A superconducting boron allotrope featuring anticlinal pentapyramids

Abstract

Finding allotropes with novel structures and intriguing properties is of great interest from both fundamental and applicable standpoints. Here, we propose a hitherto unknown boron allotrope, possessing intrinsic superconductivity and hardness at ambient pressure. With the aid of swarm-intelligence structure search calculations, stable boron-rich Sr–B binary compounds at high pressures were explored. In addition to the known SrB₆, an orthogonal compound, SrB₈, which is stable above 134.6 GPa was found. Its remarkable structural feature is that it contains a channel-like boron covalent framework constituted of anticlinal pentapyramids. More interestingly, the removal of Sr atoms, located at the center of the channel, leads to the formation of a novel o-B₁₆ boron allotrope at ambient pressure. It is metallic with a superconducting transition temperature of 14.2 K. In addition, o-B₁₆ demonstrates an inherently hard nature with an estimated Vickers hardness of 19.4 GPa.