Analysis of the electric field gradient in the perovskites SrTiO₃ and BaTiO₃: Density functional and model calculations

Анотація

We analyze recent measurements [ R. Blinc, V. V. Laguta, B. Zalar, M. Itoh and H. Krakauer J. Phys.: Condens. Matter 20 085204 (2008)] of the electric field gradient on the oxygen site in the perovskites SrTiO₃ and BaTiO₃, which revealed, in agreement with calculations, a large difference in the electric field gradient (EFG) for these two compounds. In order to analyze the origin of this difference, we have performed density functional electronic-structure calculations within the local-orbital scheme FPLO. Our analysis reveals the counter-intuitive behavior that the EFG increases upon lattice expansion. Application of the standard model for perovskites, the effective two-level p-d Hamiltonian, cannot explain the experimentally observed and theoretically predicted behavior. In order to describe the EFG dependence correctly, a model beyond the usually sufficient p-d Hamiltonian is needed. We demonstrate that the counter-intuitive increase in the EFG upon lattice expansion can be explained by an s-p-d model containing the contribution of the oxygen 2s states to the crystal field on the Ti site. The proposed model extension is of general relevance for all related transition-metal oxides with similar crystal structure.