VOH and VOD centers in alkaline-earth oxides:: An ab initio supercell study

The V-OH and V-OD centers in MgO, CaO, and SrO have been investigated at the ab initio quantum-mechanical level, by using the CRYSTAL98 periodic program, the Hartree-Fock Hamiltonian, the supercell scheme, and a localized basis set. The defects, formally obtained by substituting a hydrogen atom for a cation, exhibit a large relaxation of H from the perfect lattice position towards one of the oxygens, and the formation of a strong O-H covalent bond, as documented by the calculated bond populations and O-H vibrational frequencies, which are in good agreement with infrared frequencies. The unpaired electron is fully localized on the oxygen opposite to H with respect to the M (M = Mg, Ca, Sr) vacancy. The first evaluations of the Fermi contact, the anisotropic component of the hyperfine coupling tensor, and the electric-field gradient at the H atom are reported and compared successfully with electron paramagnetic resonance and electron-nuclear double resonance experimental data. The defect is also characterized in terms of charge- and spin-density maps, band structures, defect formation, and relaxation energies. The stability of the results as a function of the supercell size is documented.