Activation of N≡N bonds by CN4 tetrahedron leading to energetic carbon polynitrides under high pressure

The activation of the N≡N triple bond in N2 is a fascinating topic in nitrogen chemistry. The transition metals have been demonstrated to effectively modulate the reactivity of N2 molecules under high pressure, leading to nitrogen-rich compounds. However, their use often results in a significant reduction in energy density. In this work, we propose a series of low-enthalpy nitrogen-rich phases in CNx (x = 3, …, 7) compounds using a first-principles crystal structure search method. The results of calculations reveal that all these CN compounds are assembled from both CN4 tetrahedra and Nx (x = 1, 2, or 5) species. Strikingly, we find that the CN4 tetrahedron can effectively activate the N≡N bond through weakening of the π orbital of N2 under a pressure of 40 GPa, leading to stable CN polynitrides. The robust structural framework of CN polynitrides containing C–N and N–N bonds plays a crucial role in enhancing their structural stability, energy density, and hardness. Among these polynitrides, CN6 possesses not only a very high mass density of 3.19 g/cm3, but also an ultrahigh energy density of 28.94 kJ/cm3, which represents a significant advance in the development of energetic materials using high-pressure methods. This work provides new insights into the mechanism of N2 activation under high pressure, and offers a promising pathway to realize high-performance energetic materials.