Superhard monoclinic BN allotrope in M-carbon structure

•Two superhard boron nitrides have been designed based on M-carbon structure.•Cm-BN-1 and Cm-BN-2 are dynamically stable and direct bandgap semiconductors.•Cm-BN-1 and Cm-BN-2 could be potential superhard materials. Superhard materials have always attracted people's interesting due to their extensive industrial applications. In this work, two reasonable superhard monoclinic allotropes of boron nitrides with space group of Cm have been designed based on previously proposed M-carbon structure using first-principles calculations. Our results show that Cm-BN-1 and Cm-BN-2 are dynamically stable, and they are direct semiconductors with bandgap of 2.69 and 3.90 eV, respectively. Moreover, they could be potential superhard materials with Vickers hardness of 58.0 and 60.4 GPa, respectively. This work provides insights for exploring new superhard boron nitrides materials.