Novel superhard materials, especially those with superior thermal and chemical stability, are needed to replace diamond. Carbon nitrides (C-N), which are likely to possess these charac- teristics and have even been expected to be harder than diamond, are excellent candidates. Here we report three new superhard and thermodynamically stable carbon nitride phases. Based on a systematic evolutionary structure searches, we report a complete phase diagram of the C-N system at 0-300 GPa and analyze the hardest metastable structures. Surprising- ly, we find that at zero pressure, the earlier proposed graphitic-C3N4 structure (P-6m2) is dynamically unstable and the lowest energy form of structures based on s-triazine unit and s-heptazine unit have similar topology and belong to the same space group.
@article{arxiv.1408.2901,
title = {The Phase Diagram and Hardness of Carbon Nitrides},
author = {Huafeng Dong and Artem R. Oganov and Qiang Zhu and Guang-Rui Qian},
journal= {arXiv preprint arXiv:1408.2901},
year = {2015}
}