4. Thermodynamic measurement of physical stability, the driving force behind crystallization

At a given pressure P, a crystal melts at temperature T _m . The possible existence, below T _m , of both states, crystalline or subcooled liquid (which is amorphous), raises the question of the relative stability of these states. For transformations occurring at a given temperature T and pressure P, the stability of a state is measured by the Gibbs free energy G, also called free enthalpy. G is defined by :

The differential form of G (T, P) as T and P vary is given by :