The role of entropy in crystal structures

Polymorphism, the formation of different crystal structures by a single compound, is of critical importance in numerous applications, most notably in the solid forms used for pharmaceuticals. It is a very common feature of organic solids, and it is important because different solid forms have different physical properties such as solubility, morphology or tabletting characteristics. The existence of polymorphism is often understood by looking for energetically competitive intermolecular interactions, such as hydrogen bonds. This approach emphasises enthalpy. The effect of entropy is often neglected, so that the free energy characteristics of a polymorphic system is only partially explored.