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

Because of their particular characteristics, searching for potential sp 2 – sp 3 hybridized carbon allotropes has become the foundation of new functional materials in the future. Here, we design a novel carbon phase constructed by sp 2 –sp 3 hybridized bonding networks with a 36-atom body-centered orthorhombic unit cell, termed o C 36 . o C 36 shows thermodynamic stability comparable to most of the theoretically predicted sp 3 carbons. The elastic and dynamical stability of o C 36 is confirmed by the calculation results of elastic constants and phonon spectra. Analysis of electronic properties reveals that o C 36 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 sp 2 hybridized nonbonding carbon interactions as the lattice constant decreases. Electron–phonon coupling calculations indicate the superconductivity of o C 36 under 25 GPa, and its λ and T C values are 0.50 and 10.7 K, respectively. Strikingly, the estimated hardness, tensile strength, and shear strength of o C 36 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 sp 2 –sp 3 carbon allotropes and provides new ideas for the later theoretical design of carbon allotropes with switchable properties.