Abstract
Conditions and ranges of stability of nonstoichiometric compounds are thermodynamically reviewed focusing the separation of phase based on the equilibrium pressure-composition isotherm. The recent precedent isotherms for the some of transition metal oxides and sulfides are chosen in this introductory paper.
Nonstoichiometric compounds are essentially high temperature species with wide homogeneity range. At low temperature these high temperature species separate to the super structure species or the equilibrium state of two phases coexistence.
The observations of separation of phase are very significantly for the characterization of chemical structure of solids. Especially the developments of detective technique or high resolution observation and in situ experimental proceedures will give us the much more precise informations concerning the separation of phase and a new phase with a proper crystal structure.
The well characterized nonstoichiometric high purity compounds are very valuable in the field of material science.
