Density functional theory study of impurity levels in Cr-substituted alumina at different Hubbard parameters

Abstract

A theoretical model of the electronic structure of chromium-substituted aluminum oxide (Cr_xAl_2–xO₃}) is needed to understand the optoelectronic properties of ruby from first principles. However, simple density functional theory (DFT) is unable to completely describe the localized d-electron states of transition metal impurities because of its limited treatment of strong electron correlations. To address this problem, one can employ a Hubbard U correction (DFT+U) to account for the energy cost of double occupation. In this study, we explore the effects of increasing Hubbard repulsion on the electronic structure of the impurity levels of chromium-substituted alumina. We performed calculations using both conventional DFT and DFT+U methods to obtain projected densities of states about the band gap. Preliminary results show that Hubbard repulsion is a major contributor to the splitting of a flat donor level that originates from the chromium t_2g orbital of 3d electronic states. This incremental result is intended to guide ongoing efforts to construct a theoretical model for the well-studied optical spectrum of ruby.