In a series of large-capacity oxygen storage materials using the redox between Ln 2 O 2 SO 4 (S 6+ ) and Ln 2 O 2 S(S 2-), the Pr system can work at as low as 600 °C, compared to g650 °C required for Ln ) La, Nd, and Sm. The exceptional character of the Pr system has been studied from physicochemical points of view by means of thermogravimetric analysis, X-ray photoelectron spectroscopy, X-ray diffraction, Rietveld analysis, Fourier transform infrared spectroscopy, and Raman spectroscopy. Unlike the other Ln oxysulfates/oxysulfides, the Pr system contained a considerable amount of tetravalent cation (Pr 4+ ) on the surface. The smooth redox between Pr 3+ and Pr 4+ would promote the oxidation of bulk Pr 2 O 2 S to Pr 2 O 2 SO 4 . On the other hand, the smooth reduction of Pr 2 O 2 SO 4 to Pr 2 O 2 S appears to be associated with a local structure of sulfate. The X-ray and spectroscopic analysis predicted that the tetrahedral SO 4 unit of Pr 2 O 2 SO 4 is more distorted than that of La 2 O 2 SO 4 . Instability caused by the stronger distortion of SO 4 would lead to the easier reduction to S 2species. A synergy of these two different effects on the redox process of the Pr system seems to be a possible reason for the successful oxygen storage/release cycles at lowest possible temperatures.

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