Translational and rotational mode coupling in disordered ferroelectric KTa(1-x)Nb(x)O(3) studied by Raman spectroscopy

The coupling of translational modes to the reorientational motion is an essential property of systems with internal orientational degrees of freedom. Due to their high complexity most of those systems (molecular crystals, glasses...) present a major puzzle for scientists. In this paper we analyze the Raman scattering of a relatively simple ferroelectric system, KTa(1-x)Nb(x)O(3), which may serve as a model for more complicated cases. We are showing that there is a strong coupling between translational and reorientational motion in the crystal. Our data suggest that this coupling is the main reason for the depolarized component of the second order Raman spectra and that it is also responsible for lowering of the frequency (softening) of the transverse acoustic mode down to the third of three transitions, below which reorientational motion is no longer allowed.