Stable Rb-B compounds under high pressure

As a frontier issue of physics and material, the structures and related properties of borides have been extensively investigated in fundamental science. The search for pressure-induced stable compounds has become a feasible approach to acquire borides that are inaccessible at atmospheric pressure. Combined with state-of-the-art swarm intelligence structure prediction and first-principles calculations, we systematically explored the Rb-B system and uncovered a series of unprecedented RbB, Rb2B3, RbB3, RbB6, RbB8, and RbB10 under high pressure. It is found that the catenation of boron evolves from linear chain to layered sheets, clusterlike units, and further to three-dimensional tunnel structures with increasing boron content. Among them, RbB6 and RbB8 are expected phonon-mediated superconductors with Tc of -12 K and superhard material with a hardness of -37 GPa at ambient pressure, respectively. Additionally, RbB8 is a suitable precursor for obtaining the superconducting o-B16 boron allotrope by removing Rb due to its better stability than isomorphic SrB8. The current results provide insights into the design of unforeseen borides and illustrate intriguing B-B bonding features originating from Rb B charge transfer under pressures.