Hexagonal BC₂N with Remarkably High Hardness

Pursuit of materials with desirable mechanical properties is an eternal theme. The reported BC 2 N 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 R 3 m BC 2 N 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 R 3 m BC 2 N are in good agreement with the experimental measurement ones. Moreover, R 3 m BC 2 N is more stable than the earlier proposed BC 2 N phases. These results indicate that our predicted BC 2 N is the most likely experimental candidate structure, awaiting experimental confirmation. R 3 m BC 2 N 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 sp 3 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 R 3 m BC 2 N has a large band gap of 5.0 eV. Our work is also important to fully understand the ternary B–C–N compounds.