Aview and a review of melting of alkali metal halide crystals

Theoretical models for the melting of solids are inadequate because relatively little is known about the structures of liquids formed and the factors that control this phase transformation. In the present analysis of fusion phenomenon, usually considered to be a physical change, it is pointed out that, for many solids (e.g., metals and some simple ionic salts) melting involves the redistribution of primary valence bonds. Accordingly, this review includes examination some more chemical aspects of the controls of melting. The available data show that enthalpy and density changes during liquefaction and solidification of the metallic elements and of the alkali halides are small. From quantitative consideration of these values, it is concluded that ordered packing arrangements of atoms, ions, or molecules, comparable with those of crystals, must be extensively retained into the melt. The energy and molar volume changes on melting are too small to allow significant departure, in the liquid, from the regular, efficient space-filling arrays that characterize crystalline solids. The set/liq model for melting (dynamic equilibria between alternative ordered structures) is proposed to account for the properties of the liquid. A detailed and critical comparison of melting with solid state decompositions considers the kinetics and the mechanisms of the changes that occur during the supply/removal of energy to/from the melt/crystal contact interface. Other relevant aspects of melting are discussed including the factors that determine the magnitudes of the melting points of individual solids.