First-principles study of the superconductivity of quaternary ABIrH6 (A, B = Be, Mg, Ca, Sr, Ba) compounds under ambient pressure

• Two novel quaternary superconductors were discovered. • The stability of five novel quaternary metal hydrides was verified. • Ambient-pressure superconductivity in MgCaIrH 6 (33.4 K) and MgSrIrH 6 (23.2 K). • The factors influencing electron–phonon coupling were thoroughly analyzed. Superconducting materials have long attracted interest, but their practical applications are constrained by the requirement for extremely low transition temperatures or high pressures. In this study, inspired by the ambient-pressure superconductor Mg 2 IrH 6 , a series of quaternary ABIrH 6 compounds (where A, B = Be, Mg, Ca, Sr, Ba) were designed through binary substitution of Group II elements. Using first-principles calculations, we evaluated the thermodynamic, mechanical, and dynamical stability of these hydrogen-rich metallic compounds, identifying five stable structures. Notably, MgCaIrH 6 and MgSrIrH 6 exhibit superconducting transition temperatures of 33.4 and 23.2 K, respectively. Electron–phonon coupling (EPC) analysis reveals that phonons in the 0–18 THz range contribute considerably to the EPC, with hydrogen accounting for nearly half of the total contribution. The superconducting phases exhibit a higher hydrogen density of states at the Fermi level, indicating enhanced coupling. The complex Fermi surface topology and the relatively low electron localization between adjacent Ir–H octahedra represent key differences between the superconducting and non-superconducting phases within the same structural framework. Additionally, although some non-superconducting compounds exhibit higher Debye temperatures, the low electronic density of states at the Fermi level suppresses the electron–phonon coupling