First-principles calculations of atomic and electronic structure of SrTiO3 (001) and (011) surfaces

We present and discuss the results of the calculations of surface relaxation and rumpling on SrTiO3 (001) and (011) surfaces. We consider both SrO and TiO2 terminations of the (001) surface, and three terminations (Sr, TiO, and O) of the polar (011) surface. The calculations are based on hybrid Hartree-Fock and density-functional theory exchange functionals by using Becke's three-parameter method combined with the nonlocal correlation functionals of Perdew and Wang. We find that all top-layer atoms for TiO2 and SrO-terminated SrTiO3 (001) surfaces relax inward, with the exception of SrO-terminated surface O atoms, whereas all second-layer atoms relax outward. The surface rumpling for the TiO-terminated SrTiO3 (011) surface, which is 11.28% of the bulk lattice constant, is considerably larger than the relevant surface rumplings for SrO and TiO2-terminated (001) surfaces. The surface rumplings for the SrO and TiO2-terminated (001) surfaces are in excellent agreement with relevant low-energy electron diffraction and reflection high-energy electron diffraction experimental data, and the surface relaxation energies on both surfaces are similar. In contrast, the different terminations of the (011) surface lead to large differences in relaxation energies. The O-terminated (011) surface has the lowest surface relaxation energy (- 1.32 eV). The TiO-terminated (011) surface has a much higher surface relaxation energy of -1.55 eV, while the Sr-terminated (011) surface has the highest surface relaxation energy (-1.95 eV). Our calculations indicate a considerable increase in the Ti-O bond covalency (0.130e) near the TiO-terminated (0 11) surface relative to the bulk (0.088e), which is much larger than that for the (001) surface (0.118e). The Ti-O bond populations are considerably larger in the direction perpendicular to the TiO-terminated (011) surface (0.188e) than in the plane (0.130e).