Superconductivity of metastable dihydrides at ambient pressure

Hydrogen in metals is a significant research area with far-reaching implications, encompassing diverse fields such as hydrogen storage, metal-insulator transitions, and the recently emerging phenomenon of room-temperature superconductivity under high pressure. Hydrogen atoms pose challenges in experiments as they are nearly invisible, and they are considered within ideal crystalline structures in theoretical predictions, which hampers research on the formation of metastable hydrides. Here, we propose pressure-induced hydrogen migration from tetrahedral (T-) site to octahedral (O-) site, forming \({{\rm{LaH}}}_{x}^{{\rm{O}}}{{\rm{H}}}_{2-x}^{{\rm{T}}}\) in cubic LaH_2. Under decompression, it retains \({{\rm{H}}}_{x}^{{\rm{O}}}\) occupancy, and is dynamically stable even at ambient pressure, enabling a synthesis route of metastable dihydrides via compression-decompression process. We predict that the electron-phonon coupling strength of \({{\rm{LaH}}}_{x}^{{\rm{O}}}{{\rm{H}}}_{2-x}^{{\rm{T}}}\) is enhanced with increasing x, and the associated T_c reaches up to 10.8 K at ambient pressure. Furthermore, we calculated stoichiometric hydrogen migration threshold pressure (P_c) for various lanthanides dihydrides (RH₂, where R = Y, Sc, Nd, and Lu), and found an inversely linear relation between P_c and ionic radii of R. We propose that the highest T_c in the face-centered-cubic dihydride system can be realized by optimizing the O/T-site occupancies.