni are disordered over three sets of crystallographic sites. The positional parameters and thermal vibration parameters of the three Ni/Li sites were refined initially as rigid units with the occupancy factors set initially to 0.6 and 0.4 for Li and Ni, respectively, according to the chemical composition Li,Ni,TaO,. Positional param- eters of Li/Ni were refined first followed by oxygen positions. Attempts to refine both the nickel and lithium site occupancies and their Uiso values simultaneously were unsuccessful. As a result, it was necessary to choose sensible invariant values for the thermal vibration parameters and allow occupancies to vary with the restriction that total individual site occupancies must be unity. The occupancy factors obtained for Li and Ni corresponded approximately to the total expected number of ions per unit cell. Finally, isotropic thermal vibration param- eters of Ta and 0 were refined.

The final observed and difference patterns are shown in Fig. 1; R,=7.56%, R,,=10.25% andR,=6.81%. Ashortened list of indexed powder X-ray data for Li,Ni,TaO6 is shown in Table 1 and crystallographic data obtained by Rietveld refinement in Table 2.

The structure may be considered as a rock salt superstruc-

10all 7500

i! 25 50 75 100

28ldegrees

Fig. 1 (a) Observed and (b) difference powder diffraction pattern of Li,Ni,Ta06

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1304 J . MATER. CHEM., 1994, VOL. 4

Table 1 X-oRay powder $iffraction data (I for Li,Ni,TaO,: a = 8.4259( 3) A, b = 5.9073( 3) A, c = 17.7329( 6) A

4.6658 4.4339 3.8052 3.7436 2.8601 2.8024 2.5110

2.4 192 2.3310 2.2441 2.2167 2.1230 2.1067 2.0893 2.0635 1.9064

1.8243 1.7925 1.6840

4.6665 4.4332 3.8052 3.7434 2.8601 2.8023 2.51 10 2.4195 2.4185 2.3310 2.2441 2.2166 2.1231 2.1065 2.0892 2.0632 1.9063 1.8247 1.8239 1.7925 1.6838

100 33 51 57 32 22 19

60 13 12 6

17 33 67 11 9

13

5 11

111 004 022 113 115 202 131 026 220 133 117 008 224 040 206 135 311 119 313 137 242

b -11.36 38.88 39.86 38.88

C

A / 39'86 14.61

B /

/ C

A

/ I B

0 Ta 0 Li/Ni 0 oxygen

Fig. 2 Crystal structure of Li3Ni,Ta0, projected onto bc plane

Table 3 Ta-Li/Ni distances/A

distance to

Li/Ni ( 1 ) 2.974( x 2) 2.957( x 4)

Li/Ni ( 2) 3.015( x 4)

Li/Ni( 3) 2.954( x 2)

ture with eight formula units per unit cell. The relationship of supercell and subcell is: a = J2aSub, b = 2aSub, c = 3420,"~. The superstructure arises as a consequence of Ta ordering on the 8a site in an attempt to minimise Ta-Ta repulsions. The other octahedral sites are filled non-randomly by Li,Ni. The Li/Ni( 3) site shows a strong preference for Li whereas Li/Ni( 1) shows a slight preference for Ni. The occupancy of Li/Ni(2) is very close to the statistical ratio 0.60 : 0.40.

Fig. 2 shows a projection along the [loo] zone axis. The TaO, octahedra are isolated from each other but share edges with various Li/Ni octahedra. Average bopd distances are Ta-0, 1.96(2) A; Li/Ni(l)-0, 2.11(3) A; Li/Ni(2)-0, 2.13(4) A; Li/Ni(3)-0, 2.17(8) A. Cubic-close-packed oxide layers in two orientations can be seen in Fig. 2. Between any pair of close packed layers, the octahedral sites are occupied by Ta and Li/Ni in the ratio 1: 5.

The higher Li content of the Li/Ni( 3) site may be associated with cation<ation repulsion effects since the distance from this site to Ta, 2.954A, is the shortest of all the Li/Ni to Ta distances, Table 3. These diffraction results do, of course, give only an average structure and it is possible that a more complex structure, with local cation order, is in fact present.

Conductivity measurements were made on a sintered pellet of Li,Ni,TaO, with Au electrodes, using Solart ron 1250/1286 and Hewlett Packard 4192 impedance instrumentation. The impedance data showed an essentially bulk response, with relatively small grain-boundary impedances. Little evidence of electrode polarisation effects was seen, indicating the charge carrier to be electronic. A conductivity Arrhenius plot is

-21 0, 0 -

-5 -

- 6 1 1.0 1.2 1.4 1.6 1.8

1 O ~ W T

Fig. 3 Conductivity Arrhenius plot (E, = 0.77 eV)

Table 2 Structure refinement data for Li,Ni,TaO,

atom site xla Ylb c/z uiso occupancy (n) ~

Ta(1) 8 (4 118 1/8 1/8 0.0064( 7) 1 .oo Li( l)/Ni( 1) 1 6 k ) 1/8 1.8 0.2928( 7) 0.0225 0.550( 9)/0.450( 9) Li(2)/Ni(2) 1 m ) 1/8 5/8 0.2864( 7) 0.0225 0.%6( 9)/0.414( 9) Li(3)/Ni( 3 ) 8 (b) 1/8 518 1/8 0.0225 0.73( 1)/0.27( 1)

O(1) 16(f) 1/8 0.357( 2) 118 0.02 1 (6) 1 .oo O(2) 32(4 0.1 10( 2) 0.378( 2) 0.2969( 9) 0.024 (4) 1 .oo

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shown in Fig. 3, which was reversible on heat/cool cycles; the References

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1

2

data indicate a very low level of electronic conductivity. Thus, in spite of the fact that the crystal structure contains considerable disorder in positions of Li' ions, this disorder is static and does not lead to significant levels of ionic conductivity. 3

4 We thank SERC for financial support (A.R.W.) and the British Council for supporting the Aberdeen-Mexico exchange pro- gramme and we gratefully acknowledge financial support for project UNAM IN 101893 (M.C.) from the Mexican authorities.

M. Castellanos, A. R. West and W. B. Reid, Acta Crystallogr., Sect. C, 1985,41,1707. M. Castellanos, M. Chavez Martinez and A. R. West, 2. Kristallogr., 1990, 190, 161. E. Posnjak and T. F. W. Barth, Phys. Rev., 1931,38,2234. M. Castellanos, J. A. Gard and A. R. West, J. Appl. Crystallogr., 1982, 15, 116.

Paper 4/01963F; Received 31st March, 1994

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