Unravelling the Oxygen Dynamics at Gas‐Solid Interface of CaGd ₂ (MoO ₄ ) ₄ :Yb,Er Scheelite Ceramics via Upconversion Luminescence

Abstract Controlling oxygen vacancies in ceramics via atmosphere‐assisted thermal annealing is a widely adopted and impactful strategy, yet understanding the oxygen dynamics at the gas‐ceramic interface remains a significant challenge. In this study, Er ³⁺ ions are demonstrated to serve as effective luminescent probes for monitoring oxygen adsorption/evolution, redox reactions, and diffusion at the gas‐solid interface of scheelite CaGd ₂ (MoO ₄ ) ₄ :10%Yb,5%Er (CGMO:Yb/Er) ceramics under various atmospheres at 500 °C. Time‐dependent upconversion luminescence measurements reveal that slow diffusion of oxygen ions/vacancies within the lattice predominantly governs oxygen equilibrium during the reduction process (O ₂ +8 e ′+2V·· O→2O ²⁻ ), irrespective of oxygen partial pressure. It is attributed to the relatively rapid reaction and intrinsic electron carriers within the mixed ionic‐electronic conductor of CGMO:Yb/Er. Conversely, the oxidation process (2O ²⁻ →O ₂ +8 e ′+2V·· O), accelerated by an external power supply for electron extraction, has a more substantial impact on oxygen vacancy formation. This is evidenced by an increase in luminescence intensity and a prolonged equilibration time. Reducing the oxygen partial pressure has a similar effect as lowering temperature, both leading to extended oxidation balance time. These findings provide valuable insights into oxygen dynamics at the gas‐ceramic interface during thermal annealing via an in situ optical approach, enabling the targeted design of oxide ceramics with controlled oxygen vacancies and enhanced performance.