ABSTRACT
Random arrangements of defects implies a high entropy and weak interactions, while ordered arrays of defects means that the entropy contribution is small, the enthalpy of the interactions is high and the phase is likely to be a stoichiometric compound. The two extremes can be brought together by using two important variables; the interactions between the defects and the structure of the non-stoichiometric phase. On the left of the diagram the situation in normal stoichiometric materials is considered. The phase will be non-stoichiometric in an operational sense, which means that experimentally the material will behave as if it contained point-defect populations. An ideal way of linking the theoretical ideas about non-stoichiometric compounds with the experimental determination of defect structures and microstructures is to evaluate the theoretical predictions numerically. This is possible because of the rapid increase in the power of computers to carry out large numbers of arithmetical calculations in reasonably short time spans.
