Ordering in Metal Halide Melts

Abstract

The crystal structures formed by compounds of strongly electropositive and strongly electronegative elements are described crystallographically in terms of interpenetrating Bravais lattices occupied by the various chemical species. The coexistence of crystalline long-range and chemical up to melting can be explained by electronic charge transfer and local compensation of positive and negative ionic charges. An examination of the local structures found in crystalline metal halides shows that charge compensation occurs in two qualitatively distinct ways. The first involves halogen sharing and high coordination numbers for the metal ions; the structure of alkali, alkaline-earth, and lanthanide metal halides are ex­ amples of this type of order. In the second, charge compensation takes place within well-defined molecular units, either monomeric ones, as for example in HgC12 and SbCI3, or dimerie ones, as in AlBr3' Studies of molten salts have generally been based on the assumption that the melting transition, which involves the loss of crystalline long­ range order largely preserves the type of local found in the crystal. Determinations of the structure of metal halide meits carried out over the past two decades with neutron diffraction have largely substantiated this