Polymorphism and Polytypism in Crystals

Since the discovery of x-ray diffraction in 1912 immense strides have been made in determining the structures of crystals, from sodium chloride in the beginning to vitamins and proteins in the 1960's. However, progress has been made not only in the organic field; inorganic crystals have also revealed some extraordinary structures. It is remarkable to what lengths nature sometimes goes with simple proportions of atoms in order to produce a stable structure.

Polymorphism and phase transformations in crystals have attracted the attention of solid-state scientists for nearly a century. Earlier investigations of the phenomenon were based on thermodynamic considerations which provided only a partial understanding. It is only after the development of x-ray diffraction methods of structure determination and the subsequent knowledge of the stable atomic configuration of a solid is essential to the understanding of its properties, most of which are structure sensitive. X-ray investigations of polymorphic modifications formed under different temperatures and pressures have revealed the structural changes that occur during the phase transformations between them. This has led to a classification of polymorphic transformations on a structural basis.

Contents:
1. Introduction
2. Polymorphism--the phase aspect
3. Polymorphism--the structural aspect
4. Polytypism and the description of polytypic structures
5. Silicon carbide and other polytypic substances
6. Structure determination of polytypes
7. Dislocations and the spiral growth of crystals
8. Theories of polytypism
9. Recent observations and polytypism and present position