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Metal borides are of significant interest because of their high strength, high melting point, superconductivity and magnetism. Here, a combination of swarm-intelligence structural search and first-principle calculations propose a pressure-stabilized hitherto unknown manganese boride, MnB6, that is recoverable under ambient conditions and exhibits versatile properties. MnB6 contains unique B∞ ribbons along the c-axis, which are linked to each other in the a-b plane by B–B bonds to form open channels filled with Mn atoms. MnB6 is ferromagnetic at its onset energetically stable pressure of 127 GPa and undergoes a transition from magnetic to superconducting at 160 GPa, with an estimated superconducting critical temperature of 15.9 K. Under ambient pressure, MnB6 exhibits increased ferromagnetism with a magnetic moment of 1 μB per Mn atom and a Curie temperature as high as 520 K. Moreover, MnB6 possesses good mechanical properties with an estimated Vickers hardness of 25 GPa and high ductility, which stems from the peculiar structural arrangements in MnB6 driven by the ability of boron to form unusually versatile bonding states. [ABSTRACT FROM AUTHOR]

Materials possessing both superhard and metallic properties are beneficial for the creation of multifunctional devices under extreme conditions. Here, we report the formation of a new metallic superhard boron nitride at high pressure with stoichiometry B2N3 through first-principles calculations and structure searching. At ambient pressure, B2N3 has layered structures (h-B2N3) consisting of hexagonal B4N4 layers intercalated by triply bonded N2 molecules. With the pressure increasing to ∼10 GPa, h-B2N3 transforms to a three-dimensional tetragonal structure (t-B2N3) with the formation of single N–N bonds. Calculations reveal that t-B2N3 can be recovered under ambient conditions in view of the dynamical, thermal and mechanical stability. Interestingly, t-B2N3 is proposed to be a superhard material with an estimated Vicker's hardness of ∼52 GPa by performing stress–strain calculations. More importantly, electronic calculations show unique two-dimensional metallicity in t-B2N3, which originates from the π orbitals of N–N bonds spreading in the ab plane. In addition, the energy density of ∼2.95 kJ g−1 makes t-B2N3 a potential high-energy density material. [ABSTRACT FROM AUTHOR]