chemically individual materials of variable composition. The existence of compounds whose composition does not obey the laws of stoichi-ometry was predicted by C. L. Berthollet in the early 19th century. In the early 20th century, N. S. Kurnakov proved that nonstoichiometric compounds are formed in some binary metal alloys, and he called them berthollides.
The latest physical methods of research have made possible demonstration of the widespread distribution of nonstoichiometric compounds among such important classes of inorganic compounds as the hydrides, oxides, sulfides, nitrides, carbides, and complexes. Naturally occurring nonstoichiometric compounds include feldspars, zeolites, and spinels. Nonstoichiometric compounds are denoted by a tilde in front of the stoichiometric formula or by a line over it (for example, ~ FeS or FeS). The quantitative limits of the composition of nonstoichiometric compounds are indicated by the degree of nonstoichiometricity x (for example, α-FeSx, where 1.02 < x< 1.10, and β-FeSx, where 1.11 < x< 1.14).
S. A. POGODIN