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The formation of the fluorite phase Y0.8Ta0.2O1.7 and other related phases in the system Y2O3–Ta2O5–MO (M=Mg, Ca, Sr or Ba) has been studied. The single fluorite phase formed when up to 12 mol% MgO was added to the fluorite phase; however, MgO appeared as the second phase as well as the main fluorite phase when more than 16 mol% MgO was added. When more than 8 mol% CaO was added to Y0.8Ta0.2O1.7, Ca2YTaO6 and Y2O3 were produced as the second phases as well as the main fluorite phase. Ba2YTaO6 and Sr2YTaO6 of the perovskite-type ordered structure and Y2O3 were produced as well as the main fluorite phase when only 4 mol% BaO or SrO were added to Y0.8Ta0.2O17. The region of the fluorite single phase was found in the system Y2O3–Ta2O5–CaO. The formation of the fluorite phase is assumed to be related to the cation radius of the doped alkaline-earth oxide.