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.