High-pressure recrystallisation - a route to new polymorphs and solvates

The recrystallisation of organic compounds from solution under high-pressure conditions is shown to be a versatile method for the formation of new polymorphs and solvates. The technique is illustrated by the crystallisation of a new polymorph of phenanthrene from dichloromethane at a pressure of 0.7 GPa, and the crystallisation of a novel dihydrate of paracetamol from water at a pressure of 1.1 GPa. These phases have been characterised by single crystal X-ray diffraction. We also demonstrate that the technique can be used to prepare a polymorph that is metastable under ambient conditions. Thus the orthorhombic form of paracetamol was crystallised from ethanol at a pressure of 1.1 GPa.