Modelling of the Gibbs adsorption at transition-metal–oxide interfaces: effect of the oxygen chemical potential on interfacial bonding, interfacial energy and equilibrium precipitate shape

Abstract

Non-reactive ternary metal–oxide interfaces are thermodynamically stable over extended ranges of oxygen activities and temperatures. At each condition within this range, the interface adopts a different equilibrium structure and chemistry. A continuum model of the Gibbs adsorption–desorption at transition-metal–oxide interfaces is developed, which predicts interfacial chemistry and modifications in the specific free interfacial energy as a function of oxygen activity. Three oxygen activity domains can be distinguished according to this model: the upper part of the metal–oxide coexistence range characterized by an enrichment in interfacial oxygen, established by adsorption of oxygen at structural vacancies in the case of polar interfaces or by desorption of the less noble metal in the case of mixed interfaces; an intermediate-oxygen-activity range with the interface remaining free of adsorption; a lower-oxygen-activity range, where the interface is enriched in less noble metal by adsorption of excess less noble metal at structural vacancies or by desorption of oxygen. Largest excess concentrations are reached at the limits of the coexistence range; absolute values depend on the ability of the transition metal to undergo partial charge transfer. Any charge transfer across the interface imposes a formal charge in the terminating oxide plane and leads to the formation of a space charge layer in the oxide. In the present work, defect concentration profiles in the space charge layer are calculated for different oxygen activities and their influence on the cohesive energy is evaluated. The general model is applied to the MgO–Cu system. Computed interfacial occupancies are compared with experimental observations of the chemical bonding at polar and mixed topotactical MgO–Cu interfaces at different oxygen activities (electron-energy-loss near-edge structure studies). The evolution of relative specific free interfacial energy ratios, inferred from the equilibrium shape of MgO precipitates within a copper matrix and of liquid copper inclusions in a MgO matrix, is compared with model predictions of the interfacial energy of different facets and their evolution with oxygen chemical potential. Qualitative and quantitative agreement between model and experimental results is found.