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 sp$^{2}$-sp$^{3}$ mixed hybrid carbon allotrope
through Density functional theory calculations. The proposed carbon phase contains 36 atoms in an orthorhombic unit cell with
\emph{Pmmm} symmetry. In present structure (namely poC$_{36}$), the sp$^{2}$ bonds are wrapped around inside the sp$^{3}$ bonded network. At 0 GPa it is dynamically stable and energetically more favourable than fullerene C$_{60}$, graphene, Orth-C$_{10}$, 
orth-C$^{`}_{10}$, oC$_{36}$, C$_{48}$, C20-sc, C21-sc, M-carbon, W-carbon etc. At 47.2 GPa pressure it's 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 1-D conductivity in poC$_{36}$. These, interesting features make poC$_{36}$ 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.