Extending the family of oxygen ion conductors isostructural with La2Mo2O9

► Phase formation in the La2Mo2O9-Pr2W2O9-Pr2Mo2O9 system has been studied. ► Phase formation in the La2Mo2O9-Nd2W2O9-“Nd2Mo2O9” system has been studied. ► Compounds isostructural with La2Mo2O9 were found in wide single-phase regions. ► Tungsten stabilizes Nd2Mo2−2xW2xO9 compounds with the La2Mo2O9 structure. ► The α→β phase transition of La2Mo2O9 is suppressed and the cubic phase is stabilized.