Is carbon nitride harder than diamond? No, but its girth increases when stretched (negative Poisson ratio)

It has been believed that C3N4 ceramic would be harder than diamond. Predictions are reported of the mechanical properties based on a force field derived from ab initio quantum mechanics. The predicted structure for α-C3N4 is in good agreement with transmission electron diffraction (TED) studies, supporting the identity of the films as α-C3N4. The bulk modulus of α-C3N4 is calculated to be 189 GPa, significantly less than diamond (436 GPa calculated, 442 GPa experimental). However, α-C3N4 is predicted to have a negative Poisson ratio. This may lead to interesting applications.