Stabilization of polynitrogen anions in tantalum‐nitrogen compounds at high pressure

The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN5 via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN5 is the first example of a material containing branched all-single-bonded nitrogen chains [N55- ]∞ . Apart from that we discover two novel Ta-N compounds: TaN4 with finite N44- chains and the incommensurately modulated compound TaN2-x , which is recoverable at ambient conditions.