Pentadiamond-like Metallic Hard Carbon Nitride

The design and synthesis of superhard carbon nitrides have always been a focus of attention. Recently, an experiment has revealed the existence of the previously predicted three-dimensional strongly covalent carbon-rich Pnnm-CN structure with high incompressibility. This discovery promotes the study of new carbon-rich C–N compounds with various stoichiometric ratios. In this paper, we investigated the electronic and mechanical properties of carbon-rich C–N compounds with the structural analogues of a very recently proposed pentadiamond. The carbon-rich C10N and C19N3were constructed by replacing the carbon atoms of the pentadiamond with nitride atoms at specific positions. The elastic and dynamic stabilities were confirmed by the calculations of the elastic constants and the phonon spectra. The C19N3possessed a bulk modulus of 257.6 GPa, a shear modulus of 168.4 GPa, a Young’s modulus of 414.8 GPa, and a hardness of 21.4 GPa, which were similar to the properties of the C10N. The analyses of the band structure and the density of states showed that the C10N and the C19N3were both metallic, completely different from the semiconducting pentadiamond. The detailed electron orbital analysis showed that the C10N and C19N3had different conductive behaviors due to the different positions of nitrogen atoms in the structures. In addition, our research expands the scope of C–N compounds and provides new ideas for the later theoretical design of carbon-rich C–N compounds.