A superhard sp2–sp3 hybridized orthorhombic carbon allotrope with conductive property under extreme pressure

Abstract

Superhard materials with conductive properties are extremely important. They have potential applications in multifunctional devices under extreme natural conditions. Here we present a superhard and conductive sp2–sp3 mixed hybrid carbon allotrope through Density functional theory calculations. The proposed carbon phase contains 36 atoms in an orthorhombic unit cell with Pmmm symmetry. In present structure (namely poC36), the sp2 bonds are wrapped around inside the sp3 bonded network. At 0 GPa it is dynamically stable and energetically more favourable than fullerene C60, graphene, Orth-C10, orth-C 10′ , oC36, C48, C20-sc, C21-sc, M-carbon, W-carbon etc. At 47.2 GPa pressure it is energy becomes lower than graphite. The Vickers hardness value is 76.32 GPa, which is higher than cubic boron nitride, the second hardest material. At 0 GPa it is an indirect band gap semi-conductor with band gap 0.08 eV. At around 11 GPa pressure, the valence band crosses the conduction band generating 1D conductivity in poC36. These, interesting features make poC36 a useful material for mechanical tools and electronic devices. The Raman spectra exhibits a diamond-like band, alongside graphite-like G and D bands, all of which undergo rightward shifts with increasing pressure, indicating structural changes. X-ray diffraction at 0 GPa resembles diamond, but at 47 GPa, four peaks vanish while six new ones emerge, signifying significant structural alterations under high pressure.