Phonon dispersion and lattice dynamics ofKTaO3from 4 to 1220 K

A complete set of transverse and longitudinal phonon branches in each of the three principal directions (〈100〉, 〈110〉, and 〈111〉) and polarizations have been measured up to 60 meV in the cubic perovskite ${\mathrm{KTaO}}_3$. Results were obtained at 296 K and at several other temperatures between 4 and 1220 K. The data have been analyzed in terms of a shell model which includes a fourth-order core-shell interaction at the ${\mathrm{O}}^{2\mathrm{-}}$ ion. Within the self-consistent mean-field approximation a very accurate description of the lattice dynamics over the whole temperature range is obtained. It is shown that the nonlinear polarizability of ${\mathrm{O}}^{2\mathrm{-}}$ along the Ta-O-Ta chains is primarily responsible for most of the phonon shifts, particularly of the ferroelectric soft mode, and for the strong second-order Raman effect.