Hexagonal Bc2n With Remarkably High Hardness

Pursuit of materials with desirable mechanical properties is an eternal theme. The reported BC2N exhibiting high hardness and superior stability is deemed as the potential alternative to diamond. However, its crystal structure has not been unambiguously determined thus far. Here, we identify a hitherto unknown R3m BC2N with the second high hardness value of 71 GPa through first-principles swarm structure calculations. Intriguingly, the simulated X-ray diffraction patterns, K-edge spectra, and hardness value of R3m BC2N are in good agreement with the experimental measurement ones. Moreover, R3m BC2N is more stable than the earlier proposed BC2N phases. These results indicate that our predicted BC2N is the most 15 likely experimental candidate structure, awaiting experimental confirmation. R3m BC2N shows interesting structural features such as the honeycomb C sublattice and the hexagonal B-N layer. Its unusual hardness can be attributed to the full spa bonding character and the higher number of C-C and B-N bonds compared to C N and B-C. Electronic property analysis shows that R3m BC2N has a large band gap of 5.0 eV. Our work is also important to fully understand the ternary B-C-N compounds.