Polymorphism

Polymorphism, which is defined as the ability of a compound to exist in more than one crystalline modification is a widespread phenomenon  and frequently found in all kinds of crystalline solids. Because different polymorphic modifications exhibits different physical properties, this phenomenon is of extremely importance in in academic and industrial reseach like e.g. pharmaceutical development. In this context the phenomenon of pseudopolymorphism is also of importance, which refers to compounds that contains additional solvent like e.g. hydrates or generally solvates, frequently called cocrystals.

As the chemical composition of the different crystalline forms is identical, changes in properties of these forms can directly be traced back to differences in their crystal structure. In view of this, investigations on polymorphism provide information on intermolecular interactions in crystals and allows to study structure-property-relationships in crystalline solids. However, in this context not only the structural aspects of polymorphism are of importance because also the thermodynamic and kinetic properties should be investigated. Therefore, important questions has to be answered like. e. g. how many modifications exists and how they can be prepared? It also must be investigated which is the thermodynamic stable and which the metastable form and in which temperature range some specific form is stable or metastable. Furthermore it is of interest how the different forms can be transformed into each other and if these forms behave enantiotropic or monotropic.

In our own research our interest is focused mainly on the polymorphism and pseudopolymorphism of coordination compounds and of drugs. This includes the search for new modifications (polymorphic screening) as well as investigations on their thermodynamic properties and their transition behaviour. These investigations are frequently be performed using different thermoanalytical methods like simultaneous differential thermoanalysis and thermogravimetry coupled to mass spectroscopy (DTA-TG-MS), differential scanning calorimetry (DSC) and thermomicroscopy as well as X-ray single crystal and powder diffraction (XRPD).