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.