High-Temperature Superconductivity of Thermodynamically Stable Fluorite-Type Hydrides at Ambient Pressure

Hydride superconductors have attracted significant attention, yet achieving superconductivity at ambient pressure remains a key challenge. Here, a family of high-T_c (superconducting critical temperature, T_c) hydrides based on the fluorite-type AXH₈ structure, exhibiting thermodynamic and dynamic stability at low to atmospheric pressure, is proposed. Through comprehensive screening of 150 ternary systems, eight stable hydrides below 35 GPa are identified. Notably, AcRhH₈ and BaRhH₈ show ambient-pressure stability, while AcIrH₈ and BaIrH₈ are stable at 3 GPa, demonstrating great potential for practical applications. Several compounds surpass the liquid-nitrogen T_c threshold, including AcRhH₈ (78 K at 0 GPa), LaRhH₈ (94 K at 24 GPa), LaOsH₈ (83 K at 35 GPa), and CeOsH₈ (106 K at 31 GPa). Mechanistic analysis reveals that large-radius A-site atoms serve as electron donors, stabilizing the framework and promoting weak covalent connections among [XH₈]^n- anionic units. This leads to a 3D weak covalent network that enhances electron delocalization, increases the density of states at the Fermi level, and strengthens electron-phonon (EPC). Moreover, a linear correlation between minimum ELF at H₈ tetrahedral connections and EPC constant further highlights the role of hydrogen-framework charge delocalization in boosting superconductivity.