A fresh class of superconducting and hard pentaborides

On the basis of the current theoretical understanding of boron-based hard superconductors under ambient conditions, numerous studies have been conducted with the aim of developing superconducting materials with favorable mechanical properties using boron-rich compounds. In this paper, first-principles calculations reveal the existence of an unprecedented family of tetragonal pentaborides MB5 (M = Na, K, Rb, Ca, Sr, Ba, Sc, and Y), comprising B20 cages and centered metal atoms acting as stabilizers and electron donors to the boron sublattice. These compounds exhibit both superconductivity and high hardness, with the maximum superconducting transition temperature Tc of 18.6 K being achieved in RbB5 and the peak Vickers hardness Hv of 35.1 GPa being achieved in KB5 at 1 atm. The combination of these properties is particularly evident in KB5, RbB5, and BaB5, with Tc values of ∼14.7, 18.6, and 16.3 K and Hv values of ∼35.1, 32.4, and 33.8 GPa, respectively. The results presented here reveal that pentaborides can provide a framework for exploring and designing novel superconducting materials with favorable hardness at achievable pressures and even under ambient conditions.