Phase Evolution in Pd-Capped SmH x Thin Films during Electrochemical Hydrogenation

Samarium hydride thin films are of interest for switchable optical and thermal-management devices because hydrogenation can induce large changes in optical transmittance and thermal conductivity. Here, we investigate the phase evolution of Pd-capped SmH x thin films during electrochemical hydrogenation and dehydrogenation using in situ synchrotron X-ray diffraction. During hydrogenation, the thin film evolves from cubic SmH 2 to a fluorite-based solid-solution phase, SmH 2+δ, and then to a noncubic hydrogen-rich phase with anisotropic lattice expansion. In contrast to the equilibrium bulk phase relationship, no detectable formation of hexagonal SmH 3 is observed; instead, the hydrogen-rich phase is most consistent with an expanded Sm 3 H 7 -like intermediate stabilized under thin-film conditions. Together with previous reports of large optical-transmittance and thermal-conductivity changes in samarium hydride films, these findings suggest that the transition to the expanded Sm 3 H 7 -like phase, rather than the formation of an end-member SmH 3 phase, underlies the switching functionality. These results provide a mechanistic basis for the electrochemically driven hydrogenation/dehydrogenation and highlight the importance of intermediate-phase formation for the thin-film-based switchable optical, electrical, and thermal-management devices.