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

 
K. Koch,
 
K. Koepernik,
 
I. V. Kondakova,
 
H. Rosner
 

I. M. Frantsevich Institute for Problems of Materials Science of the NAS of Ukraine, Krzhizhanovsky str., 3, Kyiv, 03142, Ukraine
Phys. Rev. B - Maryland, USA: American Physical Society (APS), 2009, #80
http://www.materials.kiev.ua/article/646

Abstract

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.