Superhard conductive orthorhombic carbon polymorphs

Materials with superhard and conductive properties are valuable and have great potential applications in multifunctional devices under extreme conditions. Here we propose two carbon polymorphs with superior superhard and conductive properties via first-principle calculations. These two carbon phases, called Orth-C10 and Orth-C10’, contain 10 carbon atoms buckled through sp2-sp3 hybridized bonds in an orthorhombic unit cell with Pmmm and Pmm2 symmetry, which are energetically more stable than fullerene C60 at 0 GPa, and more favorable than graphite at the pressure above 34.4 and 45.4 GPa, respectively. More importantly, their Vickers hardness are as high as 62.2 and 59.9 GPa, respectively, close to the hardness of cubic boron nitride, the second-hardest material. Specially, Orth-C10’ owns ultra-high axial incompressibility even beyond that of diamond at a pressure above 40 GPa. Furthermore, both Orth-C10 and Orth-C10’ are metallic, which is rare in superhard carbon polymorphs. These outstanding natures make Orth-C10 and Orth-C10’ potential materials for electronic devices and mechanical tools.