Prediction of superhard C1+xN1−x compounds with metal-free magnetism and narrow band gaps

Abstract

The scarcity of superhard materials with magnetism or a narrow band gap, despite their potential applications in various fields, makes it desirable to design such materials. Here, a series of C_1+xN_1−x compounds are theoretically designed by replacing different numbers of nitrogen atoms with carbon atoms in the synthesized C₁N₁ compound. The results indicate that the compounds C₅N₃ and C₇N₁ possess both superhardness and antiferromagnetic ordering due to the introduction of low-coordinated carbon atoms. The hardness of the two compounds is about 40.3 and 54.5 GPa, respectively. The magnetism in both compounds is attributed to the unpaired electrons in low-coordinated carbon atoms, and the magnetic moments are 0.42 and 0.39 μ_B, respectively. Interestingly, the magnetism in C₅N₃ remains unaffected by the external pressure used in this study, whereas C₇N₁ becomes nonmagnetic when the pressure exceeds ∼80 GPa. Electronic calculations reveal that both compounds behave as indirect band gap semiconductors, with narrow energy gaps of about 0.30 and 0.20 eV, respectively. Additionally, the other two compounds, C₆N₂-I and C₆N₂-III, exhibit nonmagnetic ordering and possess hardness values of 52.6 and 35.0 GPa, respectively. C₆N₂-I behaves as a semiconductor with an energy gap of 0.79 eV, and C₆N₂-III shows metallic behavior. Notably, the energy gaps of C₅N₃ and C₆N₂-I remain nearly constant under arbitrary pressure due to their porous and superhard structure. These compounds fill the gap in magnetic or narrow band gap superhard materials, and they can be used in the spintronic or optoelectronic fields where conventional superhard materials are not suitable.