Pressure-Induced Structural Stabilities and Superconductivity in Rhodium Borides.

Abstract

Transition metal borides have garnered significant research interest due to their versatile properties, including superconductivity and exceptional hardness. This study examines the stable crystal structures of Rhodium-Boron (Rh-B) compounds under high pressure using first-principles structural searching. Beyond the previously known Rh₂B, RhB₂, and RhB₄ phases, three new boron-rich phases-C2/m-RhB₆, Amm2-RhB₆, and Cmca-RhB₈-are identified, each characterized by three-dimensional covalent bonding networks. Their mechanical and thermodynamic stability is validated through elastic property assessments and phonon dispersion calculations. Surprisingly, these phases exhibit low bulk and shear moduli, ruling them out as candidates for hard materials. The metallic character of these borides is evident from their electronic density of states, which exhibits a sharp peak at the <i>E</i>_F-a signature often associated with superconducting systems. Indeed, our calculations predict <i>T</i>_c values of 8.93 K and 9.36 K for Amm2-RhB₆ and Cmca-RhB₈, respectively, at 100 GPa.