The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab initio total-energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with ΔE=-90.2 kJ ${\mathrm{mol}}^{\mathrm{-}1}$. We conclude that protonation stabilizes the otherwise unstable (111) surface and that this, not the bare (001), is the most stable surface of MgO under ambient conditions.

• Received 5 July 1995

DOI:https://doi.org/10.1103/PhysRevB.52.10823