Evidence of defect-induced polarization clusters in nominally pure KTaO3from low-temperature Raman and hyper-Raman spectra

A comparison is made between low-temperature Raman and hyper-Raman spectra of six single crystals of KTaO 3 grown as pure according to the present state of the art by top-seeded solution and spontaneous-nucleation techniques. All samples differ in four spectral features: (a) first-order Raman intensity, (b) hyper-Rayleigh intensity, (c) width of the soft-mode hyper-Raman line, (d) soft-mode frequency. While (a), (b) and (c) are approximately linear functions of each other, (d) turns out to vary with the square of (a) or (b). No correlation is found between my of the four quantities under study and the photoluminescence at 687 run recently assigned to Ta 3+ near oxygen vacancies. The results are interpreted in terms of polarization clusters formed by the host crystal in response to static or quasi-static defects of the random local-field type. Since the radius of a single polarization cluster reaches the magnitude of about five lattice constants, the specific defect structure at the centre is screened, so that the observed relationships between (a), (b), (c) and (d) become rather general.