The stability of peroxide ion O22− at (110), (210) and (001) surfaces of MgO, CaO and SrO periodic ab initio calculations

The stability of peroxide ion O22− resulting from the interaction between an adsorbed oxygen O and surface oxygen O2− at the (110), (210) and (001) five layer surface films of the alkaline earth oxides MgO, CaO and SrO has been investigated using periodic density functional theory calculations. The 1:1 supercell model in which two incoming oxygens were added per each supercell, one at each side of the film, was employed in the calculations. The incoming oxygen/solid interaction energies exhibit exothermic character at the three considered films. The interaction energies at the fully relaxed surfaces increase monotonically from MgO to CaO to SrO, mainly from (110) to (210) to (001) planes, and are explainable in terms of the basicities and ionization potentials of the cations as well as the acceptor property of the incoming oxygen. Based on charge electron density maps, electrostatic potential contours and Mulliken population analysis the results confirm: (i) a genuine charge transfer from the surface to the adsorbed oxygen, (ii) the overlap population between the adsorbed oxygen and the surface oxygen is much larger than that between two oxygens in the crystal bulk, (iii) the overlap population between adsorbed oxygen and surface oxygen increases from MgO to CaO to SrO, being consistent with predicted order of stability of the peroxy bond.