First-principles investigation of the Metal-insulator transitions in Ti-substituted tetragonal rutile VO2

Abstract

The effect of Ti-substitution on the structural, electronic and magnetic properties of VO2 in its high-temperature rutile phase is investigated. The crystal symmetry reduces with Ti concentration in V1-xTixO2 (x = 0.25, 0.5, 0.75) due to the breaking of mirror symmetries in the VO6 and TiO6 octahedra. The compounds V0.75Ti0.25O2 and V0.5Ti0.5O2 are found to be ferrimagnetic metals. These two compounds encounter metal-to-half-metal transition for the application of Coulomb repulsion of U = 1 eV and 0.5 eV respectively, exhibiting ferromagnetism. On the other hand, VO2 encounters metal-to-half-metal transition for x = 0.75 in V1-xTixO2 (the corresponding compound is V0.25Ti0.75O2), showing ferromagnetism. More interestingly, all three substituted compounds encounter metal-insulator transition (MIT) for U = 4 eV, 2.5 eV and 2 eV respectively, preserving ferromagnetism. In the absence of U, orbital fluctuation is observed among the three V-t2g states (V-dx2-y2, dyz, dxz). The sharing of the available single V-3d electron by the three V-t2g states results in the metallicity of V0.75Ti0.25O2 and V0.5Ti0.5O2, and half-metallicity of all three compounds. In the insulating phases, electron correlation is triggered by the application of U which diminishes the orbital fluctuation among the three V-t2g states. The single V-3d electron occupies the V-dx2-y2 state, leading to V4+-V4+ and Ti4+-Ti4+ charge ordering. Therefore, the simultaneous effect of structural distortion, U and V4+-V4+, Ti4+-Ti4+ charge ordering is responsible for the MIT in V0.75Ti0.25O2, V0.5Ti0.5O2 and V0.25Ti0.75O2. The cooperative effect of p-d hybridization and V–O antiferromagnetic coupling results in ferromagnetism in the insulating phases of all three compounds.