Nonstoichiometry in oxides and its control

Abstract

A point–defect scheme is methodically outlined for description of oxygen nonstoichiometry in extended structures of oxides. Two cases of nonstoichiometry are treated; a narrow-range nonstoichiometry centered at an integer valence state of the non-oxygen atom, and a wide-range nonstoichiometry extending between two integer structures. The latter term is introduced as a counterpart to the integer valence (stoichiometry), because for doped oxides these two situations do not coincide on the oxygen partial-pressure scale. The description of the narrow nonstoichiometry is exemplified on a corundum-type oxide (Cr 2O 3; chosen for its nontrivial atom ratio and clear red–ox properties whereas no actual experimental data are treated) and includes the effects of doping. The wide-range nonstoichiometry model is exemplified by YBa 2Cu 3O 6+ w and tested on experimental data from the literature. Techniques of the oxygen nonstoichiometry control are sorted out according to the controlled variables. Types of analyses of the bulk oxygen nonstoichiometry and of its distribution across the sample are described.