Computational Prediction of a Novel Superhard sp^3 Trigonal Carbon Allotrope with Bandgap Larger than Diamond

Searching for new carbon allotropes with superior properties has been a longstanding interest in material sciences and condensed matter physics. Here we identify a novel superhard carbon phase with an 18-atom trigonal unit cell in a full-sp^3 bonding network, termed tri-C_18 carbon, by first-principles calculations. Its structural stability has been verified by total energy, phonon spectra, elastic constants, and molecular dynamics simulations. Furthermore, tri-C_18 carbon has a high bulk modulus of 400 GPa and Vickers hardness of 79.0 GPa, comparable to those of diamond. Meanwhile, the simulated x-ray diffraction pattern of tri-C_18 carbon matches well with the previously unexplained diffraction peaks found in chimney soot, indicating the possible presence of tri-C_18 carbon. Remarkably, electronic band structure calculations reveal that tri-C_18 carbon has a wide indirect bandgap of 6.32 eV, larger than that of cubic diamond, indicating its great potential in electronic or optoelectronic devices working in the deep ultraviolet region.