A surface divacancy on MgO(100), and its reactivity with water, were studied by first-principles simulations. Among the most frequent point defects on MgO(100), the divacancy was estimated to be stable with respect to the formation of neutral O and Mg vacancies in moderate O environments. A single water molecule dissociated spontaneously at the divacancy. The final product of this exothermic reaction consisted of a surface Mg vacancy with 2 adsorbed hydrogens; the (VMg,2H) complex which exhibited peculiar atomic and electronic structures. In particular, the stretching frequencies of the hydroxyl groups were noticeably down-shifted, which could explain recent infra-red data collected on wet MgO powders. The trend towards an aggregation of the brucite-like intrusions (VMg,2H) was pointed out in relation to the complex behavior of the MgO(100) surface upon exposure to water.

Water Dissociation on a Defective MgO(100) Surface - Role of Divacancies. B.Ealet, J.Goniakowski, F.Finocchi: Physical Review B, 2004, 69[19], 195413 (9pp)