A tetragonal phase of superhard $BC2N$

Based on first-principles calculation, we predict a tetragonal phase of $BC2N$ $(t-BC2N)$⁠, which matches the experiment well because the estimated Vickers hardness and the simulated X-ray diffraction (XRD) and Raman patterns are in excellent agreement with the experimental results besides the low total energy and formation energy. In particular, we acquire the underlying physics behind the apparent lack of the transverse optical mode in the measured Raman spectrum and find the interesting electronic structure transition among the polymorph of superhard $BC2N$ compounds.