Xuhui Xu

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.

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.