Ab initio calculations of the atomic and electronic structure of CaTiO ₃ (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.