Low-Temperature Oxygen Storage of Cr-Cr Mixed-Valence YCrPO Driven by Local Condensation around Oxygen-Deficient Orthochromite.

The oxygen storage capability and related defect structure of tetrahedral orthochromite(V) compound YCrPO (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. YCrPO started releasing oxygen as heated from 50 °C under ambient atmosphere, with reduction of Cr to Cr, while the reduced YCrPO 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 YCrPO phases were stabilized by forming linking polyhedral CrO via corner sharing between oxygen-deficient CrO and adjacent CrO. YCrPO was found to have an extremely low reduction enthalpy of about 20 kJ mol 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.