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Polymorphism

It was said earlier in this pamphlet that crystal nucleation and growth are quite often under kinetic control. The final product, the (single) crystal, may result from less stable but faster growing nuclei; the transformation to the thermodynamically stable phase is hindered by an energy barrier, because the forces holding the metastable phase together have to be overcome, so that molecules can rearrange into the stable crystalline form. In some favourable (and almost exceptional) cases, the spatial rearrangement is so simple that a highly cooperative single-crystal to single-crystal transformation can occur.

The natural outcome of all this is polymorphism, or the ability of a given compound to crystallize in different crystal structures. Thermodynamics holds that only one structure is the stable one at a given temperature and pressure but, not surprisingly, kinetics sometimes allows many coexisting phases^[16]. A typical enthalpy difference between polymorphs for an organic compound is 4-8 kJ/mole, which, for transition temperatures of the order of 300 K, implies entropy differences of the order of 10-20 J K^-1 mole^-1 (G = 0 = H-TS). These figures are now at the borderline of the accuracy of both detection apparatus and theoretical methods^[17,18].

A further influence on crystal growth in terms of shape and structure is that of the presence of impurities. Crystal morphology is affected by the adsorption of impurity molecules on to particular faces of the crystal with a consequent slowing in their growth rate. Impurities trapped in the host crystal, especially those that are close in chemical structure to the bulk compound, may cause it to adopt a different structure. A few cases are known of opposite enantiomers adopting different structures although grown under similar conditions.

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