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.

中文翻译：

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一类新的超导和硬五硼化物

基于当前对环境条件下硼基硬超导体的理论理解，人们进行了大量研究，旨在利用富硼化合物开发具有良好机械性能的超导材料。在本文中，第一性原理计算揭示了前所未有的四方五硼化物 MB5 系列（M = Na、K、Rb、Ca、Sr、Ba、Sc 和 Y）的存在，其中包含 B20 笼和充当稳定剂的中心金属原子和硼亚晶格的电子供体。这些化合物同时表现出超导性和高硬度，RbB5 的最大超导转变温度 Tc 为 18.6 K，KB5 在 1 atm 下的峰值维氏硬度 Hv 为 35.1 GPa。这些特性的结合在 KB5、RbB5、和BaB5，Tc值分别为~14.7、18.6和16.3 K，Hv值分别为~35.1、32.4和33.8 GPa。这里提出的结果表明，五硼化物可以为探索和设计新型超导材料提供框架，该材料在可达到的压力甚至环境条件下具有良好的硬度。