RESUMO
First-principles calculations were carried out on recently synthesized Re2N and Re3N as well as hypothetical Tc and Mn nitrides. It is found that structure and covalent bonds play an important role in determining mechanical properties. Under a large strain along (0001)<1010> direction, Re2N undergoes a phase transformation with a slight increase in ideal shear strength. On the other hand, it is transformed into a phase with weaker mechanical properties, if the strain is along Re2 <1210> direction. Mn2N can be synthesized under moderate conditions due to its more negative formation energy. Re2N, Re3N, and Mn2 N show structure-related mechanical property under larger strains to ReB2 but exhibit much lower ideal strengths, which is attributed to the larger ionicity of cation-anion bond. Three-dimensional framework of strong covalent bonds is thus highly recommended to design superhard materials.
