Mechanisms driving robust high-temperature superconductivity in complex metal hydrides under moderate pressure

Abstract: The discovery of near-room-temperature superconductivity in compressed hydrides has sparked intensive research efforts to identify superconducting hydrides stable at low or even ambient pressures. Herein, we demonstrate a new mechanism for achieving robust superconductivity in complex metal hydrides under moderate pressure, using Li3IrH9 as a paradigmatic example. This compound displays unique electronic structural characteristics where the broadening and overlap between antibonding electronic bands of [IrH8]2- and adjacent H- orbitals not only drive the intrinsic metallicity of the hydrogen sublattice, generating hydrogen-dominated electronic states at the Fermi level, but also soften hydrogen-related optical phonon modes, inducing strong electron-phonon coupling that remains robust even under high-pressures. First-principles calculations predict that Li3IrH9 maintains thermodynamic stability at 100 GPa while exhibiting a consistently high Tc exceeding 100 K across a broad pressure range (8-150 GPa). Through high-throughput computational screening, we have identified a new superconducting family based on this structural prototype, including Li3RhH9 (Tc = 124 K at 20 GPa) and Li3CoH9 (Tc = 80 K at 10 GPa). This work provides a new platform and original theoretical insights for the development of complex metal hydride superconductors that exhibit robust high-temperature superconductivity and promising practical applications.