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 [N-5(5-)](infinity). Apart from that we discover two novel Ta-N compounds: TaN4 with finite N-4(4-) chains and the incommensurately modulated compound TaN2-x, which is recoverable at ambient conditions.Funding Agencies|Ministry of Science and Higher Education of the Russian Federation [K2-2020-026]; SFO in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; DOE Office of ScienceUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; National Science Foundation - Earth SciencesNational Science Foundation (NSF) [EAR-1634415]; Department of Energy, GeosciencesUnited States Department of Energy (DOE) [DE-FG02-94ER14466]