Coexistence of ionic and metallic bonding in noble-metal oxides

Abstract

We investigate the nature of bonding in noble-metal oxides ${\mathrm{Cu}}_{2}\mathrm{O}$, ${\mathrm{Ag}}_{2}\mathrm{O}$, and ${\mathrm{Au}}_{2}\mathrm{O}$ using a combination of first-principles density-functional-based methodologies including self-interaction correction, polarization theory, and the electron localization function. A long-standing question concerns the type of bonding operating in these compounds. They are prevailingly ionic, but their observed cubic structure is stabilized by weak cation-cation interactions whose nature is not fully understood. A currently credited picture is that electron promotion out of the filled cation $d$ shells produces nonspherical charge distributions on the cations, which interact in a van der Waals fashion according to some or form covalent cation-cation bonds according to others. In this work we give a detailed and consistent picture of bonding in ${\mathrm{Cu}}_{2}\mathrm{O}$ (and related compounds) at some variance with that hitherto assumed. First, we show that no trace of interstitial covalency is to be found, which negates the covalent bonding hypothesis. Second, we show the cation on-site $s\text{\ensuremath{-}}d$ hybridization causes only a marginal deviation from the exact sphericity of the electron charge and contributes negligibly to the stabilization of the cubic phase. Third, we show that the cubic phase is stable because of incomplete ionic charge transfer, leaving on the cation an excess of delocalized charge which establishes weakly metallic cation-cation bonding.