Crystallization of Organic Compounds, 2nd Edition

Chapter 3Polymorphism

Polymorphism, as applied to the solid state, can be defined as the ability of the same chemical substance to exist in different crystalline structures (Findlay et al. 1951) (regular, repeating arrangement of atoms or molecules in the solid state). The different structures are generally referred to as polymorphs, polymorphic modifications, crystal forms, or forms (Verma and Krishna 1966). Strict adherence to this definition of polymorphism excludes solvates and hydrates (specific water solvate) as polymorphs because they correspond to different chemical substances. Solvates and hydrates are sometimes referred to as pseudopolymorphs. Molecule A is a different chemical substance than molecule A coordinated with a solvent. Similarly, when molecule A forms salts or co‐crystals, its salts or co‐crystals are considered as different chemical substances from molecule A. Also, salts or co‐crystals can form pseudopolymorphs.

3.1 PHASE RULE

This difference is further reinforced by application of The Phase Rule to the equilibrium between two strictly defined polymorphs of a compound or the equilibrium between a compound and a corresponding solvate of that compound. In the former case; there is only one component (in The Phase Rule sense—the compound), there are two phases (the two polymorphs) and therefore, there is only one degree of freedom for equilibrium between two polymorphs by application of The Phase Rule equation.

(3.1)

where F is the degrees of freedom ...