Finite-Temperature Atomic Structure of 180^o Ferroelectric Domain Walls in PbTiO_3

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

lower free energy than Pb-centered domain walls and hence are more

likely to be seen at finite temperatures.