Hyper-Raman scattering from the incipient ferroelectric KTaO3

Hyper-Raman scattering from KTa${\mathrm{O}}_3$ is studied to obtain the frequencies and linewidths of the seven optical $\Gamma$ phonons with a high degree of accuracy. Particular attention is paid to the soft-mode region below 150 ${\mathrm{cm}}^{-1\mathrm{}}$. We confirm and extend the data found by electric-field-induced Raman spectroscopy. On the other hand, considerable discrepancies exist between hyper-Raman and infrared results. We also discuss the origin and temperature dependence of the hyper-Rayleigh line.