Phase Diagrams: Materials Science and Technology

The representation and interpretation of phase relations in systems containing oxides of transition-metal oxides as components is more complicated than in systems of noble-metal oxides, because the former oxides are relatively unstable and because the elements commonly occur in several states of oxidation. The stability range of the various phase assemblages, therefore, are functions not only of temperature but also of the oxygen pressure* of the system. In order to describe these systems adequately, provisions must be made in the phase diagrams to include the oxygen pressure as a parameter in addition to those usually represented in phase diagrams, i.e., temperature and composition of the condensed phases. For binary systems, this is easily accomplished by superimposing on the temperature-composition diagram a family of curves of equal oxygen pressures (i.e., oxygen isobars). For ternary and quaternary systems, however, this approach would result in diagrams that are practically illegible because of excessive cluttering of lines. In the latter case, therefore, it is found more practical to present supplementary diagrams to represent the oxygen isobars.