Polar fluctuations and first-order Raman scattering in highly polarizableKTaO3crystals with off-center Li and Nb ions

Abstract

Relatively low concentrations of randomly substituted off-center ions are known to induce ferroelectric order in highly polarizable crystals. In the case of potassium tantalate (${\mathrm{KTaO}}_3$) doped with lithium or niobium, the results of first-order Raman scattering from two hard phonon modes (${\mathrm{TO}}_2$ and ${\mathrm{TO}}_3$) show that polar microregions are formed at relatively high temperatures. At lower temperatures, and above a certain critical dopant concentration, the Raman results are consistent with the occurrence of a structural phase transition. These results can be reasonably explained by using a random-molecular-field theory that takes into account the indirect dipolar interactions mediated by the soft lattice. This approach accounts for the characteristic asymmetry of the ${\mathrm{TO}}_2$ Raman scattering detected at high temperatures and for the subsequent increase in the integrated scattering intensity with decreasing temperature. An analysis of the Raman results provides a means for determining the size of the polar microregions formed in the highly polarizable ${\mathrm{KTaO}}_3$ lattice. The good agreement obtained between the theoretical and experimental results provides support to the physical concepts embodied in the present theoretical approach to describing the onset of order in these systems.