First-principles study of novel monoclinic BC2N polymorphs with distinct electronic and anisotropic mechanical properties

In this paper, two novel three-dimensional BC2N polymorphs with monoclinic symmetry, denoted as mBC2N-I and mBC2N-II, were predicted by first-principles calculations. Structurally, both phases consist of sp2-/sp3-hybridized graphite-like units assembled in monoclinic symmetry. Both structures are mechanically and dynamically stable at ambient pressure, while their formation enthalpies suggest possible thermodynamic accessibility under high pressure conditions. Electronic structure calculations reveal that mBC2N-I is a semiconductor with a direct bandgap of 0.90 eV, whereas mBC2N-II is metallic and exhibits a one-dimensional conduction channel mainly derived from sp2-hybridized C atoms. Mechanical calculations further show pronounced anisotropy in both phases, with the maximum tensile strengths reaching 109.1 and 108.3 GPa along the [010] direction for mBC2N-I and mBC2N-II, respectively. Although the predicted hardness values are below the superhard threshold, the distinct structural, electronic, and anisotropic mechanical features of these polymorphs suggest that they may be promising functional BC2N materials.