Thermodynamic stability and unusual strength of ultra-incompressible rhenium nitrides

We report on a comprehensive study of thermodynamic and mechanical properties as well as a bond-deformation mechanism on ultra-incompressible Re₂N and Re₃N. The introduction of nitrogen into the rhenium lattice leads to thermodynamic instability in Re₂N at ambient conditions and enhanced incompressibility and strength for both rhenium nitrides. Rhenium nitrides, however, show substantially lower ideal shear strength than hard ReB₂ and superhard c-BN, suggesting that they cannot be intrinsically superhard. An intriguing soft "ionic bond mediated plastic deformation" mechanism is revealed to underline the physical origin of their unusual mechanical strength. These results suggest a need to reformulate the design concept of intrinsically superhard transition-metal nitrides, borides, and carbides.