VO2: Orbital competition, magnetism, and phase stability

Abstract

The relative phase stability of VO${}_{2}$ is one of the most fundamental issues concerning the metal-insulator transition in this material but has been so far largely unexplored theoretically. We investigate the relative stability of various phases of VO${}_{2}$ using different levels of energy functionals within density functional theory (DFT). It is found that straightforward applications of several popular energy functionals, including the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional, result in a wrong prediction for the ground state of VO${}_{2}$. In particular, although the HSE and DFT+$U$ methods are able to produce a band gap in the M${}_{1}$ phase, they strongly favor the formation of local magnetic moments, a result that clearly disagrees with experiments. We also examine the effect of the occupation and the redistribution of the $d$ derived ${t}_{2g}$ (i.e., ${d}_{xz}$, ${d}_{yz}$, and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$) orbitals of V atoms on the calculated relative phase stability of VO${}_{2}$. We find that a small change in $d$ occupation can result in a drastically different theoretical prediction. With the introduction of an orbital-dependent potential, a complete separation between the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ derived valence band and ${d}_{xz}$ and ${d}_{yz}$ derived conduction bands in the M${}_{1}$ phase is achieved, resulting in a slight redistribution of the $d$ occupation and a more faithful account of the polarization of the ${t}_{2g}$ orbitals. This slight rearrangement of the $d$ occupation also leads to a relative phase stability of VO${}_{2}$ (including structural and magnetic phases) that agrees well with experiment.