long to understand that the phenomena associated with the redistribution of interstitial atoms and structural vacancies in non stoichiometric compounds represent an excellent model of analogous phenomena arising in solid-state systems with mutual substitution of components. Therefore methods of analysis and description proposed and developed for nonstoichiometric compounds are applicable to a number of systems with substitutional disorder. In 1980, when we embarked on this work, ordering in nonstoichiometric compounds was assumed to be a rare and accidental effect, which hardly deserved close...