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Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap., Funding Agencies|Army Research Office; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation [K22020-026, 211]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRC, the Swedish Research Council (VR) [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [201605156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; National Science Foundation.Earth SciencesNational Science Foundation (NSF) [EAR.1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DEAC02-06CH11357]; [W911NF-192-0172]

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap., Funding Agencies|Army Research Office; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation [K22020-026, 211]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRC, the Swedish Research Council (VR) [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [201605156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; National Science Foundation.Earth SciencesNational Science Foundation (NSF) [EAR.1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DEAC02-06CH11357]; [W911NF-192-0172]

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]

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]

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap., Funding Agencies|Army Research Office; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation [K22020-026, 211]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRC, the Swedish Research Council (VR) [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [201605156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; National Science Foundation.Earth SciencesNational Science Foundation (NSF) [EAR.1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DEAC02-06CH11357]; [W911NF-192-0172]

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]

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap., Funding Agencies|Army Research Office; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation [K22020-026, 211]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRC, the Swedish Research Council (VR) [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [201605156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; National Science Foundation.Earth SciencesNational Science Foundation (NSF) [EAR.1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DEAC02-06CH11357]; [W911NF-192-0172]

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nano-electronics. In this work we report the synthesis of nickel diazenide NiN2 which possesses atomic-thick layers comprised of Ni2N3 pentagons forming Cairo-type tessellation. The layers of NiN2 are weakly bonded with the calculated exfoliation energy of 0.72 J/m(2), which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (P4/mbm) and possesses significant anisotropy of elastic properties. The single-layer NiN2 is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN2 to be a precursor of a pentagonal 2D material with a tunable direct band gap., Funding Agencies|Army Research Office; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation [K22020-026, 211]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRC, the Swedish Research Council (VR) [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant [201605156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; National Science Foundation.Earth SciencesNational Science Foundation (NSF) [EAR.1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DEAC02-06CH11357]; [W911NF-192-0172]

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN4. A triclinic phase of beryllium tetranitride tr-BeN4 was synthesized from elements at similar to 85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated pi systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN4 layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions., Funding Agencies|Army Research Office; Deutsche Forschungsgemeinschaft (DFG)German Research Foundation (DFG) [DU 954-11/1, DU 393-9/2, DU 393-13/1]; Federal Ministry of Education and Research, Germany (BMBF)Federal Ministry of Education & Research (BMBF) [05K19WC1]; Alexander von Humboldt FoundationAlexander von Humboldt Foundation; Russian Science FoundationRussian Science Foundation (RSF) [18-12-00492]; Ministry of Science and Higher Education of the Russian Federation of Increase Competitiveness Program of NUST MISIS [K2-2020-026, 211]; Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant) [KAW-2018.0194]; Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; SeRCAgency for Science Technology & Research (ASTAR); Swedish Research Council (VR)Swedish Research Council [2019-05600]; Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) [2016-05156]; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-07213]; JTC-FLAGERA Project GRANSPORT; National Science Foundation-Earth SciencesNational Science Foundation (NSF) [EAR-1634415]; Department of Energy-GeosciencesUnited States Department of Energy (DOE) [DE-FG0294ER14466]; DOENNSAs Office of Experimental Sciences; DOE Office of ScienceUnited States Department of Energy (DOE) [DE-AC02-06CH11357]; [W911NF-19-2-0172]

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]

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]

The structural and chemical properties of the bi-molecular, hydrogen-bonded, nitrogen-rich energetic material triaminoguanidinium 1-methyl-5-nitriminotetrazolate C3H12N12O2 (TAG-MNT) have been investigated at room pressure and under high pressure isothermal compression using powder x-ray diffraction and Raman and infrared spectroscopy. A stiffening of the equation of state and concomitant structural relaxation between 6 and 14 GPa are found to correlate with Raman mode disappearances, frequency discontinuities, and changes in the pressure dependence of modes. These observations manifest the occurrence of a reversible martensitic structural transformation to a new crystalline phase. The onset and vanishing of Fermi resonance in the nitrimine group correlate with the stiffening of the equation of state and phase transition, suggesting a possible connection between these phenomena. Beyond 15 GPa, pressure induces irreversible chemical reactions, culminating in the formation of a polymeric phase by 60 GPa., Published in the Journal of Chemical Physics, v137 article ID 054501, 1 Aug 2012. Prepared in cooperation with Howard University, Washington, DC and the Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC. Sponsored in part by DoE DE-FC52-08NA28554, DE-FG02-94ER14466, DE-AC02-06CH11357, DE-AC02-98CH10886 and DE-FC03-03N00144.