Modeling of Ruddlesden-Popper surfaces and oxygen vacancies in strontium titanate

Strontium titanate (STO) is an oxide crystallizing with cubic perovskite-type of structure that exhibits a high tunability of dielectric, electric, mechanical and optical properties by means of defects. Apart from dopants, also intrinsic oxygen vacancies or ordered stacking faults, e.g. Ruddlesden-Popper (RP) phases SrO(SrTiO3)n, influence these properties.
We have investigated structural stability, electronic properties and surface energies of such RP phases up to n = 5 by means of density-functional theory. We find a significant gain of formation energy up to n = 3 and can approximate the interaction range of neigh-boring stacking faults to 11.7 Angs.. Surfaces in [001] and [100] direction with all possible unreconstructed crystal terminations have been modeled and stability is compared. In contrast to pure STO, the near-surface SrO-OSr stacking fault can be employed to control surface roughness by adjusting SrO and TiO2 surface rumpling, to stabilize SrO termination in SrO-rich surrounding or to increase the band gap for TiO2 termination.
Further, we have theoretically verified a rever-sible elastic softening along an O-deficient 001 direction recently found in nano-inden-tation of SrTiO3 under influence of an electric field. Results from an isotropic and anisotropic model of a SrTiO3-d super cell are discussed.