Neutron powder diffraction study of the intermediate-temperature form of lithium tantalum oxide (1977)

M-LiTa30 8, monoclinic, C2/c, a = 9.410 (1), b = 11.521 (1), c = 5 .0506(5) /k, f l = 91 .108(5) ° , Z = 4. The location of the lithium ion in the structure has been determined by profile analysis of powder diffraction data. Introduet|on. M-LiTa30 8, stable between 800 and 1100°C, has been identified by Roth, Parker, Brower & Minor (1974) during an investigation of the LiTaO 3Ta205 system. Its crystal structure has been previously analyzed by X-ray single-crystal techniques (Gatehouse, Negas & Roth, 1976). The compound was found to be crystallographically similar to the mineral wodginite. However, it was not possible to determine the position of the lithium ion in the structure because of the dominant scattering of the Ta 5+ ions. The Li + ions were, therefore, approximately located in octahedral gites by analogy with related compounds and on the basis of other crystallochemical considerations such as the weak tendency of the Ta 5+ ions to share octahedral edges and the unlikely possibility of face sharing. With this assignment the proposed structure becomes an ordered variant of the ~t-PbO2 structure type with the Li + and Ta 5+ ions more equidistantly ordered than in the structure of LiNb308. As the values of the neutron Table 1. Experimental conditions used to measure the powder pattern of M-LiTa30 8 Monochromatic beam: reflection 220 of a Cu monochromator Mean neutron wavelength: 1.358 (1) A Take-off angle: 638 ° Monochromator mosaic spread: 20' arc Horizontal divergences (a) In-pile collimator Cq = 21' arc (b) Monochromatic beam collimator Ct 2 = 24' arc (c) Diffracted beam collimator~ 3 = 23' arc Theoretical values of full width at half maximum (fwhm) parameters* U=0.72, V=-0.47, W=0.24deg 2 Sample container: vanadium can of ~ 1 cm diameter 2 0 1 n i t l a ~ = 9.0 °, 20nnal = 780 °, step = 0-1 o Number of independent Bragg reflections: 247 Number of observations above background: 641 * The parameters U, V and 14 appear in the expression H 2 = U t a n 20 + V tanO + I4, where H is the fwhm of a reflection with Bragg angle O. Their theoretical values are calculated with the expressions given by Caglioti, Paoletti & Ricci (1958). scattering amplitudes of the atoms in the structure do not differ greatly from each other, a neutron diffraction analysis of the title compound was carried out to determine the position of the Li ÷ ion and thus confirm or reject the L i O coordination polyhedra assumed in the X-ray work. M-LiTa30 8 was prepared by mixing LiTaO 3 and Ta20 5 under acetone, drying overnight and then heating the mixture at 1025 o C for several hours. The product of this treatment was allowed to cool slowly, then ground and the entire operation repeated until X-ray powder patterns of samples obtained in successive operations showed no change. Equilibrium was assumed to have been obtained after four remixings and after a total of 50.5 h at 1025°C. Neutron diffraction measurements were made at room temperature on a powder diffractometer of intermediate resolution at the National Bureau of Standards Reactor. The experimental conditions used to take the powder pattern are indicated in Table 1. The refinement of the structure was based on the space group C2/c and was carried out with the method of total profile analysis proposed by Rietveld (1969). The neutron scattering amplitudes were b(Ta) = 0.70, b(O) = 0.58, b(Li) = 0 . 2 1 5 x 10 -12 cm (Bacon, 1972). The contribution to the profile from the background was determined at portions of the powder pattern free from diffraction effects, and estimated by linear extrapolation from these regions at high scattering angles, where overlapping is most severe. The initial values of the structural and lattice parameters were those determined in the X-ray study, and those of the profile parameters U, V and W were calculated theoretically (Caglioti, Paoletti & Ricci, 1958). In the first stage of the analysis the coordinates and temperature factors of all atoms were kept fixed and two refinements of the scale factor and of the lattice and profile parameters were carried out, one with the Li ÷ ion in the position postulated by Gatehouse, Negas & Roth (1976), and one with the Li ÷ ion removed from the structure. The R factors obtained in the two refinements were 6.1 and 10-6% respectively. This result clearly shows that the location of the Li ÷ ion was postulated correctly in the X-ray work. In the final refinement a total of 30 structural, lattice and profile parameters were refined simultaneously until all shifts were less than 0.300. 3946 T H E I N T E R M E D I A T E T E M P E R A T U R E F O R M OF L I T H I U M T A N T A L A T E - 4[ l_,ithium Tantalate • Observed Value [ 3t Intermediate Temperature Form Calculated Profile] / ~ Ii 7 2 / {/ J l i~ 10 15 20 25 30 35 40 45 50 55 60 65 70 75 DIFFRACTION ANGLE (DEGREES) Fig. 1. Comparison of the calculated and observed neutron diffraction profiles for M-LiT%O 8. Table 2. Results of the total profile refinement of the structure of M-LiTa30 8