ENERGETICS OF METAL-OXIDE SOLID-SOLUTIONS

Excess energies of metal oxide solid solution formation have been calculated via tight-binding calculations on a set of 27 rock-salt-based superstructures. From these values three important factors which control the energetics of solid solutions have been identified: (i) a two-atom term, which may be identified with the volume change of solution formation; (ii) a three-atom term, which may be identified with the bulk of the metallic substitution process and understandable in terms of the regular solution model, and (iii) an asymmetry term, which is interpreted as arising via the interruption of a delocalized orbital network and is probably largely a four-atom term. The electron count dependence of the three terms and electronic insights provided via the method of moments enables an understanding of the reversal in the sign of DELTAH(xs)(x) found experimentally on going from (Fe,Mg)O to (Ni,Mg)O.