Periodic unrestricted Hartree-Fock study of corundumlike Ti₂O₃ and V₂O₃

The ground-state electronic and magnetic properties of rhombohedral Ti₂O₃ and V₂O₃ have been investigated by ab initio all-electron periodic Hartree-Fock calculations. Both unrestricted open-shell and restricted closed-shell methods have been employed, with basis sets of atomic orbitals represented by contracted Gaussian-type functions. The t_2g degeneracy of d electrons is removed by the rhombohedral field, giving rise to orbital ordering between a_1g and e_g^π levels. The self-consistent-field variational solutions are spin-polarized insulating states with single (a_1g) and double (e_g^π) electron occupations for Ti₂O₃ and V₂O₃, respectively. Conducting or semiconducting states, with different relative energies of a_1g and e_g^π bands, have also been obtained by changing the c/a ratio of the hexagonal unit cell. The charge transfer into e_g^σ levels is discussed and compared to the Cr₂O₃ and Fe₂O₃ behavior.