Polymorphism is the ability of a substance to crystallise in different structures. For example, carbon can crystallise as diamond, graphite or C₆₀. Different polymorphs have different properties, which makes polymorphism of great importance in the manufacture of crystalline materials, in particular in the pharmaceutical industry. 

In the latest CrystEngComm Highlight article, Lian Yu at the University of Wisconsin-Madison, US, reviews recent progress in understanding how crystallisation occurs in a liquid that is capable of forming polymorphs. 

According to Yu, the traditional views of crystallisation assume that the initial nucleation in a liquid defines the polymorph of the end product. But Yu goes on to explain that a fast-nucleating polymorph might be slow-growing. Therefore, a slower-nucleating, but faster-growing, polymorph could catch-up or overtake. 

For this to be successful, the second, slower-nucleating polymorph must be able to nucleate on the first, a phenomenon termed 'cross-nucleation between polymorphs'. 

'While the initial nucleation has received considerable attention in studies of crystal polymorphism, the other two properties [cross-nucleation and relative growth rates] have not,' said Yu. 

Theoretical work on the prediction of the growth rates crystal faces of single polymorphs is in progress. 'Further studies may offer a unified theory capable of predicting not only the growth morphologies of one polymorph but also the relative growth rates of different polymorphs,' added Yu.