Stable Rb-B compounds under high pressure

Abstract

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, Rb_{2}B_{3}, RbB_{3}, RbB_{6}, RbB_{8}, and RbB_{10} 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, RbB_{6} and RbB_{8} are expected phonon-mediated superconductors with T_{c} of ∼12 K and superhard material with a hardness of ∼37 GPa at ambient pressure, respectively. Additionally, RbB_{8} is a suitable precursor for obtaining the superconducting o-B_{16} boron allotrope by removing Rb due to its better stability than isomorphic SrB_{8}. 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.