THE INFLUENCE OF DEFECT-INDUCED POLARIZATION CLUSTERS ON FREQUENCY AND WIDTH OF THE SOFT MODE IN NOMINALLY PURE KTAO3

Hyper-Raman spectroscopy is used to compare the soft-mode frequencies OMEGA and damping constants gamma of two samples of nominally pure KTaO3 as functions of temperature between 5 and 300 K. While the hyper-Rayleigh scattering efficiencies of the two samples differ by a factor of about 2 in the whole range of temperature, measurable discrepancies in OMEGA and gamma are found only below 60 K reaching a low-temperature limit around 2 cm-1. The discrepancies are interpreted to result from isolated, quasistatic polarization clusters of the host crystal around symmetry-breaking defects being present in both samples with different concentrations or strengths of coupling to the host lattice. Since the extension of the polarization clusters is determined by the correlation length r(c) of the soft-mode polarization, the differences in OMEGA and gamma follow some power of r(c) and hence decrease with increasing temperature.