Pressure-induced nitrogen-rich GeN20 with pentazolate units

Polynitride compounds have recently attracted great attention because of their unique properties, including exotic nitrogen motifs and high energy densities. Among them, metal-bearing nitrogen-rich compounds with pentazolate anion (cyclo-N5-) are highly desirable in basic research and applications. In this article, we conduct a comprehensive simulation of the pressurized germanium-nitrogen system based on the strategy of introducing a small amount of metal into nitrogen and designing a previously unknown unconventional stoichiometric material, GeN20, being stable at 49 GPa through a first-principles structure search method. Strikingly, the cyclo-N5- anionic unit, acting as an energy storage carrier, is revealed in Ge(N5)4, the formation mechanism of which is attributed to strong covalent N-N bonds and charge transfer from Ge to N. Furthermore, the robust dynamic, mechanical, and thermal stabilities of Ge(N5)4 predict its feasibility of synthesis in the future. In addition, Ge(N5)4 has a relatively high energy density (4.1 kJ g-1), which is comparable to the energy density of TNT (4.2 kJ g-1). Remarkably, Ge(N5)4 has a high detonation pressure (619 kbar) and a high explosion velocity (11.42 km s-1), which are approximately three times the detonation pressure of TNT (190 kbar) and twice the explosion velocity of TNT (6.9 km s-1), respectively, and it produces a more environmentally friendly detonation product composition. Moreover, Ge(N5)4 hosts an indirect bandgap of 3.0 eV and exhibits optical absorption performance. These findings have valuable implications for the rational design and synthesis of novel multifunctional nitrogen-based materials.