Pressure-Induced Phase Stability and Superconductivity of Ruthenium Borides

Abstract

Ruthenium borides have garnered significant attention due to their diverse configurations, intriguing electronic structures, and promising applications. By combining first-principles calculations with crystal-structure searches, we systematically investigate the Ru–B system over the pressure range of 0–400 GPa. The results show that RuB and RuB2 remain thermodynamically stable across the entire pressure range, whereas RuB3 is metastable at ambient pressure and becomes thermodynamically stable above approximately 100 GPa. Among the considered phases, P-6m2-RuB3 exhibits the highest calculated superconducting critical temperature (Tc) of 7.70 K at 0 GPa. Under compression, its Tc evolves nonmonotonically: it first decreases to 3.06 K at 180 GPa, then increases to 6.75 K at 200 GPa, and finally decreases to 4.45 K at 213.7 GPa. Above 213.7 GPa, P-6m2-RuB3 is predicted to transform into P21/m-RuB3, where Tc drops to 0.58 K and decreases further under compression, accompanied by weakened electron–phonon coupling and changes in Ru–B and B–B bonding. Finally, pressure–temperature phase diagrams for RuB, RuB2, and RuB3 are constructed within the quasiharmonic approximation. Increasing boron content promotes the formation of boron-rich frameworks, thereby enhancing structural stability, modifying the electronic structure, and tuning Tc. These findings provide guidance for the design of novel superconducting materials.