IntroductionMaterials with an ordered arrangement of oxide ion vacanciesattract much interest as oxide ion or proton-conducting materi-als [1–4], CO2absorption materials [5,6] and so on. Some of themshow a structural transition from a phase with ordered oxide ionvacancies to a phase with a random distribution of oxide ion vacan-cies, accompanied by a marked variation of material properties [7].Therefore, analysis of such structural phase transitions is inevitablefor the application of these materials.