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 H_{2} at a pressure of 18 GPa, we form RbH_{9}-I, which is stable upon decompression down to 8.7 GPa, the lowest stability pressure of any known superhydride. At 22 GPa, another polymorph, RbH_{9}-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 RbH_{5}, which exhibits two polymorphs (RbH_{5}-I and RbH_{5}-II). All of the crystal structures comprise a complex network of quasimolecular H_{2} units and H^{-} anions, with RbH_{5} providing the first experimental evidence of linear H_{3}^{-} anions.