TEMPERATURE-DEPENDENCE OF THE LOCAL-STRUCTURE AROUND THE TANTALUM ATOM IN POTASSIUM TANTALATE

The local structure around the Ta atom in potassium tantalate KTaO3 ha s been investigated using extended x-ray absorption fine-structure (EX AFS) measurements. EXAFS spectra near the Ta L(III) edge were observed at 24, 40, 100, 150, 200, 250 and 298 K. The Debye-Waller factor has been analysed in detail as also have other local structure parameters. Anharmonicity of the thermal vibration of the Ta atom which is locate d at the centre of an oxygen octahedron is almost negligible below roo m temperature, while anisotropy of the thermal vibration of the Ta ato m increases gradually with decreasing temperature. It has been found t hat the anisotropic thermal vibration in the [001] directions at low t emperatures, which was estimated by EXAFS analysis, corresponds to the softening of the transverse optical phonon at the Brillouin zone cent re found by an inelastic neutron scattering study.