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

The local structure around the Ta atom in potassium tantalate KTaO3 has been investigated using extended x-ray absorption fine-structure (EXAFS) 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 located at the centre of an oxygen octahedron is almost negligible below room temperature, while anisotropy of the thermal vibration of the Ta atom increases gradually with decreasing temperature. It has been found that the anisotropic thermal vibration in the [001] directions at low temperatures, which was estimated by EXAFS analysis, corresponds to the softening of the transverse optical phonon at the Brillouin zone centre found by an inelastic neutron scattering study.