High-temperature superconductivity below 100 GPa in ternary C-based hydride MC2H8 with molecular crystal characteristics (M = Na, K, Mg, Al, and Ga)

To explore the high-temperature superconductivity of hydrogen-rich compounds at low pressures, we have investigated the crystal structures, electronic and dynamical properties, electron-phonon interactions, and possible superconductivity of the ternary hydride MC2H8 (M = Na, K, Mg, Al, and Ga) in the low-pressure range of 0-100 GPa based on the first-principles calculations. The results show that there is no imaginary frequency in phonon spectra for MC2H8 at selected pressures which indicates that MC2H8 is dynamically stable. Furthermore, according to the Eliashberg spectral function under pressures, MC2H8 is predicted to be superconducting at low pressure. Especially, the superconducting critical temperature (T-c) of MgC2H8 is higher than 55 K at 40 GPa and the T-c in AlC2H8 reaches 67 K at 80 GPa. Electronic and phonon states and the electron-phonon interactions show that H has a considerable contribution to this ternary hydride superconductor and suggest that increasing the contribution of H to total electron-phonon coupling is a way to design materials with high T-c. Our study shows that it is one of the feasible routes to explore the low-pressure and high-temperature superconductivity in ternary carbon-based hydrides.