  We present first principles calculations of the phonon dispersions of Bi2Te3
and discuss these in relation to the acoustic phonon interface scattering in
ceramics. The phonon dispersions show agreement with what is known from neutron
scattering for the optic modes. We find a difference between the generalized
gradient approximation and local density results for the acoustic branches.
This is a consequence of an artificial compression of the van der Waals bonded
gaps in the Bi2Te3 structure when using the generalized gradient approximation.
As a result local density approximation calculations provide a better
description of the phonon dispersions in Bi$_{2}$Te$_{3}$. A key characteristic
of the acoustic dispersions is the existence of a strong anisotropy in the
velocities. We develop a model for interface scattering in ceramics with
acoustic wave anisotropy and apply this to Bi2Te3 and compare with PbTe and
diamond.
