Polymorphism is a phenomenon often observed in crystals. It is defined as a property of substance to exist in more than one crystal structures. By changing the physical conditions of crystallization (pressure, temperature) or the environment in which process occurs (concentration, solvent), different polymorphic phases of a given substance are obtained. The crystallization process may be driven by slow evaporation of aqueous solutions or by cooling a melted solid up to room temperature. Usually the recrystallization is applied as a method to remove the impurities of a substance in solid phase. The maintenance of exactly the same condition of nucleaction and growth of the crystals is necessary to obtain the same polymorphic phase of a substance by recrystallization. From the chemical point of view polymorphism can be treated as a reaction of isomerization.

The various polymorphic forms of a crystal have different crystal lattices and thereby different physical and chemical properties. Frequently to distinct the polymorphic forms of crystals such properties like: density, hardness, solubility, electric, magnetic and optical features are investigated. An especially important property of the group of medical substances is the bioavailability of different polymorphic forms. By way of digression it is worth saying that more frequently a largest bioavailability have pharmaceutic substances in the amorphous state. An amorphous solid does not have the regular arrangement of elements (atoms, ions, molecules) in space and can be compared to the liquid phase or the glassy one.

In addition to polymorphism and amorphism the term pseudopolymorphism is also used in the literature. It has association to hydrates and solvates in which the molecules of water or the solvent are build into the crystal lattice. The dehydration and desolvation processes effect a change of the crystal lattice type. The polymorphic, pseudopolymorphic as well as amorphous transformations can occur in a reversible or irreversible way. The quantitative knowledge of those transformations pass are not generally known. There is a level of imagination about the transformation of order-disorder which results in an increase of the crystalographic symmetry with the increase of temperature.

The majority of polymorphic transformations is very fast since they proceed without diffusion. A slow transformation process causes a greater change in the group symmetry of the crystal and, moreover such a process is usally irreversible. The method of thermal analysis, microcalorymetric and spectroscopic methods are usually used to investigate the polymorphic transformation of a crystals. During the last time, the method of acoustic emission was shown to be useful in the investigation of the dynamics of phase transformations. This method is utilized as an independent method or as a one coupled to the DTA method.

Ammonium nitrate is of great practical importance as the component of fertilizers and explosives. NH₄NO₃ forms five polymorphic phases and, moreover there is also a phase transformation of higher order. It results from the fact that the unmovable NO₃^- anion with plain structure over some temperature level reach the possibility of rotation. This phase can exists above 125,2 ⁰C in the regular crystal structure only when both the ions ( NH₄⁺ i NO₃^-) rotate in a way preserving the spherical symmetry of the crystal. The polymorphic phases of ammonium nitrate with respect to the ranges of temperature are as in TABLE.