High-T_c superconductivity of polyhydride Rb₂MgH₁₈ with a layered hydrogen structure at high pressure

Since near room-temperature superconductivity is measured in pressurized H3S and LaH10, H-rich hydrides have served as promising candidates for high-temperature superconductors. Given the remarkable superconductivity stems from the unique H units of these hydrides, the search for new hydrides with novel H units remains a pressing task. Herein, we predicted a metastable structure of Rb2MgH18 at 235 GPa with an intriguing H layer composed of H-6 and H-12 rings, using the first-principles structure prediction method in conjunction with density functional theory calculations. As a result of electron-phonon coupling simulations, this structure exhibits superconductivity with an estimated critical temperature, T-c, of 278 K at 235 GPa and 261 K at 300 GPa. Our in-depth analysis reveals that the vibrations of atoms in the H layer are the main contributor to the superconducting behavior. The present study largely enriches the fundamental knowledge of the chemistry of hydrogen and superconducting properties in high-pressure hydrides.