Stability diagrams for fourfold coordination of polyvalent metal ions in molten mixtures of halide salts

The stability of local fourfold coordination for divalent and trivalent metal ions in liquid mixtures of polyvalent metal halides and alkali halides is classified by means of structural coordinates obtained from properties of the elements. In parallel with earlier classifications of compound crystal structures and molecular shapes, the elemental properties are taken from first-principles calculations of valence electron orbitals in atoms in the form of (i) the nodal radii of Andreoni, Baldereschi and Guizzetti or (ii) the pseudopotential radii of Zunger and Cohen (1979). As a third alternative a classification based on Pettifor's phenomenological chemical scale of the elements is also considered. The alternative structural classification schemes that are developed from these elemental properties are generally successful in distinguishing molten mixtures in which the available experimental evidence indicates long-lived fourfold coordination of polyvalent metal ions. In addition, Pettifor's chemical scale scheme (1986) is useful in sorting out finer details of local coordination in the liquid state.