QUANTUM CHEMISTRY OF OXIDE SURFACES: FROM CO CHEMISORPTION TO THE IDENTIFICATION OF THE STRUCTURE AND NATURE OF POINT DEFECTS ONMgO

The electronic structure and chemisorption properties of the surface of ionic crystals are reviewed, with emphasis on two topics: a critical overview of the experimental and theoretical studies of the adsorption of CO on single crystal and polycrystalline MgO, and a discussion on the most important defect centers at the MgO surface — low-coordinated sites, single oxygen and magnesium vacancies, divacancies, and impurity or substitutional atoms. The two subjects are to some extent interconnected. From the detailed theoretical and experimental study of the adsorption of a nonreactive molecule like CO and from the comparison of experiments done on single crystal or thin films and on powder samples, one can learn about the nature and concentration of the defects at the surface. A more precise characterization of defects requires, however, a careful spectroscopic investigation and a direct comparison with quantum-chemical calculations of both geometric structure and observable properties. The combined theoretical–experimental approach offers new opportunities for a better understanding of the complexity of oxide surfaces.