Superhard three-dimensional carbon with metallic conductivity

There has been a long-standing search for 3D metallic, superhard carbon, stable under ambient conditions. Here, we report the discovery of a new 3D orthorhombic phase (denoted C14-diamond below) from first-principles calculations. This allotrope is metallic and superhard and is built from nano-layered sp3 carbon with the cubic diamond structure, connected by proper sp2-bonded ethene-type CC links. This unique configuration of sp2 carbons makes C14-diamond conductive and allows one-dimensional electronic conduction along the c-axis. The high hardness of C14-diamond is further confirmed by its higher density compared with other metallic carbon allotropes. It is dynamically stable and more favorable than most other theoretically predicted carbons in energy. Interestingly, the simulated x-ray diffraction pattern of C14-diamond is similar to that of the experimentally reported while yet unresolved carbon phase obtained by shock-compression of tetracyanoethylene (TCE) powder, indicating that it could be a potential candidate for this carbon phase. We further propose a possible transition process from TCE to C14-diamond, suggesting that conductive superhard material might be synthesized by this way. [Display omitted]