Melting and Polymorphism

Phase equilibria between condensed phases, like melting and crystal polymorphic transitions, have no mass-dependent terms (no equilibrium constants) since the activity of pure condensed phases is unity, and hence the equilibrium thermodynamics is represented by the simple relationships  

Enthalpy differences between polymorphic crystal forms of the same substance [18,19] can be directly measured by calorimetry or by the difference in enthalpies of dissolution. A critical survey of these experiments shows that these enthalpy differences are very small, of the order of 0-10 kj mol-1, as expected since they must be a small fraction of the enthalpy of melting. In many cases the measured value is undistinguishable from experimental noise. These enthalpy differences can be estimated by just taking the differences in lattice energy between pairs of crystal phases whose complete structure is known, but the intrinsic uncertainty of such a calculation is also of the same order of magnitude of the property it tries to simulate. 

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Jayaraman AG, Kennedy C, Klement W (1963) Melting and polymorphic transitions for some group 2-6 compounds at high pressures. Phys Rev 130 2277... 

Menyhard A, Varga J. The effect of compatibiUzers on the crystallisation, melting and polymorphic composition of p-nucleated isotactic polypropylene and polyamide 6 blends. Eur Polym J 2006 42 3257-68. 

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