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)(pi) levels. The self-consistent-field variational solutions are spin-polarized insulating states with single (a(1g)) and double (e(g)(pi)) electron occupations for Ti2O3 and V2O3, respectively. Conducting or semiconducting states, with different relative energies of a(1g) and e(g)(pi) bands, have also been obtained by changing the c/a ratio of the hexagonal unit cell. The charge transfer into e(g)(sigma) levels is discussed and compared to the Cr2O3 and Fe2O3 behavior.