Theory of PbTiO3, BaTiO3, and SrTiO3 surfaces

First-principles total-energy calculations are carried out for (001) surfac es of the cubic perovskite ATiO(3) compounds PbTiO3, BaTiO3, and SrTiO3. Bo th AO-terminated and TiO2-terminated surfaces are considered, and fully-rel axed atomic configurations are determined. In general, BaTiO3 and SrTiO3 ar e found to have a rather similar behavior, while PbTiO3 is different in man y respects because of the partially covalent character of the Pb-O bonds. P bTiO3 and BaTiO3 are ferroelectrics, and the influence of the surface upon the ferroelectric distortions is studied for a tetragonal ferroelectric dis tortion parallel to the surface. The surface relaxation energies are found to be substantial, i.e., many times larger than the bulk ferroelectric well depth. Nevertheless, the influence of the surface upon the ferroelectric o rder parameter is modest, and is qualitatively as well as quantitatively di fferent for the two materials. Surface energies and electronic properties a re also computed. It is found that for BaTiO3 and SrTiO3 surfaces, both AO- terminated and TiO2-terminated surfaces can be thermodynamically stable, wh ereas for PbTiO3 only the PbO surface termination is stable.