Comparative ab initio calculations of SrTiO3 and CaTiO3 polar (111) surfaces


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We present the results of calculations of SrTiO3 and CaTiO3 polar (111) surface relaxations, rumplings, energetics, optical band gaps, and charge distributions using the ab initio code CRYSTAL and a hybrid description of exchange and correlation. We have calculated the surface relaxation of the two possible terminations (Ti and SrO3 or CaO3) of the SrTiO3 and CaTiO3 (111) surfaces. According to our calculations, atoms of the first surface layer relax inwards for Ti-, SrO3-, and CaO3-terminated (111) surfaces of both materials. The only exception is outward relaxation of the SrO3-terminated SrTiO3 (111) surface upper layer Sr atom. For both Ti-terminated SrTiO3 and CaTiO3 (111) surfaces our calculated second layer Sr and Ca metal atom inward relaxations are approximately four and two times larger than the upper layer Ti atom inward relaxation. Our calculated optical band gap for the SrO3- and Ti-terminated SrTiO3, as well as for CaO3-terminated CaTiO3 (111) surfaces, becomes smaller with respect to the bulk optical band gap. Our calculated surface energies for SrO3- and CaO3-terminated SrTiO3 and CaTiO3 (111) surfaces (6.30 and 5.86 eV) are considerably larger than the surface energies for Ti-terminated SrTiO3 and CaTiO3 (111) surfaces (4.99 and 4.18 eV). Our B3LYP calculations indicate a considerable increase of Ti–O chemical bond covalency near the SrTiO3 and CaTiO3 (111) surface (+0.098e and +0.094e) relative to the SrTiO3 and CaTiO3 bulk (+0.088e and +0.084e).