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

Abstract

Reducing the stabilization pressure of superconducting hydrides remains a major challenge for their practical application. While complex metal hydrides are recognized for their stability at low pressures, their promise as high-temperature superconductors has been largely overlooked. Here, we identify a family of superconducting complex transition metal hydrides (CTMHs), represented by Li₃IrH₉, and demonstrate a new mechanism for achieving robust superconductivity in this class of materials. In Li₃IrH₉, the broadening and overlap between the 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 pressure. Li₃IrH₉ is predicted to be thermodynamically stable at 100 GPa and maintains a high T_c above 100 K across a broad pressure range (8–150 GPa). Other isostructural compounds such as Li₃RhH₉ (T_c = 124 K at 20 GPa) and Li₃CoH₉ (T_c = 80 K at 10 GPa) also exhibit prominent superconductivity at moderate pressures. This work provides a new platform and original theoretical insights for the development of CTMH superconductors that exhibit robust superconductivity and promising practical applications.