A view and a review of the melting of alkali metal halide crystals |
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Authors: | A K Galwey |
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Affiliation: | (1) Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa |
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Abstract: | Summary A representational model, proposed to account for the physical changes that accompany the melting of alkali halides, was described
in Part 1 1]. The liquid is portrayed as undergoing continual dynamic structural reorganization of its constituent ions between
individual small domains, zones of various regular, crystal-type arrays. These alternative arrangements are stabilized by
the enthalpy of melting, which, in liquids, relaxes the restriction for solids that only the single, most stable, crystal
structure can be present. The dynamic character of the melt accounts for its fluid character and the loss of long-range order
1, 2]. This model is extended here to consider the phase diagrams of binary, common ion, alkali halide mixtures comprehensively
reviewed in 3]. Factors determining whether each of these yields a eutectic, or a solid solution, on cooling are discussed
and several trends in the 70-phase diagrams are identified. Eutectic formation, involving maintenance of the liquid state
below the melting points of the pure components, is ascribed to the participation, in an extended dynamic equilibrium, of
additional domains having the regular structures characteristic of double salts. The known crystalline double binary halides
3], Li/Cs or Rb/F, Cl, Br or I, melt at temperatures well below those of the simpler pure component salts. It is concluded
that the set/liq model for melting, proposed in 1, 2], accounts for some important properties of the phase diagrams presented
in 3]. |
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Keywords: | melting fusion binary alkali halide mixture melting eutectic formation solid solution formation |
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