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, ${\mathrm{Rb}}_{2}{\mathrm{B}}_{3}$, ${\mathrm{RbB}}_{3}$, ${\mathrm{RbB}}_{6}$, ${\mathrm{RbB}}_{8}$, and ${\mathrm{RbB}}_{10}$ 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, ${\mathrm{RbB}}_{6}$ and ${\mathrm{RbB}}_{8}$ are expected phonon-mediated superconductors with ${T}_{c}$ of \ensuremath{\sim}12 K and superhard material with a hardness of \ensuremath{\sim}37 GPa at ambient pressure, respectively. Additionally, ${\mathrm{RbB}}_{8}$ is a suitable precursor for obtaining the superconducting $o\text{\ensuremath{-}}{\mathrm{B}}_{16}$ boron allotrope by removing Rb due to its better stability than isomorphic ${\mathrm{SrB}}_{8}$. The current results provide insights into the design of unforeseen borides and illustrate intriguing B-B bonding features originating from Rb \ensuremath{\rightarrow} B charge transfer under pressures.