Yuta Inoue

Epitaxial strain in perovskite oxide thin films is conventionally understood through lattice parameter modulation, characterized by the c / a ratio of the pseudocubic cell. However, internal atomic arrangements such as octahedral tilting can relax independently, a phenomenon that has remained largely unexplored. Here, we demonstrate that electrochemical operation induces internal structural relaxation in epitaxial La 0.7 Sr 0.3 CoO 3 films while in-plane lattice parameters remain pinned to the substrate. Synchrotron x-ray diffraction reveals that prolonged electrochemical treatment drives the octahedral tilt mode from the as-grown strained configuration toward the bulk structure without conventional lattice parameter changes. This relaxation is enabled by ionic oxygen migration during electrochemical reactions, which reduces energy barriers between structural configurations. Our findings reveal a relaxation pathway in epitaxial oxides that operates independently of conventional lattice parameter relaxation, and demonstrate the importance of monitoring internal atomic arrangements under operating conditions. This establishes electrochemically driven ionic migration as a versatile tool for controlling internal structure in epitaxial oxides, opening opportunities for designing strain-engineered functional materials.

Mixed-anion compounds are emerging as promising oxygen electrocatalysts; however, the impact of anion-site engineering across various transition metals remains underexplored. Here, we investigate the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) activities of Ruddlesden–Popper-type layered perovskite oxychlorides, Sr2MeO3Cl (Me = Mn, Fe, Co, and Ni), and compare them with the corresponding LaSrMeO4 benchmark oxides. For the OER, chloride substitution universally enhanced the catalytic activity across all investigated transition metals. Density functional theory and X-ray photoelectron spectroscopy revealed that the oxygen p-band center serves as a universal OER activity descriptor, correlating strongly with surface oxygen vacancies and shifting closer to the Fermi level upon chloride incorporation. Conversely, the ORR pathway depends on the transition metal species. While the benchmark oxides predominantly proceed via a two-electron reduction of O2, Sr2MnO3Cl and Sr2CoO3Cl oxychlorides exhibit apparent four-electron pathways. Specifically, Sr2MnO3Cl drives both direct four-electron and sequential (two-electron reduction followed by peroxide decomposition) pathways, whereas Sr2CoO3Cl exclusively utilizes the sequential pathway, facilitated by abundant oxygen vacancies. These findings establish fundamental design principles for tuning bifunctional oxygen electrocatalysis via anion-site engineering.

The surface structure of the La_0.6Sr_0.4CoO₃ film, a typical model water-splitting catalyst, is examined under vacuum and electrochemical conditions using surface X-ray diffraction. The pristine sample has a two-unit-cell-thick strongly polarized SrCoO₃ layer at the surface, and the surface termination is predominantly a CoO₂ layer with apical oxygen atoms. After electrochemical treatment, the surface was covered with an additional edge-shared CoO₆ double layer. The polarization in the SrCoO₃ region was greatly reduced. These structural changes were accompanied by an increase in the working electrode current, suggesting a strong relationship between surface structure modulation and catalytic activity. Reversible structural modulation induced by the film's electric potential was observed and was qualitatively understood as atomic displacements caused by the local electric field and change in the Co ionic radii.

Perovskite oxides have been extensively investigated as active electrocatalysts for the oxygen evolution reaction (OER) in alkaline solution. While the OER activity of some perovskite oxides is positively influenced by Fe ions in the electrolyte, the impact of other transition metal ions in the electrolyte remains unclear. In this study, we compared the influence of Co ions intentionally added to the electrolyte on the OER activities of two Fe‐based perovskite oxides (Ba_0.5Sr_0.5FeO_3−δ and LaFeO₃). While the OER activity of Ba_0.5Sr_0.5FeO_3−δ is significantly enhanced by adding Co ions to the electrolyte, LaFeO₃ showed little difference in the OER behavior between the Co‐free and Co‐containing electrolytes. In the case of Ba_0.5Sr_0.5FeO_3−δ, an amorphous layer was formed, and the Co ions from the electrolyte were incorporated on the surface as a result of OER cycling. On the other hand, Co ions were also detected on the surface of LaFeO₃, but its crystalline structure remains unchanged during the OER. Our study suggests that synergistic interplay between the perovskite oxides undergoing a structural transformation at the surface and transition metal ions in the electrolyte can improve the OER activity.