Ab initio calculations of SrTiO₃, BaTiO₃, PbTiO ₃, CaTiO₃, SrZrO₃, PbZrO₃ and BaZrO₃ (001), (011) and (111) surfaces as well as F centers, polarons, KTN solid solutions and Nb impurities therein

In this paper, the review of recent results of calculations of surface relaxations, energetics, and bonding properties for ABO₃ perovskite (001), (011) and (111) surfaces using mostly a hybrid description of exchange and correlation is presented. Both AO and BO₂-terminations of the nonpolar (001) surface and A, BO, and O terminations of the polar (011) surface, as well as B and AO₃-terminations of the polar (111) surface were considered. On the AO-terminated (001) surface, all upper-layer A atoms relax inwards, while all second layer atoms relax outwards. For the BO ₂-terminated (001) surface, in most cases, the largest relaxations are on the second-layer metal atoms. For almost all ABO₃ perovskites, the surface rumpling is much larger for the AO-terminated than for the BO ₂-terminated (001) surface, but their surface energies are always quite similar. In contrast, different terminations of the (011) ABO₃ surface lead to very different surface energies for the O-terminated, A-terminated, and BO-terminated (011) surface, respectively. A considerable increase in the Ti-O or Zr-O, respectively, chemical bond covalency near the (011) surface as compared both to the bulk and to the (001) surface in ABO ₃ perovskites were predicted. According to the results of ab initio calculations for Nb doped SrTiO₃, Nb is a shallow donor; six nearest O ions are slightly displaced outwards from the Nb ion. The F center in ABO ₃ perovskites resembles electron defects in the partially-covalent SiO₂ crystal rather than usual F centers in ionic crystals like MgO and alkali halides. The results of calculations for several perovskite KNb _xTa_1-xO₃ (KTN) solid solutions, as well as hole and electron polarons in ABO₃ perovskites are analyzed.