Semi-empirical simulations of surface relaxation for perovskite titanates

The (100) and (110) surface relaxations are calculated for SrTiO₃ and BaTiO₃ perovskite thin films. By means of a semi-empirical shell model, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are calculated. Surface rumpling and surface-induced dipole moments are calculated for all possible surface terminations. Our results for the (100) surface structure are in good agreement with ab-initio plane-wave pseudopotential calculations and LEED experiments. The surface energy for the Ba-, Sr-, TiO-terminated (110) surfaces is found to be much larger than that for the (100) surface. In contrast, the surface energy for the asymmetric O termination, where outermost O atoms are strongly on-plane-displaced, is the lowest for all (110) terminations and thus the most stable.