Ab initio calculations of BaTiO3 and PbTiO3 (001) and (011) surface structure

TL;DR

This study uses ab initio calculations to analyze surface relaxations and energies of BaTiO3 and PbTiO3 (001) and (011) surfaces, revealing differences in relaxation behaviors and increased Ti-O covalency at the surfaces.

Contribution

First ab initio study comparing surface relaxations and energies of BaTiO3 and PbTiO3 (001) and (011) surfaces with detailed analysis of terminations.

Findings

Inward relaxation of AO layers on (001) surfaces.

Larger relaxation energies for TiO-terminated (011) surfaces.

Increased Ti-O covalency near (011) surfaces.

Abstract

We present and discuss the results of calculations of surface relaxations and rumplings for the (001) and (011) surfaces of BaTiO3 and PbTiO3, using a hybrid B3PW description of exchange and correlation. On the (001) surfaces, we consider both AO (A = Ba or Pb) and TiO2 terminations. In the former case, the surface AO layer is found to relax inward for both materials, while outward relaxations of all atoms in the second layer are found at both kinds of (001) terminations and for both materials. The surface relaxation energies of BaO and TiO2 terminations on BaTiO3 (001) are found to be comparable, as are those of PbO and TiO2 on PbTiO3 (001), although in both cases the relaxation energy is slightly larger for the TiO2 termination. As for the (011) surfaces, we consider three types of surfaces, terminating on a TiO layer, a Ba or Pb layer, or an O layer. Here, the relaxation energies are much larger for the TiO-terminated than for the Ba or Pb-terminated surfaces. The relaxed surface energy for the O-terminated surface is about the same as the corresponding average of the TiO and Pb-terminated surfaces on PbTiO3, but much less than the average of the TiO and Ba-terminated surfaces on BaTiO3. We predict a considerable increase of the Ti-O chemical bond covalency near the BaTiO3 and PbTiO3 (011) surface as compared to both the bulk and the (001) surface.