DFT study of Cl/MgO(1 0 0), influence of the stoichiometry

We have performed DFT–GGA periodic calculations for Cl/MgO(1 0 0). Chlorine is an example of electron-acceptor radical adsorbate. The adsorption on a perfect stoichiometric surface (one atom per unit cell) takes place at the O site. It is weak and explained by a three-electron interaction, preserving the radical character. The adsorption on the oxygen-defective surface is much stronger, Cl being bound as Cl− to the Mg2+ ions in the vacancy or at the surface. This occurs through a redox process (electron transfer from the reduced cations of the lattice to the Cl atoms). The ratio of two chlorine atoms per O vacancy is optimal and restores the energy gap of the perfect metal oxide.