The dynamics of the hydride ion in MgO single crystals

Diffusion of H- ions in thermochemically reduced MgO crystals with a large concentration of hydrogen has been investigated both experimentally and theoretically. Infrared absorption of the [H-](+) centers is used to monitor the concentration and to study their diffusion rates in crystals heated at elevated temperatures in flowing oxygen. Characteristic parameters such as activation energy and diffusion coefficients were determined. The diffusion coefficient is approximate to 10(-6) cm(2)/s at 1600 K. The activation energy is estimated experimentally to be (1.5 +/- 0.3) eV. Analysis of the decay curves of [H-](+) center concentrations during isothermal anneals indicates that the annealing-out of the [H-](+) centers does not follow a first-order kinetic reaction. The H- ion diffusion was simulated using an ab-initio Hartree-Fock cluster approach. The mobile defect is more compatible with the H- ion than with the proton. Theoretical modeling for the direct interstitial H- hops along the [100] axis gives an activation energy twice as large as the experimental value. It is proposed that either a collinear interstitialcy mechanism for diffusion, or motion of the H--oxygen vacancy complex are involved.