Ab initio calculations of the atomic and electronic structure of CaTiO3 (001) and (011) surfaces

We present the results of calculations of surface relaxations, energetics, and bonding properties for CaTiO3 (001) and (011) surfaces using a hybrid description of exchange and correlation. We consider both CaO and TiO2 terminations of the nonpolar (001) surface and Ca, TiO, and O terminations of the polar (011) surface. On the (001) surfaces, we find that all upper-layer atoms relax inward on the CaO-terminated surface, while outward relaxations of all atoms in the second layer are found for both terminations. For the TiO2-terminated (001) surface, the largest relaxations are on the second-layer atoms. The surface rumpling is much larger for the CaO terminated than for the TiO2-terminated (001) surface, but their surface energies are quite similar at 0.94 and 1.13 eV/cell, respectively. In contrast, different terminations of the (011) CaTiO3 surface lead to very different surface energies of 1.86, 1.91, and 3.13 eV/cell for the O-terminated, Ca-terminated, and TiO-terminated (011) surface, respectively. Our results for surface energies contrast sharply with those of Zhang [Phys. Rev. B 76, 115426 (2007)], where the authors found a rather different pattern of surface energies. We predict a considerable increase in the Ti-O chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface.