Metal–insulator transition in the high pressure cubic $$\hbox {CaF}_{2}$$-type structure of $$\hbox {CrO}_{2}$$

Abstract

The total energy, magnetic moment, density of states and energy band structures of \(\hbox {CrO}_{2}\) in the high pressure cubic \(\hbox {CaF}_{2}\) (Fm-3m)-type structure are investigated for the first time by using first principles electronic structure calculations based on density functional theory in the local spin density approximation (LSDA) as well as using LSDA + U method. It is revealed from the present study that \(\hbox {CrO}_{2}\) is a ferromagnetic nearly half-metal in the LDA calculation due to almost occupied \(\hbox {Cr-3de}_{\mathrm{g}}\) orbitals in the spin majority species and a tiny electron contribution at the Fermi level (\(\hbox {E}_{\mathrm{F}}\)) for spin minority species. This compound encounters metal–insulator transition (MIT) upon the application of Hubbard-type Coulomb repulsion \(U = 1 \, \hbox {eV}\). Ferromagnetism is preserved in this transition. Application of U increases the electron correlation, which polarizes \(\hbox {e}_{\mathrm{g}}\) orbitals far below \(\hbox {E}_{\mathrm{F}}\) for spin majority channel while in the spin minority channel \(\hbox {e}_{\mathrm{g}}\) orbitals are polarized well above \(\hbox {E}_{\mathrm{F}}\), resulting in opening of a band gap (\(E_{\mathrm{g}} \sim 1.05\,{\mathrm{eV}}\)) in the vicinity of \(\hbox {E}_{\mathrm{F}}\). Coulomb repulsion increases hybridization between occupied \(\hbox {Cr-3de}_{\mathrm{g}}\) and O-2p orbitals, which induces ferromagnetism both in half-metallic and insulating phases of cubic \(\hbox {CrO}_{2}\).