  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.

Materials Science