Spin and orbital occupation and phase transitions in V2O3

Polarization dependent x-ray-absorption measurements were performed on pure and Cr-doped V2O3 single crystals in the antiferromagnetic insulating, paramagnetic insulating, and metallic phases. The orbital occupation of the V 3d(2) ion is found to depend appreciably on the phase, but always with the S= 1 character, requiring an explanation which is beyond the elegant simplicity of the pure one-band Hubbard model or of models in which the a(1g) orbital is projected out by means of a simple dimerization. The results reveal the critical role of the spin and orbital dependence of the on-site 3d-3d Coulomb energy, and a mechanism is proposed to explain the closing or opening of the band gaps which are of much higher energy scale than the transition temperatures.