Stabilization of hexazine and pentazolate rings in polynitrides of alkali metals at high pressure

Synthesis and characterization of polynitrogen species is of great importance for the design of novel high energy density materials as these compounds possess extremely energetic low-order nitrogen–nitrogen bonds. However, synthesis and stabilization of such compounds at ambient conditions remain a big challenge because they often require very high pressure to synthesize them. Here we report the synthesis of polynitrogen planar N₆^2- hexazine dianions, stabilized in K₂N₆, and several oxygen-free cyclo-N₅^- pentazolate salts stabilized in K and Na compounds, some of which are also containing slightly elongated dinitrogens N₂ [1,2]. The experiments have been performed in a laser heated diamond anvil cell above 45 GPa on NaN₃ and KN₃ azides using also N₂ as a medium and reagent in some runs. Synchrotron single-crystal and powder X-ray diffraction and Raman spectroscopy were used to identify this material, which have also distinct appearance and optical properties. The hexazine and pentazolate compounds documented here remain metastable down to approximately 20 GPa, suggesting that some of them may be metastable at low temperature.  The nitrogen ring compounds are likely to be present in many other high energy density materials stabilized by pressure.