1. Definitions and issues

Polymorphs, or polymorphic varieties, are solids that have strictly the same chemical composition, but different crystal structures ([1] [P 1 097] ). By structure, we mean the internal arrangement of atoms, ions or molecules in crystal lattices. These networks are built on elementary meshes of different dimensions and symmetries.

The term solvates is more generally used to describe crystalline phases in which solvent molecules are found within the lattice. Solvent molecules may be localized in the crystal structure at isolated sites, in planes or channels,...