In Situ Transport Characterization of Hydride-Induced Thin-Film Reduction

Metal hydrides, such as CaH₂, have recently emerged as highly promising reducing agents for facilitating the low-temperature reduction of metal oxides. A unique advantage of hydride reduction is its capability to synthesize metastable materials that are otherwise inaccessible through conventional high-temperature reactions. Notably, hydride reduction techniques have been utilized to create unusual NiO₄ square-planar coordination in nickelates, a structure known to host superconductivity [1]. Beyond novel materials discovery, metal hydrides hold substantial potential in applied engineering, as previous studies have demonstrated that CaH₂ can lower the temperature required for H₂ reduction of iron oxide, offering benefits for clean hydrogen-based ironmaking [2]. Despite these wide-ranging applications, a pivotal scientific question remains: <i>What is the true active reducing species in hydride reduction?</i> Answering this question is crucial for unlocking the full potential of metal hydrides in both fundamental research and practical applications.<br/><br/>In this study, we investigate the CaH₂-induced reduction kinetics of metal oxides using epitaxial α-Fe₂O₃ thin films as a model system. To elucidate the intrinsic reducing capability of CaH₂, we seal the iron oxide thin-film samples along with CaH₂ powders in an evacuated quartz tube and analyze the reduction behavior within this closed system. We developed an experimental platform that enables real-time monitoring of the CaH₂ reduction process through transport measurements. Using this setup, we quantified the phase transformation kinetics from iron oxide to metallic iron by continuously tracking the evolution of electrical resistivity in the thin-film sample. Consequently, we determined the apparent activation energy of hydride reduction under conditions where samples were either in contact with or separated from CaH₂ powders. In both cases, the apparent activation energies were identical and comparable to those obtained from gas-phase H₂ reduction. These findings indicate that CaH₂ reduction predominantly occurs via solid-gas interactions, with gas-phase H₂ being the primary reducing agent. This study highlights the power of combining thin-film systems and <i>in situ</i> transport measurements to understand critical material processing reactions, which can help accelerate materials design and optimization.<br/><br/>[1] Li <i>et al.</i>, Nature, 2019.<br/>[2] Tsuchida <i>et al.</i>, Journal of Solid State Chemistry, 2023.