The effect of structure and phase transformation on the mechanical properties of Re₂N and the stability of Mn₂N.

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. 2011 Wiley Periodicals, Inc.