Some useful definitions

Polymorphism scheme

Polymorphism scheme

A compound changing from the liquid to the solid state can form two states as extremes; it can either become amorphous so that the elements, molecules or ions arrange in incidental positions with only short-range order, or it can solidify to a crystalline state in which the chemical entity forms a lattice with three-dimensional periodicity. The crystalline state can again be split up into different classes. If a crystal form contains only one chemical entity, it is called unsolvated or anhydrated. The majority of molecules can crystallise in several different unsolvated crystal forms, which are then called polymorphs or modifications. If the crystal lattice contains more than one chemical entity, co-crystals are present. These crystal forms contain two or more compounds without proton transfer occurring between them. In the case of one component being solvent molecules incorporated from the mother liquor, solvates crystallise. A special case is the incorporation of water which leads to the formation of hydrates. Any of the above mentioned co-crystals can be polymorphic, i.e. crystallise in different crystal lattices without changing the chemical composition.