Superconductivity in Diamond-Like BC₁₅

Advancing the compositional space of a compound class can result in intriguing superconductors, such as LaH₁₀. Herein, we performed a comprehensive first-principles structural search on a binary B-C system with various chemical compositions. The identified diamond-like BC₁₅, named d-BC₁₅, is thermodynamically superior to the synthesized BC₃ and BC₅. Interestingly, d-BC₁₅ shows anisotropic superconductivity resulting from three distinct Fermi surfaces. Its predicted critical temperature (T_c) is 43.6 K at ambient pressure, beyond the McMillan limit. d-BC₁₅ reaches a maximum of around 75 K at 0.43% hole doping due to the substantially enhanced density of states at the Fermi level. Additionally, d-BC₁₅ demonstrates superhard characteristics with a Vickers hardness of 75 GPa. The calculated tensile and shear stresses are 72 and 73 GPa, respectively. The combination of high superconductivity and superhardness in d-BC₁₅ offers new insights into the design of multifunctional materials.