Compositional and Structural Diversity in Cesium Superhydrides at High Pressure

The alkali metals have a propensity to form hydrides with unusually high hydrogen content, hosting a wide variety of hydrogenic species under high pressure conditions. We demonstrate that cesium exhibits unusually diverse hydride chemistry, culminating in the formation of ${\mathrm{CsH}}_{25}$ , the most hydrogen-rich metal hydride reported to date. Synthesized through laser-heated diamond anvil cell experiments at pressures of 46 GPa, ${\mathrm{CsH}}_{25}$ features icosahedral assemblies of quasi-molecular $H_2$ units stabilized within an ionic ${\mathrm{Cs}}^{+}$ framework. At lower pressures, room-temperature compression yields ${\mathrm{CsH}}_9$ , which remains stable down to 7.8 GPa, the lowest stability pressure known for any superhydride. We also identify two polymorphs of ${\mathrm{CsH}}_3$ containing linear $H_3^{-}$ anions and a body-centered tetragonal ${\mathrm{CsH}}_7$ . These results demonstrate unprecedented chemical complexity in the cesium-hydrogen system and establish cesium as a unique host for extreme hydrogenation.