Superconductivity of highly hydrogen-rich hydride CsH₂₅

Hydrogen-rich hydrides have been regarded as a crucial framework for achieving high-temperature superconductivity. Here, through random structure search combined with first-principles calculations, we predict a thermodynamically stable hydrogen-rich hydride CsH_25 in the CsH system within a pressure range of 100-400 GPa. The results demonstrate the presence of quasi-H_2 units embedded within the metallic cesium framework, where alkali metals provide a constant charge and support the hydrogen-cesium alloy framework to maintain a stable crystal structure. This structural feature exerts a significant influence at both the electronic and phononic levels. The synergistic interaction between these electronic and phononic features ultimately enables the system to exhibit a superconducting transition temperature approaching 98 K at 100 GPa. Unlike traditional high-temperature superconductors that rely on complex covalent hydrogen networks, this study demonstrates that in simple binary systems, quasi-molecular hydrogen can also achieve strong electron-phonon coupling by modulating the electronic states at the Fermi level and inducing phonon softening. In systems containing molecular hydrogen units, superconductivity arises from the synergistic regulation of the electronic structure and phonon behavior. This provides novel insights into the superconducting mechanisms in such systems and enriches research on quasi-molecular hydrides.