First-Principles Study On The Structures And Properties Of Rb-B Under Pressure

Metal borides,exhibiting outstanding physical properties such as superconductivities and superhard behaviors,have been extensively investigated in the fundamental science and even industrial fields.Among these borides,alkaline-earth metal boride MgB₂ is found as an archetypalBardeen-Cooper-Schrieffe(BCS)system and sets the T_c record of 39 K for conventional superconductor at ambient pressure.Some transition-metal borides are suggested as superhard materials as well for their three-dimensional covalent bonds,such as WB₄ tailored to be a new generation of superhard solids with superior indentation hardness above 40 GPa under small loads.In order to design newfangled borides and to explore the intrinsic nature would greatly deepen the understanding of their associated physicochemical properties and underlying mechanisms.In this work,we present an exemplary investigation on the condensed Rb-B system over a wide range of stoichiometry by combining the state-of-art swarm intelligence structure prediction and first-principles calculations and the innovation results were obtained as follows.We uncovered three thermodynamically stable stoichiometries of hexagonal RbB,monoclinic RbB₃ and cubic RbB₆ which become stable above 6.8,50 and 2.5 GPa,respectively.Specifically,RbB₆ transforms from cubic structure into orthorhombic one at 14.5 GPa,and then to monoclinic one at 70 GPa.The electronic calculation shows that hexagonal RbB,monoclinic RbB₃ are both metal phases and RbB₆ has undergone the transformation from metal to semiconductor and then to metal with the increasing pressure.Our further explorations show that RbB₆ is estimated to be a phonon-mediated superconductor with critical temperature of 7.3-11.6K at atmospheric condition.In addition,calculated hardness for the Rb-B demonstrate its intrinsic hard nature with a highest estimated Vickers hardness of 28GPa.