Revealing the role of lattice distortions in the hydrogen-induced metal-insulator transition of SmNiO3

The discovery of hydrogen-induced electronic phase transitions in strongly correlated materials such as rare-earth nickelates has opened up a new paradigm in regulating materials' properties for both fundamental study and technological applications. However, the microscopic understanding of how protons and electrons behave in the phase transition is lacking, mainly due to the difficulty in the characterization of the hydrogen doping level. Here, we demonstrate the quantification and trajectory of hydrogen in strain-regulated SmNiO₃ by using nuclear reaction analysis. Introducing 2.4% of elastic strain in SmNiO₃ reduces the incorporated hydrogen concentration from ~10²¹ cm^-3 to ~10²⁰ cm^-3. Unexpectedly, despite a lower hydrogen concentration, a more significant modification in resistivity is observed for tensile-strained SmNiO₃, substantially different from the previous understanding. We argue that this transition is explained by an intermediate metastable state occurring in the transient diffusion process of hydrogen, despite the absence of hydrogen at the post-transition stage.