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

TL;DR

This study investigates the electric field gradient differences in SrTiO3 and BaTiO3 perovskites using density functional calculations, revealing a counter-intuitive increase of EFG with lattice expansion explained by an extended s-p-d model.

Contribution

The paper introduces an extended s-p-d model that explains the EFG behavior in perovskites, surpassing the standard p-d Hamiltonian.

Findings

EFG increases with lattice expansion in SrTiO3 and BaTiO3.

Standard p-d Hamiltonian cannot explain the observed EFG behavior.

Including oxygen 2s states in the model accounts for the EFG increase.

Abstract

We analyze recent measurements [R. Blinc, V. V. Laguta, B. Zalar, M. Itoh and H. Krakauer, J. Phys. : Cond. Mat., v.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 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 yields the counter-intuitive behavior that the EFG increases upon lattice expansion. Applying the standard model for perovskites, the effective two-level p-d Hamiltonian, can not explain the observed behavior. In order to describe the EFG dependence correctly, a model beyond this usually sufficient p-d Hamiltonian is needed. We demonstrate that the counter-intuitive increase of the EFG upon lattice expansion can be explained by a 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.