  In this paper we extend the classical method of lattice dynamics to defective
crystals with partial symmetries. We start by a nominal defect configuration
and first relax it statically. Having the static equilibrium configuration, we
use a quasiharmonic lattice dynamics approach to approximate the free energy.
Finally, the defect structure at a finite temperature is obtained by minimizing
the approximate Helmholtz free energy. For higher temperatures we take the
relaxed configuration at a lower temperature as the reference configuration.
This method can be used to semi-analytically study the structure of defects at
low but non-zero temperatures, where molecular dynamics cannot be used. As an
example, we obtain the finite temperature structure of two 180^o domain walls
in a 2-D lattice of interacting dipoles. We dynamically relax both the position
and polarization vectors. In particular, we show that increasing temperature
the domain wall thicknesses increase.
