Modern Concepts of Atom Coordinations, Atom Volume and Charge Transfer in Intermetallic Compounds

Abstract

The crystal structure determination of a large number of binary intermetallic compounds has allowed to build up structural maps, by means of which atom coordinations or coordination polyhedra of a new phase can be predicted as a function of a few atomic properties of the two elements. The volume effect (generally a contraction) that accompanies the formation of a binary phase can be evaluated in several ways. Three methods are described, fulfilling the requirements of being applicable to a large number of metallic systems and easy to use. The semi-empirical Miedema’s model, based on the concept of atomic cells, applies to transition-metal compounds and emphasizes charge transfer effects. In Hafner’s approach the first - order perturbation theory and the pseudopotential concept are used to yield a simple rule for the variation of the mean atomic volume of sp binary alloys with concentration. In Merlo’s approach a three - parameters empirical formula is proposed to calculate volume contractions in compounds formed by alkaline earths and bivalent rare earths with other elements.