Low-frequency relaxation modes and structural disorder in ${\mathrm{KTa}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{O}}_3$

A light-scattering study of the low-frequency excitations in single crystals of cubic and tetragonal ${\mathrm{KTa}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Nb}}_{\mathrm{x}}$ ${\mathrm{O}}_3$ (x=0.26 and 0.28) has been carried out by employing a technique which uses an Iodine filter to remove elastically scattered light. Low-frequency Raman and Fabry-Pérot components related to structural disorder in the mixed-crystal tantalate-niobate system were observed. The spectral shape, symmetry properties, and thermal behavior of these components are consistent with an eight-site order-disorder model of the sequence of structural phase transitions observed in ferrodistortive perovskites.