Polymerization of nitrogen in two theoretically predicted high-energy compounds ScN₆ and ScN₇ under modest pressure

Materials under high pressure usually exhibit unique chemical and physical properties. Polynitrogen compounds have received widespread attention as potential high energy density materials. This paper uses CALYPSO crystal structure prediction method to study the structures of ScN₆ and ScN₇ in 0–100 GPa. Theoretical calculations show that ScN₆ is thermodynamically stable above 80 GPa, while ScN₇ is thermodynamically stable from 30 GPa to 90 GPa. Furthermore, ScN₇ is metastable under ambient conditions, demonstrating that it can be quenched to ambient conditions after high pressure synthesis. The P 1 ¯ -ScN₆ is a three-dimensional extended fold multi-nitrogen network, and the P 1 ¯ -ScN₇ contains a five-membered ring and a curved N4 molecular unit. Both P 1 ¯ -ScN₆ and P 1 ¯ -ScN₇ contain a lot of N–N single bonds and N=N double bonds. The energy densities of P 1 ¯ -ScN₆ and P 1 ¯ -ScN₇ are 3.97 kJ g⁻¹ and 3.12 kJ g⁻¹, respectively. The detonation velocity and detonation pressure of the P 1 ¯ -ScN₆ phase and P 1 ¯ -ScN₇ phase are also higher than that of TNT. Excellent energy storage properties and detonation performance show that they can be used as potential high-energy materials. These results opened up a new way for the synthesis of nitrogen-rich compounds.