Low-Temperature Oxygen Storage of Cr IV -Cr V Mixed-Valence YCr 1-x P x O 4-δ Driven by Local Condensation around Oxygen-Deficient Orthochromite.

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCr1-x Px O4 (x = 0, 0.3, 0.5, and 0.7) were investigated by employing thermal gravimetry and in situ X-ray spectroscopy for reversible oxygen store/release driven by heating-cooling cycles in the temperature range from 50 to 600 °C. YCr1-x Px O4 started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of CrV to CrIV , while the reduced YCr1-x Px O4-δ phase was significantly reoxidized via absorbing oxygen by cooling to 50 °C under ambient atmosphere, recovering the original stoichiometric phase. Operando X-ray adsorption spectroscopy and first-principles calculations demonstrate that nonstoichiometric YCr1-x Px O4-δ phases were stabilized by forming linking polyhedral CrIV 2 O7 6- via corner sharing between oxygen-deficient CrIV O3 2- and adjacent CrIV O4 4- . YCr1-x Px O4 was found to have an extremely low reduction enthalpy of about 20 kJ mol-1 probably due to the relatively high reduction potential of high-valence-state Cr(V)/Cr(IV) redox pairs, thereby resulting in reversible oxygen storage in such a low-temperature region.