Rubidium Polyhydrides Under Pressure: Emergence of the Linear H3- Species

The structures of compressed rubidium polyhydrides, RbHn with n>1, and their evolution under pressure are studied using density functional theory calculations. These phases, which start to stabilize at only P=2 GPa, consist of Rb+ cations and one or more of the following species: H- anions, H2 molecules, and ${{\rm H}{{- \hfill \atop 3\hfill}}}$ molecules. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH5 and RbH3, which metallize above 200 GPa. At the highest pressures studied, our evolutionary searches find an RbH6 phase which contains polymeric (${{\rm H}{{- \hfill \atop 3\hfill}}}$)8 chains that show signs of one-dimensional liquid-like behavior.