Lattice Dynamics

In this letter we obtain the finite-temperature structure

of 180^o domain walls in PbTiO_3 using a quasi-harmonic

lattice dynamics approach. We obtain the temperature dependence of

the atomic structure of domain walls from 0 K up to room

temperature. We also show that both Pb-centered and Ti-centered

180^o domain walls are thicker at room temperature; domain

wall thickness at T=300 K is about three times larger than that of

T=0 K. Our calculations show that Ti-centered domain walls have a

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.