Stabilization of hexazine rings in potassium polynitride at high pressure

Polynitrogen molecules are attractive for high-energy-density materials due to energy stored in nitrogen–nitrogen bonds; however, it remains challenging to find energy-efficient synthetic routes and stabilization mechanisms for these compounds. Direct synthesis from molecular dinitrogen requires overcoming large activation barriers and the reaction products are prone to inherent inhomogeneity. Here we report the synthesis of planar N62− hexazine dianions, stabilized in K2N6, from potassium azide (KN3) on laser heating in a diamond anvil cell at pressures above 45 GPa. The resulting K2N6, which exhibits a metallic lustre, remains metastable down to 20 GPa. Synchrotron X-ray diffraction and Raman spectroscopy were used to identify this material, through good agreement with the theoretically predicted structural, vibrational and electronic properties for K2N6. The N62− rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N20.75−-containing compound with the formula K3(N2)4 was formed instead. The planar hexazine dianion ring (N62–), which had previously been predicted to exist, has now been synthesized from potassium azide (KN3) under laser heating in a diamond anvil cell above 45 GPa; it remains metastable down to 20 GPa. By contrast, at 30 GPa an unusual N2-containing compound with the formula K3(N2)4 was produced.