Effect of oxidation on chemical bonding around 3d transition-metal impurities in ZnO

First-principles molecular orbital calculations for ZnO with 3d transition- metal (TM) impurities ranging from Mn to Cu have been carried out in order to investigate the relationship between TM impurities and oxygen adsorption in ZnO ceramics. Two kinds of model calculations, i.e., the TM impurities with various formal charges in the bulk and oxygen atom adsorption on the ( <10(1)over bar 0>) surface with TM impurities, were performed. For bulk cal culations, TM impurities form specific states within the band gap. The cova lent bonding around TM impurities is reinforced by the increase in formal c harges. A similar behavior is recognized for surface calculations. TM impur ities at the surface are oxidized by the adsorbed oxygen atom and the coval ent bonding around TM impurities is reinforced. On the other hand, the cova lent bonding at the surface is weakened by oxygen adsorption when TM impuri ties are absent. This result suggests that TM impurities enhance oxygen ads orption at the ZnO surface. The mechanism may be similar at the grain bound ary of polycrystalline ZnO.