Critical quasielastic light scattering in ${\mathrm{KTa}}_{0.968}$${\mathrm{Nb}}_{0.032}$${\mathrm{O}}_3$

Intense quasielastic light scattering is observed in potassium tantalate niobate at temperatures near the ferroelectric transition at $T_c$∼40 K. The central peak (CP) has a non-Lorentzian shape and consists of a sharp component with a broad tail. Near $T_c$, the sharp component narrows to 1.5 GHz (half-width at half maximum) which is close to the limit of resolution achievable when using an ${\mathrm{I}}_2$ vapor filter to eliminate elastically scattered light. It is, therefore, possible that the CP has a much narrower distribution than that of the renormalized Fabry-Perot data. Spectra obtained over a range of frequency shifts including the Raman scattering from the soft TO mode have been fitted by a coupled-mode formalism in which it is assumed that the CP derives all of its intensity from the coupling to the TO phonon. This model accounts reasonably well for the relative intensities of the CP and phonon and the large renormalized frequency of the phonon near $T_c$. The integrated intensity of the CP has a temperature dependence which is singular at $T_c$, whereas that of the TO phonon is close to a step-function increase at $T_c$. Although ordinary two-phonon difference (phonon-density-fluctuations) scattering cannot be excluded as a possible origin of the observed quasielastic scattering, several properties of the quasielastic and Raman spectra, as well as other recent experimental results, suggest a mechanism involving fluctuations due to the disorder in the polarizabilities and force constants introduced by the Nb substitution.