A stable superatomic nitrogen ring crystal under ambient conditions.

The nitrogen ring structure represented by the pentazolate anion (N5-) has received widespread attention due to its potential to form high-energy-density materials. However, the inherent high-energy properties of nitrogen rings make obtaining their stable crystals under ambient conditions a recognized key challenge. Here, we report a stable polynitrogen crystal Li4(N5)2 under ambient conditions by first-principles structural and dynamic studies. In its primitive cell, electron transfer from lithium (Li) atoms to N5 rings occupies their delocalized orbitals, forming anionic N5- in a superatomic state, where Li-N5 ionic interactions and N5-N5 van der Waals interactions jointly maintain the structural stability. These interactions result in structure anisotropy, where the ab-plane direction is governed by both ionic and covalent interactions, while the c-axis direction is dominated by ionic interactions. Our findings provide an important foundation for the development of high-energy-density materials based on nitrogen rings and for related artificial unit crystallization pathways.