Polymorphism is the ability of solid crystalline anhydrous materials to exist in various structural forms (or modifications). A directly related term is pseudo-polymorphism which concerns hydrates or solvates.
Different polymorphic forms exhibit different physico-chemical properties such as melting temperatures or specific heat capacities. This, in turn, has often an impact on the stability or bioavailability of the drug substance.
In the example, 1.23 mg of carbamazepine of modification III (with a theoretical melting temperature of 176°C) was heated at 10 K/min in a nitrogen atmosphere using Al crucibles.
At around 160°C (peak temperature), modification III converts into modification I which finally melts at approx. 190°C (melt onset temperature = extrapolated onset temperature).