Hyper-Rayleigh scattering from the bulk of nominally pureKTaO3

Second-harmonic light or hyper-Rayleigh scattering from the bulk of nominally pure ${\mathrm{KTaO}}_3$ is studied as a function of scattering configuration and temperature between 10 and 200 K. At the lower end of the temperature range the selection rules on the polarizations of incident and scattered light are similar to those of the soft-mode hyper-Raman line, while at the upper end they become reminiscent of hyper-Raman scattering by longitudinal-optical phonons. For any scattering configuration the temperature dependence of the intensity is given by (A+B${\Omega }_0^{\mathrm{-}2}$ ${)}^2$, where the soft-mode frequency ${\Omega }_0$ is the only parameter varying with temperature. We interpret our results in terms of symmetry-breaking microregions formed by quasistatic lattice distortions around a defect core. Hyper-Rayleigh scattering turns out to be unspecific with respect to the core, but to probe the activation of hyper-Raman tensors by the quasistatic lattice distortions and the associated macroscopic electric field. We compare our results with recent observations of noncubic local symmetry in ${\mathrm{KTaO}}_3$. .AE