High-pressure synthesis of ultraincompressible hard rhenium nitride pernitride Re-2(N-2)(N)(2) stable at ambient conditions

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K-0 = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re-2(N-2)(N)(2). Unlike known transition metals pernitrides Re-2(N-2)(N)(2) contains both pernitride N-2(4-) and discrete N3- anions, which explains its exceptional properties. Re-2(N-2)(N)(2) can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH4N3, in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re-2(N-2)(N)(2) turned to be at a threshold for superhard materials.Funding Agencies|Deutsche Forschungsgemeinschaft (DFG) [ME 4368/7-1]; Federal Ministry of Education and Research, Germany (BMBF) [5K16WC1]; Fonds der Chemischen Industrie (FCI), Germany; National Science Foundation-Earth Sciences [EAR-1634415]; Department of EnergyGeoSciences [DE-FG02-94ER14466]; DOE Office of Science by Argonne National Laboratory [DE-AC02-06CH11357]; Russian Science Foundation [18-12-00492]; Ministry of Science and High Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST "MISIS" [K2-2019-001, 211]; Swedish Research Council (VR) [2015-04391]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkcping University [2009-00971]; VINN Excellence Center Functional Nanoscale Materials [FunMat-2, 2016-05156]; European Research Council (ERC) [787527]