Atomically Revealing Bulk Point Defect Dynamics in Hydrogen‐Driven γ‐Fe ₂ O ₃ → Fe ₃ O ₄ → FeO Transformation

Abstract Understanding how point defects in the bulk govern redox transformations is essential for advancing hydrogen‐based metal production and designing high‐performance oxide materials. This study reveals the atomic‐scale mechanisms driving hydrogen‐induced reduction of γ‐Fe 2 O 3 to Fe 3 O 4 , focusing on how bulk vacancy dynamics dictate structural evolution and reaction kinetics. A key finding is the pronounced contrast in defect behavior between the two oxides: in γ‐Fe 2 O 3 , intrinsic Fe vacancies promote oxygen vacancy clustering, destabilizing the local lattice and driving nanopore formation. In contrast, Fe 3 O 4 exhibits a higher oxygen vacancy formation energy and lacks intrinsic Fe vacancies, suppressing vacancy aggregation and maintaining a dense, pore‐free structure. This divergence governs distinct reduction pathways—γ‐Fe 2 O 3 undergoes an interface‐reaction‐limited transformation confined to the γ‐Fe 2 O 3 /Fe 3 O 4 boundary, while Fe 3 O 4 supports a uniform increase in oxygen vacancy concentration, enabling bulk‐phase reduction to lower‐oxide FeO. Integrated in situ electron microscopy and density functional theory modeling uncover a vacancy‐mediated mechanism, where synergistic cation‐anion vacancy dynamics steer microstructure evolution and phase progression. These insights highlight the critical role of vacancy dynamics in controlling oxide reactivity and offer a pathway toward vacancy engineering to enhance reduction kinetics in hydrogen metallurgy and to tailor porosity, reactivity, and structural resilience in oxide‐based catalysts and energy materials.