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

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 H3- units. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH5 and RbH3. Pressure induces the symmetrization of H3-. The thermodynamically most stable polyhydrides metallize above 200 GPa. At the highest pressures studied, our evolutionary searches find an RbH6 phase which contains polymeric (H3-)$_\infty$ chains that show signs of one-dimensional liquid-like behavior.