Orthorhombic B₈: A Boron Allotrope with Coexisting Superhard and Superconducting Properties

Boron materials, particularly boron allotropes, have emerged as appealing potential hard and superconducting species for practical applications due to their abundant building blocks, robust chemical bonding patterns, and peculiar electron structures induced by an electron deficiency. However, the boron isomers that simultaneously exhibit superhardness above 40 GPa and superior superconductivity have yet to be recognized. Here, leveraging a first-principles intelligent structure prediction, we identify a novel bulk orthorhombic boron crystal with an eight-atom conventional cell, named o-B₈, featuring a boat-shaped boron motif consisting of triangular B₃ and quadrilateral B₄ and holding two- and four-center two-electron covalent bonds. Strikingly, o-B₈ exhibits a high Vickers hardness H_v of 48 GPa at atmospheric pressure (atm), attributed to the strong boron-boron covalent bonding. Further, o-B₈ has a superconducting transition temperature T_c of 18.2 K at 1 atm. Both electron and hole doping can facilitate the marked enhancement of superconductivity, with T_c increasing up to 27.1 (0.06 e/cell) and 21.8 K (1.0 h/cell), respectively, originating from the enhanced electron-phonon strength induced by low-frequency softened phonon modes. These findings provide significant implications for the design and experimental verification of multifunctional boron polymorphs and open up a new perspective for research on this important class of materials.