High Pressure Synthesis of Rubidium Superhydrides

Through laser-heated diamond anvil cell experiments, we synthesize a series of rubidium superhydrides and explore their properties with synchrotron x-ray powder diffraction and Raman spectroscopy measurements, combined with density functional theory calculations. Upon heating rubidium monohydride embedded in H2 at a pressure of 18 GPa, we form RbH9-I, which is stable upon decompression down to 8.7 GPa, the lowest stability pressure of any known superhydride. At 22 GPa, another polymorph, RbH9-II is synthesised at high temperature. Unique to the Rb-H system among binary metal hydrides is that further compression does not promote the formation of polyhydrides with higher hydrogen content. Instead, heating above 87 GPa yields RbH5, which exhibits two polymorphs (RbH5-I and RbH5-II). All of the crystal structures comprise a complex network of quasimolecular H2 units and H- anions, with RbH5 providing the first experimental evidence of linear H3- anions.