Water dissociation on a defective MgO(100) surface: Role of divacancies

Surface divacancy at MgO(100) and its reactivity with water are studied by first-principles simulations. Among the most frequent point defects at MgO(100), the divacancy is estimated to be stable with respect to the formation of neutral oxygen and magnesium vacancies in moderate O environments. A single water molecule dissociates spontaneously at the divacancy. The final product of this exothermic reaction consists of a surface Mg vacancy with two adsorbed hydrogens---the (VMg,2H) complex---which shows peculiar atomic and electronic structures. In particular, the stretching frequencies of the hydroxyl groups are downshifted noticeably, which can explain recent infrared data collected on wet MgO powders. The trend towards an aggregation of the brucite-like intrusions (VMg,2H) is pointed out in relation with the complex behavior of the MgO(100) surface upon exposure to water.