Rubidium polyhydrides under pressure: Emergence of the linear H ₃ ^- species

The structures of compressed rubidium polyhydrides, RbH _n 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, H ₂ molecules, and H ^- ₃ molecules. The latter motif, the simplest example of a three-center four-electron bond, is found in the most stable structures, RbH ₅ and RbH ₃, which metallize above 200 GPa. At the highest pressures studied, our evolutionary searches find an RbH ₆ phase which contains polymeric (H ^- ₃) _∞ chains that show signs of one-dimensional liquid-like behavior.