Orthorhombic C36: a sp2–sp3 carbon with pressure-induced metallization and superconductivity

Because of their particular characteristics, searching for potential sp2–sp3 hybridized carbon allotropes has become the foundation of new functional materials in the future. Here, we design a novel carbon phase constructed by sp2–sp3 hybridized bonding networks with a 36-atom body-centered orthorhombic unit cell, termed oC36. oC36 shows thermodynamic stability comparable to most of the theoretically predicted sp3 carbons. The elastic and dynamical stability of oC36 is confirmed by the calculation results of elastic constants and phonon spectra. Analysis of electronic properties reveals that oC36 changes from semiconductivity to one-dimensional conductivity with external pressure exceeding approximately 4 GPa. The conductivity arises from the overlap of the valance and conduction bands owing to sp2 hybridized nonbonding carbon interactions as the lattice constant decreases. Electron–phonon coupling calculations indicate the superconductivity of oC36 under 25 GPa, and its λ and TC values are 0.50 and 10.7 K, respectively. Strikingly, the estimated hardness, tensile strength, and shear strength of oC36 reach remarkably high values of 69.5, 79.1, and 78.9 GPa, respectively, indicating its superhardness and ultra-incompressibility. Our research expands the scope of sp2–sp3 carbon allotropes and provides new ideas for the later theoretical design of carbon allotropes with switchable properties.