Order-disorder processes in adamantine ternary ordered-vacancy compounds

Adamantine ternary ordered-vacancy compounds (OVCs) of the AB(2)X(4) family derive from the zincblende structure of binary AX compounds and share many properties with the chalcopyrite-type ternary ABX(2) compounds. AB(2)X(4) mainly crystallize in the defect chalcopyrite (or thiogallate) structure but also in other vacancy-ordered phases, like the defect stannite (DS, or defect famatinite) and the pseudocubic (PC) structures, where vacancies occupy a fixed Wyckoff site in an ordered and stoichiometric fashion. Order-disorder phase transitions have been studied in several adamantine OVCs at high temperatures and/or pressures, where OVCs undergo a phase transition to a disordered zincblende (DZ) structure on increasing temperature and to a disordered rocksalt (DR) structure on increasing pressure. It has been suggested that these order-disorder transitions should undergo via intermediate phases of partial disorder between the fully ordered phases and the fully disordered phases. In this work we study the different possible intermediate phases of partial disorder and discuss the possibility to find them with the help of vibrational spectroscopies, like Raman scattering or infrared measurements.