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In this study, we conduct extensive high-pressure experiments to investigate phase stability in the cobalt-nitrogen system. Through a combination of synthesis in a laser-heated diamond anvil cell, first-principles calculations, Raman spectroscopy, and single-crystal X-ray diffraction, we establish the stability fields of known high-pressure phases, hexagonal NiAs-type CoN, and marcasite-type CoN 2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co 3 N 2 , Pnma-CoN and two polynitrides, CoN 3 and CoN 5 , within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN 3 feature branched-type nitrogen trimers (N 3 ) and CoN 5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN 6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN 2 and CoN 3 ), and finally to isolated (in CoN 5 ). Our study provides insights into the intricate interplay between structure evolution, bonding arrangements, and high-pressure synthesis in polynitrides, expanding the knowledge for the development of advanced energy materials., (© 2024 Wiley-VCH GmbH.)

Carbon nitrides featuring three-dimensional frameworks of CN 4 tetrahedra are one of the great aspirations of materials science, expected to have a hardness greater than or comparable to diamond. After more than three decades of efforts to synthesize them, no unambiguous evidence of their existence has been delivered. Here, the high-pressure high-temperature synthesis of three carbon-nitrogen compounds, tI14-C 3 N 4 , hP126-C 3 N 4 , and tI24-CN 2 , in laser-heated diamond anvil cells, is reported. Their structures are solved and refined using synchrotron single-crystal X-ray diffraction. Physical properties investigations show that these strongly covalently bonded materials, ultra-incompressible and superhard, also possess high energy density, piezoelectric, and photoluminescence properties. The novel carbon nitrides are unique among high-pressure materials, as being produced above 100 GPa they are recoverable in air at ambient conditions., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

A series of isostructural Ln 3 O 2 (CN 3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates was synthesized under high-pressure (25-54 GPa) high-temperature (2000-3000 K) conditions in laser-heated diamond anvil cells. The crystal structure of this novel class of compounds was determined via synchrotron single-crystal X-ray diffraction (SCXRD) as well as corroborated by X-ray absorption near edge structure (XANES) measurements and density functional theory (DFT) calculations. The Ln 3 O 2 (CN 3 ) solids are composed of the hitherto unknown CN 3 5- guanidinate anion-deprotonated guanidine. Changes in unit cell volumes and compressibility of Ln 3 O 2 (CN 3 ) (Ln=La, Eu, Gd, Tb, Ho, Yb) compounds are found to be dictated by the lanthanide contraction phenomenon. Decompression experiments show that Ln 3 O 2 (CN 3 ) compounds are recoverable to ambient conditions. The stabilization of the CN 3 5- guanidinate anion at ambient conditions provides new opportunities in inorganic and organic synthetic chemistry., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The stabilization of nitrogen-rich phases presents a significant chemical challenge due to the inherent stability of the dinitrogen molecule. This stabilization can be achieved by utilizing strong covalent bonds in complex anions with carbon, such as cyanide CN - and NCN 2- carbodiimide, while more nitrogen-rich carbonitrides are hitherto unknown. Following a rational chemical design approach, we synthesized antimony guanidinate SbCN 3 at pressures of 32-38 GPa using various synthetic routes in laser-heated diamond anvil cells. SbCN 3 , which is isostructural to calcite CaCO 3 , can be recovered under ambient conditions. Its structure contains the previously elusive guanidinate anion [CN 3 ] 5- , marking a fundamental milestone in carbonitride chemistry. The crystal structure of SbCN 3 was solved and refined from synchrotron single-crystal X-ray diffraction data and was fully corroborated by theoretical calculations, which also predict that SbCN 3 has a direct band gap with the value of 2.20 eV. This study opens a straightforward route to the entire new family of inorganic nitridocarbonates., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Non-metal nitrides are an exciting field of chemistry, featuring a significant number of compounds that can possess outstanding material properties. These properties mainly rely on maximizing the number of strong covalent bonds, with crosslinked XN 6 octahedra frameworks being particularly attractive. In this study, the phosphorus-nitrogen system was studied up to 137 GPa in laser-heated diamond anvil cells, and three previously unobserved phases were synthesized and characterized by single-crystal X-ray diffraction, Raman spectroscopy measurements and density functional theory calculations. δ-P 3 N 5 and PN 2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN 6 units. The two compounds are ultra-incompressible, having a bulk modulus of K 0 =322 GPa for δ-P 3 N 5 and 339 GPa for PN 2 . Upon decompression below 7 GPa, δ-P 3 N 5 undergoes a transformation into a novel α'-P 3 N 5 solid, stable at ambient conditions, that has a unique structure type based on PN 4 tetrahedra. The formation of α'-P 3 N 5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Invited for the cover of this issue are Dominique Laniel (University of Edinburgh), Florian Trybel (University of Linköping), and their colleagues. The image depicts a bridge built of the newly discovered δ-P 3 N 5 solid with the structure featuring PN 6 units, a previously missing connection between the carbon group elements nitrides and chalcogens nitrides. Read the full text of the article at 10.1002/chem.202201998., (© 2022 Wiley-VCH GmbH.)

Two novel yttrium nitrides, YN 6 and Y 2 N 11 , were synthesized by direct reaction between yttrium and nitrogen at 100 GPa and 3000 K in a laser-heated diamond anvil cell. High-pressure synchrotron single-crystal X-ray diffraction revealed that the crystal structures of YN 6 and Y 2 N 11 feature a unique organization of nitrogen atoms-a previously unknown anionic N 18 macrocycle and a polynitrogen double helix, respectively. Density functional theory calculations, confirming the dynamical stability of the YN 6 and Y 2 N 11 compounds, show an anion-driven metallicity, explaining the unusual bond orders in the polynitrogen units. As the charge state of the polynitrogen double helix in Y 2 N 11 is different from that previously found in Hf 2 N 11 and because N 18 macrocycles have never been predicted or observed, their discovery significantly extends the chemistry of polynitrides., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf 4 N 20 ⋅N 2 , WN 8 ⋅N 2 , and Os 5 N 28 ⋅3 N 2 via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf 4 N 20 , WN 8 , and Os 5 N 28 ) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N 2 also leads to a non-centrosymmetric polynitride Hf 2 N 11 that features double-helix catena-poly[tetraz-1-ene-1,4-diyl] nitrogen chains [-N-N-N=N-] ∞ ., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

A nitrogen-rich compound, ReN 8 ⋅x N 2 , was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN 8 framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN 8 ⋅x N 2 is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] ∞ that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN 8 ⋅x N 2 provide strong support for the experimental results and conclusions., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)