Structural Aspects of Phase Transitions

There are two kinds of structural transformations in the crystalline solid state: solid state reactions, in which the product chemically different from the starting material can be isolated, and polymorphic transitions, when the phases have different organization of identical molecules in the crystal structures. As a consequence, the starting and the final phases of a solid state reaction differ in the melt and vapor, while different polymorphic modifications are identical in melt or gas phase. Some examples of the different phase transitions in the solid state are described in detail: the π-molecular complexes, the hydrogen-bond transformations and the reversible single crystal - twin transition.