Fe-N system at high pressure reveals a compound featuring polymeric nitrogen chains

All-Single Bonds Fused N18 Macro-Rings and N8 Cagelike Building Blocks Stabilized in Lanthanum Supernitrides

Single-bonded polymeric nitrogen has gained tremendous research interest because of its unique physical properties and great potential applications. Despite much progress in theoretical predictions, it is still challenging to experimentally synthesize polynitrogen compounds with novel all-single-bonded units. Herein, we have synthesized two brand-new lanthanum supernitrides LaN8, through a direct reaction between La and N2 in laser-heated diamond anvil cells at megabar pressures. Our experiments and calculations revealed that two LaN8 phases had the R-3 and P4/n symmetry characterized by a unique 2D network with N18 macro-rings and cagelike N8 building blocks, respectively. Differing from known polynitrogen structures, these two polymers were composed of single-bonded nitrogen atoms belonging to sp3 and sp2 hybridizations. In particular, P4/n LaN8 possessed the longest N-N bond length among all of the experimentally reported metal nitrides, potentially being a high-energy-density material. The present study opens a fresh, promising avenue for the rational design and discovery of new supernitrides with unique nitrogen structures via the high-pressure treatment.

Synthesis of LaCN3, TbCN3, CeCN5, and TbCN5 Polycarbonitrides at Megabar Pressures

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions encompass important classes of solids such as cyanides and carbodiimides, well known at ambient conditions and composed of [CN]- and [CN2]2- anions, as well as the high-pressure formed guanidinates featuring [CN3]5- anion. At still higher pressures, carbon is expected to be 4-fold coordinated by nitrogen atoms, but hitherto, such CN4-built anions are missing. In this study, four polycarbonitride compounds (LaCN3, TbCN3, CeCN5, and TbCN5) are synthesized in laser-heated diamond anvil cells at pressures between 90 and 111 GPa. Synchrotron single-crystal X-ray diffraction (SCXRD) reveals that their crystal structures are built of a previously unobserved anionic single-bonded carbon-nitrogen three-dimensional (3D) framework consisting of CN4 tetrahedra connected via di- or oligo-nitrogen linkers. A crystal-chemical analysis demonstrates that these polycarbonitride compounds have similarities to lanthanide silicon phosphides. Decompression experiments reveal the existence of LaCN3 and CeCN5 compounds over a very large pressure range. Density functional theory (DFT) supports these discoveries and provides further insight into the stability and physical properties of the synthesized compounds.

High-pressure synthesis of Ruddlesden-Popper nitrides

Layered perovskites with Ruddlesden-Popper-type structures are fundamentally important for low-dimensional properties, for example, photovoltaic hybrid iodides and superconducting copper oxides. Many such halides and oxides are known, but analogous nitrides are difficult to stabilize due to the high cation oxidation states required to balance the anion charges. Here we report the high-pressure synthesis of three single-layer Ruddlesden-Popper (K2NiF4 type) nitrides-Pr2ReN4, Nd2ReN4 and Ce2TaN4-along with their structural characterization and properties. The R2ReN4 materials (R = Pr and Nd) are metallic, and Nd2ReN4 has a ferromagnetic Nd3+ spin order below 15 K. Thermal decomposition gives R2ReN3 with a Peierls-type distortion and chains of Re-Re multiply bonded dimers. Ce2TaN4 has a structural transition driven by octahedral tilting, with local distortions and canted magnetic Ce3+ order evidencing two-dimensional Ce3+/Ce4+ charge ordering correlations. Our work demonstrates that Ruddlesden-Popper nitrides with varied structural, electronic and magnetic properties can be prepared from high-pressure synthesis, opening the door to related layered nitride materials.

One-Dimensional Non-coplanar Nitrogen Chains in Manganese Tetranitride under High Pressure

Transition metal nitrides have great potential applications as incompressible and high energy density materials. Various polymeric nitrogen structures significantly affect their properties, contributing to their complex bonding modes and coordination conditions. Herein, we first report a new manganese polynitride MnN4 with bifacial trans-cis [N4]n chains by treating with high-pressure and high-temperature conditions in a diamond anvil cell. Our experiments reveal that MnN4 has a P-1 symmetry and could stabilize in the pressure range of 56-127 GPa. Detailed pressure-volume data and calculations of this phase indicate that MnN4 is a potential hard (255 GPa) and high energy density (2.97 kJ/g) material. The asymmetric interactions impel N1 and N4 atoms to hybridize to sp2-3, which causes distortions of [N4]n chains. This work discovers a new polynitride material, fills the gap for the study of manganese polynitride under high pressure, and offers some new insights into the formation of polymeric nitrogen structures.

High-pressure Synthesis of Cobalt Polynitrides: Unveiling Intriguing Crystal Structures and Nitridation Behavior

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 CoN2 within the pressure range of 50-90 GPa. We synthesize and characterize previously unknown nitrides, Co3N2, Pnma-CoN and two polynitrides, CoN3 and CoN5, within the pressure range of 90-120 GPa. Both polynitrides exhibit novel types of polymeric nitrogen chains and networks. CoN3 feature branched-type nitrogen trimers (N3) and CoN5 show π-bonded nitrogen chain. As the nitrogen content in the cobalt nitride increases, the CoN6 polyhedral frameworks transit from face-sharing (in CoN) to edge-sharing (in CoN2 and CoN3), and finally to isolated (in CoN5). 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.

Stabilization of N6 and N8 anionic units and 2D polynitrogen layers in high-pressure scandium polynitrides

Nitrogen catenation under high pressure leads to the formation of polynitrogen compounds with potentially unique properties. The exploration of the entire spectrum of poly- and oligo-nitrogen moieties is still in its earliest stages. Here, we report on four novel scandium nitrides, Sc2N6, Sc2N8, ScN5, and Sc4N3, synthesized by direct reaction between yttrium and nitrogen at 78-125 GPa and 2500 K in laser-heated diamond anvil cells. High-pressure synchrotron single-crystal X-ray diffraction reveals that in the crystal structures of the nitrogen-rich Sc2N6, Sc2N8, and ScN5 phases nitrogen is catenated forming previously unknown N66- and N86- units and ∞ 2 ( N 5 3 - ) anionic corrugated 2D-polynitrogen layers consisting of fused N12 rings. Density functional theory calculations, confirming the dynamical stability of the synthesized compounds, show that Sc2N6 and Sc2N8 possess an anion-driven metallicity, while ScN5 is an indirect semiconductor. Sc2N6, Sc2N8, and ScN5 solids are promising high-energy-density materials with calculated volumetric energy density, detonation velocity, and detonation pressure higher than those of TNT.

Nonbonding Electron Delocalization Stabilizes the Flexible N8 Molecular Assembly

Electron delocalization has an important impact on the physical properties of condensed materials. However, the L-electron delocalization in inorganic, especially nitrogen, compounds needs exploitation to improve the energy efficiency, safety, and environmental sustainability of high-energy-density materials (HEDMs). This Letter presents an intriguing N8 molecule, ingeniously utilizing nitrogen's L-electron delocalization. The molecule, exhibiting a unique lollipop-shaped conformation, can fold at various angles with very low energy barriers, self-assembling into environmentally stable, all-nitrogen crystals. These crystals demonstrate unparalleled stability, high energy density, low mechanical sensitivity, and optimal electronic thermal conductivity, outperforming existing HEDMs. The remarkable properties of these designed materials are attributed to two distinct delocalized systems within nitrogen's L-shell: π- and lone pair σ-electrons, which not only stabilize the molecular structure but also facilitate interconnected 3D networks of intermolecular nonbonding interactions. This work might pave the way to the experimental synthesis of environmentally stable all-nitrogen solids.

Lanthanium nitride LaN9 featuring azide units: the first metal nine-nitride as a high-energy-density material

High-pressure phase diagrams of the La-N binary system were systematically constructed using the CALYPSO method and first-principles calculations. In addition to the pressure-induced La-N compounds reported previously, we have uncovered a hitherto unknown LaN9 structure in Pm3̄ symmetry stabilized within a narrow pressure range of 20-24.5 GPa. Notably, LaN9 stands as the first thermodynamically stable metal nine-nitrogen compound, featuring centrosymmetric linear N3 anion units and an edge-sharing LaN12 icosahedron. Charge transfer between the La and N atoms plays a crucial role in facilitating structural stability. Furthermore, we identified a novel Cm phase for LaN8, which has a lower enthalpy compared to the previously reported phase. N atoms in Cm LaN8 are polymerized into infinite N∞ chains. Calculations demonstrate the potential recoverability of LaN9 and Cm LaN8 under atmospheric conditions while preserving their initial polynitrogen configuration. From the perspective of detonation pressure and detonation velocity, LaN9 and Cm LaN8 exhibit excellent explosive performance in comparison to TNT and HMX, with estimated energy densities of 0.9 and 1.54 kJ g-1, respectively, indicating their potential utility as high-energy-density materials.

High-Pressure Synthesis of a High-Pressure Phase of MnN Having NiAs-Type Structure

A novel high-pressure phase of manganese mononitride, NiAs-type MnN, was successfully synthesized through a pressure-induced phase transition from a tetragonal distorted NaCl-type MnN at pressures above approximately 55 GPa. High-pressure experiments, including starting material preparation, were conducted using a laser-heated diamond anvil cell. This result is the first example of a nitride with a structural phase transition from the distorted NaCl-type to the NiAs-type structure. Upon decompression after the phase transition to NiAs-type structure, the NiAs-type MnN underwent a structural change to the distorted NaCl-type phase, indicating the phase transition was reversible. NiAs-type MnN has a higher density and bulk modulus in comparison to the distorted NaCl-type one. The phase transition pressure of MnN is lower than that of oxides, such as FeO and MnO, which show a structural phase transition from a NaCl-type to a NiAs-type structure. It is suggested that this is due to the lattice distortion caused by antiferromagnetic ordering.

Stabilization of Guanidinate Anions [CN3 ]5- in Calcite-Type SbCN3

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 NCN2- carbodiimide, while more nitrogen-rich carbonitrides are hitherto unknown. Following a rational chemical design approach, we synthesized antimony guanidinate SbCN3 at pressures of 32-38 GPa using various synthetic routes in laser-heated diamond anvil cells. SbCN3 , which is isostructural to calcite CaCO3 , can be recovered under ambient conditions. Its structure contains the previously elusive guanidinate anion [CN3 ]5- , marking a fundamental milestone in carbonitride chemistry. The crystal structure of SbCN3 was solved and refined from synchrotron single-crystal X-ray diffraction data and was fully corroborated by theoretical calculations, which also predict that SbCN3 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.

Purely single-bonded spiral nitrogen chains stabilized by trivalent lanthanum ions

Inspired by the single-bonded nitrogen chains stabilized by tetravalent cerium, pentavalent tantalum, and hexavalent tungsten atoms, we explored the possibility of single-bonded nitrogen polymorphs stabilized by trivalent lanthanum ions. To achieve this, we utilized the crystal structure search method on the phase diagram of binary La-N compounds. We identified three novel thermodynamically stable phases, the C2/c LaN3, P-1 LaN4, and P-1 LaN8. Among them, the C2/c phase with infinite helical poly-N6 chains becomes thermodynamically stable above 50 GPa. Each nitrogen atom in the poly-N6 chain acquires one extra electron, and the spiral chain is purely single-bonded. The C2/c phase has an indirect band gap of ∼1.6 eV at 60 GPa. Notably, the band gap exhibits non-monotonic behavior, decreases first and then increases with increasing pressure. This abnormal behavior is attributed to the significant bonding of two La-N bonds at around 35 GPa. Phonon spectrum calculations and AIMD simulations have confirmed that the C2/c phase can be quenched to ambient conditions with slight distortion, and it exhibits excellent detonation properties. Additionally, we also discovered armchair-like nitrogen chains in LaN4 and the armchair and zigzag-like mixed nitrogen chains in LaN8. These results provide valuable insights into the electronic and bonding properties of nitrides under high pressure and may have important implications for the design and development of novel functional materials.

Pressure-induced novel ZrN4 semiconductor materials with high dielectric constants: a first-principles study

In addition to Zr3N4 and ZrN2 compounds, zirconium nitrides with a rich family of phases always exhibit metal phases. By employing an evolutionary algorithm approach and first-principles calculations, we predicted seven novel semiconductor phases for the ZrN4 system at 0-150 GPa. Through calculating phonon dispersions, we identified four dynamically stable semiconductor structures under ambient pressure, namely, α-P1̄, β-P1̄, γ-P1̄, and β-P1 (with bandgaps of 1.03 eV, 1.10 eV, 2.33 eV, and 1.49 eV calculated using the HSE06 hybrid density functional, respectively). The calculated work functions and dielectric functions show that the four dynamically stable semiconductor structures are all high dielectric constant (high-k) materials, among which the β-P1̄ phase has the largest static dielectric constant (3.9 times that of SiO2). Furthermore, we explored band structures using the HSE06 functional and density of states (DOS) and the response of bandgaps to pressure using the PBE functional for the four new semiconductor configurations. The results show that the bandgap responses of the four structures exhibit significant differences when hydrostatic pressure is applied from 0 to 150 GPa.

High-pressure synthesis of fully sp2-hybridized polymeric nitrogen layer in potassium supernitride

Searching for fully sp²-hybridized layered structures is of fundamental importance because of their fascinating physical properties and potential to host topologically non-trivial electronic states. However, the synthesis of fully sp²-hybridized layered polymeric nitrogen structures remains a challenging work because of their low stability. Here, we report the synthesis of a fully sp²-hybridized layered polymeric nitrogen structure featuring fused 18-membered rings in potassium supernitride (K₂N₁₆) under high-pressure and high-temperature conditions. Bader charge analysis reveals that the potassium atomic layer stabilizes the unique sp²-hybridized polymeric nitrogen layers through the charge transfer effect in K₂N₁₆. The calculation of electronic structure indicates that K₂N₁₆ is a topological semimetal with multiple Dirac points and hosts higher-order Dirac fermions with cubic dispersion, which are contributed by the sp²-hybridized polymeric nitrogen layers arranged in P6/mcc symmetry. The high-pressure synthesis of the fully sp²-hybridized polymeric nitrogen layered structure provides promising prospects for exploring novel topological materials with effective stabilization routes.

Predicted the structural diversity and electronic properties of Pt-N compounds under high pressure

The nitrogen-rich transition metal nitrides have attracted considerable attention due to their potential application as high energy density materials. Here, a systematic theoretical study of PtN_xcompounds has been performed by combining first-principles calculations and particle swarm-optimized structure search method at high pressure. The results indicate that several unconventional stoichiometries of PtN₂, PtN₄, PtN₅, and Pt₃N₄compounds are stabilized at moderate pressure of 50 GPa. Moreover, some of these structures are dynamically stable even when the pressure release to ambient pressure. TheP1-phase of PtN₄and theP1-phase of PtN₅can release about 1.23 kJ g^-1and 1.71 kJ g^-1, respectively, upon the decomposition into elemental Pt and N₂. The electronic structure analysis shows that all crystal structures are indirect band gap semiconductors, except for the metallic Pt₃N₄withPcphase, and the metallic Pt₃N₄is a superconductor with estimated critical temperatureT_cvalues of 3.6 K at 50 GPa. These findings not only enrich the understanding of transition metal platinum nitrides, but also provide valuable insights for the experimental exploration of multifunctional polynitrogen compounds.

Aromatic hexazine [N6]4- anion featured in the complex structure of the high-pressure potassium nitrogen compound K9N56

The recent high-pressure synthesis of pentazolates and the subsequent stabilization of the aromatic [N₅]^- anion at atmospheric pressure have had an immense impact on nitrogen chemistry. Other aromatic nitrogen species have also been actively sought, including the hexaazabenzene N₆ ring. Although a variety of configurations and geometries have been proposed based on ab initio calculations, one that stands out as a likely candidate is the aromatic hexazine anion [N₆]^4-. Here we present the synthesis of this species, realized in the high-pressure potassium nitrogen compound K₉N₅₆ formed at high pressures (46 and 61 GPa) and high temperature (estimated to be above 2,000 K) by direct reaction between nitrogen and KN₃ in a laser-heated diamond anvil cell. The complex structure of K₉N₅₆-composed of 520 atoms per unit cell-was solved based on synchrotron single-crystal X-ray diffraction and corroborated by density functional theory calculations. The observed hexazine anion [N₆]^4- is planar and proposed to be aromatic.

Iron Nitride Thin Films: Growth, Structure, and Properties

The current state-of-the-art in the growth, structure, and physicochemical properties of iron nitride thin films is presented. First, different iron nitride phases are introduced based on their crystallographic structure and the Fe-N phase diagram. Second, preparation methods for thin iron nitride films are described. Next, the structure, electronic, and magnetic properties of the films are discussed. Finally, potential applications of iron nitride films, as well as the challenges to be faced in the field, are highlighted. This Review constitutes a starting point for anyone who would like to conduct research on these fascinating materials, the scientific and technological potential of which has not been fully explored to date.

Stabilization of hexazine rings in potassium polynitride at high pressure

Polynitrogen molecules are attractive for high-energy-density materials due to energy stored in nitrogen-nitrogen bonds; however, it remains challenging to find energy-efficient synthetic routes and stabilization mechanisms for these compounds. Direct synthesis from molecular dinitrogen requires overcoming large activation barriers and the reaction products are prone to inherent inhomogeneity. Here we report the synthesis of planar N₆^2- hexazine dianions, stabilized in K₂N₆, from potassium azide (KN₃) on laser heating in a diamond anvil cell at pressures above 45 GPa. The resulting K₂N₆, which exhibits a metallic lustre, remains metastable down to 20 GPa. Synchrotron X-ray diffraction and Raman spectroscopy were used to identify this material, through good agreement with the theoretically predicted structural, vibrational and electronic properties for K₂N₆. The N₆^2- rings characterized here are likely to be present in other high-energy-density materials stabilized by pressure. Under 30 GPa, an unusual N₂^0.75--containing compound with the formula K₃(N₂)₄ was formed instead.

Novel polymerization of nitrogen in zinc nitrides at high pressures

Nitrogen-rich compounds containing polynitrogen are attractive candidates for high-energy-density materials. In this work, using first-principles calculations and a particle swarm optimization structural search method, four novel nitrogen-rich structures are predicted at high pressures, i.e., two ZnN₃phases with the same space groupP1 (low-pressure phase LP-ZnN₃and high-pressure phase HP-ZnN₃),Cmm2-ZnN₅andPcc2-ZnN₆, the energy density are estimated to be 1.41 kJ g^-1, 1.88 kJ g^-1, 4.07 kJ g^-1, and 2.60 kJ g^-1, respectively. LP-ZnN₃(54-72 GPa) and HP-ZnN₃(above 72 GPa) have the lowest enthalpies in all known ZnN₃phases, and the N₆chains in LP-ZnN₃polymerize into infinite nitrogen chains in HP-ZnN₃at 72 GPa, showing a narrow-band-gap-semiconductor to metallic phase transition. Interestingly,P1-ZnN₃has a superconducting transition temperature of 6.2 K at 50 GPa and 16.3 K at 100 GPa. InCmm2-ZnN₅andPcc2-ZnN₆, nitrogen atoms polymerize into three-dimensional network structures and network layers under high pressures. Those predicted structures may enrich the phase diagram of high-pressure zinc nitrides, and provide clues for synthesis and exploration of novel stable polymeric nitrogen.

High-pressure phases of a Mn-N system

Highly compressed extended states of light elemental solids have emerged recently as a novel group of energetic materials. The application of these materials is seriously limited by the energy-safety contradiction, because the material with high energy density is highly metastable and can hardly be recovered under ambient conditions. Recently, it has been found that high-energy density transition metal polynitrides could be synthesized at ∼100 GPa and recovered at ∼20 GPa. Inspired by these findings, we have studied a high-pressure Mn-N system from the aspects of structure, stability, phase transition, energy density and electronic structure theoretically for the first time. The results reveal that MnN₄_P1̄ consisting of [N₄]_∞^2- is thermodynamically stable at 36.9-100 GPa, dynamically stable at 0 GPa and has a noticeably high volumetric energy density of 15.71 kJ cm^-3. Upon decompression, this structure will transform to MnN₄_C2/m with the transition barrier declining sharply at 5-10 GPa due to the switching of transition pathways. Hence, we propose MnN₄_P1̄ as a potential energetic material that is synthesizable above 40 GPa and recoverable until 10 GPa.

Predicted stable high-pressure phases of copper-nitrogen compounds

The nitrogen-rich compounds are promising candidates for high-energy-density applications, owing to the large difference in the bonding energy between triple and single/double nitrogen bonds. The exploration of stable copper-nitrogen (Cu-N) compounds with high-energy-density has been challenging for a long time. Recently, through a combination of high temperatures and pressures, a new copper diazenide compound (P6₃/mmc-CuN₂) has been synthesized (Binnset al2019J. Phys. Chem. Lett.101109-1114). But the pressure-composition phase diagram of Cu-N compounds at different temperatures is still highly unclear. Here, by combining first-principles calculations with crystal structure prediction method, the Cu-N compounds with different stoichiometric ratios were searched within the pressure range of 0-150 GPa. Four Cu-N compounds are predicted to be thermodynamically stable at high pressures,Pnnm-CuN₂, two CuN₃compounds with theP-1 space group (named as I-CuN₃and II-CuN₃) andP2₁/m-CuN₅containing cyclo-N₅^-. Finite temperature effects (vibrational energies) play a key role in stabilizing experimentally synthesizedP6₃/mmc-CuN₂at ∼55 GPa, compared to our predictedPnnm-CuN₂. These new Cu-N compounds show great promise for potential applications as high-energy-density materials with the energy densities of 1.57-2.74 kJ g^-1.

The Effect of Physiological Incubation on the Properties of Elastic Magnetic Composites for Soft Biomedical Sensors

Magnetic micro- and nanoparticles (MPs)-based composite materials are widely used in various applications in electronics, biotechnology, and medicine. This group of silicone composites have advantageous magnetic and mechanical properties as well as sufficient flexibility and biocompatibility. These composites can be applied in medicine for biological sensing, drug delivery, tissue engineering, and as remote-controlled microrobots operating in vivo. In this work, the properties of polydimethylsiloxane (PDMS)-based composites with different percentages (30 wt.%, 50 wt.%, 70 wt.%) of NdFeB microparticles as a filler were characterized. The novelty of the work was to determine the influence of the percentage of MP content and physiological conditioning on the properties of the PDMS-MP composites after in vitro incubation. An important essence of the work was a comprehensive study of the properties of materials important from the point of view of medical applications. Materials were tested before and after conditioning in 0.9 wt.% NaCl solution at a temperature of 37 °C. Several studies were carried out, including thermal, physicochemical, and rheological tests. The results show that with an increase of the incubation time, most of the measured thermal and physicochemical parameters decreased. The presence of the magnetic filler, especially at a concentration of 70 wt.%, has a positive effect on thermal stability and physicochemical and rheological properties. The performed tests provided important results, which can lead to further research for a broader application of magnetic composites in the biomedical field.

Low-dimensional magnetism in calcium nitridonickelate(II) Ca2NiN2

Calcium nitridonickelate(II) Ca₂NiN₂ has been prepared through a high-temperature and high-pressure azide-mediated redox reaction, demonstrating that this method can stabilise nitrides of late transition metals in relatively high oxidation states. Ca₂NiN₂ crystallizes in the Na₂HgO₂ structure type and displays low-dimensional antiferromagnetic ordering of Ni²⁺ spins.

Preparation of Bulk-Phase Nitride Perovskite LaReN3 and Topotactic Reduction to LaNiO2 -Type LaReN2

While halide and oxide perovskites are numerous and many display outstanding properties, ABN₃ perovskite nitrides are extremely rare due to synthetic challenges arising from the low chemical potential of nitrogen and a tendency to form low-coordination nitridometallate anions. We report the preparation of a perovskite nitride LaReN₃ through azide-mediated oxidation at high pressure. High-resolution synchrotron diffraction shows that LaReN₃ has a low-symmetry, triclinic, perovskite superstructure resulting from orbital ordering with strong spin-orbit coupling distortions. Topotactic reduction of LaReN₃ above 500 °C leads to layered tetragonal LaReN₂ via a probable LaReN_2.5 intermediate, which is the first reported example of nitride defect perovskites. Magnetisation and conductivity measurements indicate that LaReN₃ and LaReN₂ are both metallic solids. The two chemical approaches presented are expected to lead to new classes of ABN₃ and defect ABN_3-x nitride perovskite materials.

High-Pressure Synthesis of the β-Zn3N2 Nitride and the α-ZnN4 and β-ZnN4 Polynitrogen Compounds

High-pressure nitrogen chemistry has expanded at a formidable rate over the past decade, unveiling the chemical richness of nitrogen. Here, the Zn-N system is investigated in laser-heated diamond anvil cells by synchrotron powder and single-crystal X-ray diffraction, revealing three hitherto unobserved nitrogen compounds: β-Zn₃N₂, α-ZnN₄, and β-ZnN₄, formed at 35.0, 63.5, and 81.7 GPa, respectively. Whereas β-Zn₃N₂ contains the N^3- nitride, both ZnN₄ solids are found to be composed of polyacetylene-like [N₄]_∞^2- chains. Upon the decompression of β-ZnN₄ below 72.7 GPa, a first-order displacive phase transition is observed from β-ZnN₄ to α-ZnN₄. The α-ZnN₄ phase is detected down to 11.0 GPa, at lower pressures decomposing into the known α-Zn₃N₂ (space group Ia3̅) and N₂. The equations of states of β-ZnN₄ and α-ZnN₄ are also determined, and their bulk moduli are found to be K₀ = 126(9) GPa and K₀ = 76(12) GPa, respectively. Density functional theory calculations were also performed and provide further insight into the Zn-N system. Moreover, comparing the Mg-N and Zn-N systems underlines the importance of minute chemical differences between metal cations in the resulting synthesized phases.

Cobalt-Nitrogen Compounds at High Pressure

The high-pressure phase diagram of Co-N compounds is enriched by proposing five stable phases (Pnnm-Co₂N, Pmn2₁-Co₂N, Pmna-CoN, Pnnm-CoN₂, and P1̅-CoN₄) and two metastable phases (P3̅1c-CoN₈ and P1̅-CoN₁₀). A systematic study has been performed for revealing the novel polymeric nitrogen structure and the outstanding properties of predicted polynitrides, such as structural characterization, energy analysis, stability analysis, and electronic analysis. P3̅1c-CoN₈ with the novel layer-shaped N-structure and P1̅-CoN₁₀ with the novel band-shaped N-structure are first reported in this work. Moreover, P3̅1c-CoN₈ (6.14 kJ/g) and P1̅-CoN₁₀ (5.18 kJ/g) with high energy density can be quenched down to ambient conditions. The proposed seven high-pressure phases are all metallic phases. A weak ionic bond interaction is observed between the Co and N atoms, while a strong N-N covalent bond interaction is observed in the Pnnm-CoN₂, P1̅-CoN₄, P3̅1c-CoN₈, and P1̅-CoN₁₀ phases. The N atoms in the polynitrides hybridize in the sp² state, for which the hybrid orbitals are constructed by the σ bond or lone electronic pair. The charge transfer between the Co and N atoms plays an important role to the structural stability. Moreover, the vibrational analysis of P3̅1c-CoN₈ and P1̅-CoN₁₀ phases is performed to guide the future experimental study.

Two-Dimensional Fe8N Nanosheets: Ferromagnets and Nitrogen Diffusion

We perform a first-principles study and identify two intriguing ferromagnets, hollow-Fe₈N (H-Fe₈N) and bridge-Fe₈N (B-Fe₈N) monolayers, by extensive structural searches. Both H-Fe₈N and B-Fe₈N nanosheets are buckled triangular lattices with a similar motif, but they are distinguishable by the positions of N atoms. The magnetic and electronic properties show that H-Fe₈N is a low-spin ferromagnet; in contrast, B-Fe₈N is a high-spin ferromagnet, which originates from the 3d orbital splitting of the Fe atom due to the low/high symmetric crystal field. Surprisingly, two stable Fe₈N monolayers can be transferred to each other by N atom diffusion from the bridge position to the hollow position with the migration energy barrier of 1.5 eV. The energy barrier is affected by introduced Fe defects and rare earth metal dopants. These findings introduce a new tactic to regulate the 2D Fe-nitride monolayers at the atomic scale.

Crystal and Electronic Structure of U7Te12-Type Tungsten Nitride Synthesized under High Pressure

Tungsten nitride continues to drive fundamental interests because of its potential as a functional compound, which combines features such as high hardness together with thermal, chemical, and wear resistance. Here, we report a novel tungsten nitride phase synthesized from MoC-type WN_0.6 and molecular nitrogen after laser irradiation at P = 70 GPa in a diamond anvil cell. This novel phase is quenchable at ambient pressure and determined to be U₇Te₁₂-type tungsten nitride and crystallizes in the hexagonal space group (P6) having lattice parameters of a = 8.2398(3) Å, c = 2.94948(14) Å, and V = 173.423(13) ų. Tungsten atoms are coordinated to eight and nine nitrogen atoms, higher than previously reported tungsten nitrides. The bulk modulus is determined to be K₀ = 312 (5) GPa (K₀^' = 4.0 fixed), and DFT calculations predict that U₇Te₁₂-type W₇N₁₂ has a metallic nature.

In situ N K-edge XANES study of iron, cobalt and nickel nitride thin films

A prototype in situ X-ray absorption near-edge structure (XANES) system was developed to explore its sensitivity for ultra-thin films of iron-nitride (Fe-N), cobalt-nitride (Co-N) and nickel-nitride (Ni-N). They were grown using DC-magnetron sputtering in the presence of an N₂ plasma atmosphere at the experimental station of the soft XAS beamline BL01 (Indus-2, RRCAT, India). XANES measurements were performed at the N K-edge in all three cases. It was found that the N K-edge spectral shape and intensity are greatly affected by increasing thickness and appear to be highly sensitive, especially in low-thickness regions. From a certain thickness of ∼1000 Å, however, samples exhibit a bulk-like behavior. On the basis of the obtained results, different growth stages were identified. Furthermore, the presence of a molecular N₂ component in the ultra-thin regime (<100 Å) was also obtained in all three cases studied in this work. In essence, this prototype in situ system reveals that N K-edge XANES is a powerful technique for studying ultra-thin films, and the development of a dedicated in situ system can be effective in probing several phenomena that remain hitherto unexplored in such types of transition metal nitride thin films.

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN2: High-Pressure Route to Pentagonal 2D Materials

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 nanoelectronics. 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/m², 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.

Synthesis of Highly Energetic PolyNitrogen by Nanosecond-Pulsed Plasma in Liquid Nitrogen

We report on an experimental study of nanosecond-pulsed plasma treatment of liquid nitrogen demonstrating synthesis of a highly energetic nitrogen material. Raman, FTIR analysis of gas phase products of decomposition, and the material explosion characteristics suggest synthesis of polymeric (amorphous) nitrogen compound which is stable at ambient pressure up to temperatures of about −150 °C. Addition of adsorbents with relatively large characteristic pore sizes (>5 nm) allows marginally improved recovery of the material as determined by temperature-dependent Raman measurements. By analyzing the shock wave propagation resulting from the explosions, we estimated the energy density of the material to be 13.3 ± 3.5 kJ/g, close to the previously predicted value for amorphous polymeric nitrogen.

Advanced Inorganic Nitride Nanomaterials for Renewable Energy: A Mini Review of Synthesis Methods

Inorganic nitride nanomaterials have attracted widespread attention for applications in renewable energy due to novel electrochemical activities and high chemical stabilities. For different renewable energy applications, there are many possibilities and uncertainties about the optimal nitride phases and nanostructures, which further promotes the exploration of controllable preparation of nitride nanomaterials. Moreover, unlike conventional nitrides with bulk or ceramic structures, the synthesis of nitride nanomaterials needs more accurate control to guarantee the target nanostructure along with the phase purity, which make the whole synthesis still a challenge to achieve. In this mini review, we mainly summarize the synthesis methods for inorganic nitride nanomaterials, including chemistry vapor deposition, self-propagation high-temperature synthesis, solid state metathesis reactions, solvothermal synthesis, etc. From the perspective of nanostructure, several novel nitrides, with nanostructures like nanoporous, two-dimensional, defects, ternary structures, and quantum dots, are showing unique properties and getting extensive attentions, recently. Prospects of future research in design and synthesis of functional inorganic nitrides are also discussed.

Polymerization of Nitrogen in Nitrogen-Fluorine Compounds under Pressure

A wide range of polynitrogen species have attracted much attention because of their potential applications as high-energy-density materials. Until now, predicted polynitrogen was found to be negatively charged, with charge transfer from introduced atoms to nitrogen in nitrogen-bearing compounds. Using an evolutionary algorithm combined with first-principles calculations, stoichiometries and structures in nitrogen-fluorine compounds at pressures ranging from 0 to 200 GPa are investigated. In addition to two fluorine-rich compounds NF3 and NF5, two other compounds, NF and N6F, emerge with increasing pressure. N6F, as a nitrogen-rich compound, will become stable at pressures greater than 180 GPa with a positively charged nitrogen network. Above 120 GPa, the NF compound with polymeric zigzag nitrogen chains is discovered, and it is quenchable to the ambient conditions, acquiring the highest energy density of 5.38 kJ/g among reported binary covalent nitrogen compounds. These newly predicted N-F compounds are useful in understanding the chemistry of polynitrogen.

Arylpentazoles with surprisingly high kinetic stability

The more-than-one-century-old arylpentazoles can only be used in situ in generating the pentazole anion due to their unfavourable kinetic stability. We successfully increased the N2-leaving barrier to reach hitherto the highest value of 40.83 kcal mol-1 at the CBS-QB3 level via a newly proposed co-stabilization method, making the broader applications of arylpentazoles feasible.

Magnetic, Electronic, and Mechanical Properties of Bulk ε-Fe2N Synthesized at High Pressures

We sintered bulk trigonal ε-Fe₂N (space group: P312) with the high-pressure and high-temperature method. Structural refinements by the Rietveld method result in a trigonal unit cell with parameters of a = 4.7767(1) Å and c = 4.4179(3) Å. ε-Fe₂N is ferromagnetic with a Curie temperature of ∼250 K, a saturation magnetization (M _s) value of up to 1.2 μ_B/formula units (f.u.), and comparatively low coercive field. The Vickers hardness was measured, and the results showed that the asymptotic hardness of bulk ε-Fe₂N is about 6.5 GPa with a load of 1000 g. Thermogravimetric (TG) analysis shows that ε-Fe₂N is thermally stable below 670 K. ε-Fe₂N exhibits good metal conductivity, and the electron transport measurements show that the resistivity of it is 172 μΩ cm at room temperature. The theoretical calculations suggest that the conducting states are mainly derive from Fe-3d states.

How stable can the pentanitrogen cation be in kinetics?

For the 22 year-old pentanitrogen cation N5+ (01), we surprisingly found that the previously reported transition states (TSs) do not correspond to N2-extrusion. We located the real N2-extrusion TS, which can well reconcile the hitherto remaining inconsistency between the gas-phase and salt-like forms of 01 both in structure and energetics.

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN_{4} Polymorph

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 BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85  GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} 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 BeN_{4} 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.

Pentagonal B2N3-based 3D metallic boron nitride with high energy density

Different from conventional insulating or semiconducting boron nitride,metallicBN has received increasing attention in recent years as its intrinsic metallicity grants it great potential for broad applications. In this study, by assembling the experimentally synthesized pentagonal B₂N₃units, we have proposed the first pentagon-based three-dimensional (3D) metallic boron nitride, labeled penta-B₄N_7.First-principles calculations together with molecular dynamics simulations and convex hull diagram show that penta-B₄N₇is not only thermally, dynamically and mechanically stable, but also three dimensionally metallic. A detailed analysis of its electronic structure reveals that the intrinsic metallicity comes from the delocalized electrons in the partially occupied antibonding N-Nπorbitals. Equally important, the energy density of penta-B₄N₇is found to be 4.07 kJ g^-1, which is the highest among that of all the 3D boron nitrides reported so far.

X-ray Free Electron Laser-Induced Synthesis of ε-Iron Nitride at High Pressures

The ultrafast synthesis of ε-Fe₃N_1+x in a diamond-anvil cell (DAC) from Fe and N₂ under pressure was observed using serial exposures of an X-ray free electron laser (XFEL). When the sample at 5 GPa was irradiated by a pulse train separated by 443 ns, the estimated sample temperature at the delay time was above 1400 K, confirmed by in situ transformation of α- to γ-iron. Ultimately, the Fe and N₂ reacted uniformly throughout the beam path to form Fe₃N_1.33, as deduced from its established equation of state (EOS). We thus demonstrate that the activation energy provided by intense X-ray exposures in an XFEL can be coupled with the source time structure to enable exploration of the time-dependence of reactions under high-pressure conditions.

Energy density of high-pressure nitrogen-rich MNx compounds

In the past decade, a large number of nitrogen-rich MN_x compounds have been discovered under high-pressure conditions. In this work, we have evaluated the energy densities of MN_x structures with thermodynamic and dynamical stability through first-principles calculations. The results show that the energy densities of MN_x consisting of alkali metals and cyclo-N₅^- are less than ∼0.5 TNT equivalence, whereas the group-III metal nitrides have high-energy density regardless of the type of nitrogen oligomers in the structures. To clarify the energy density difference for MN_x composed of different impurities, bivariate Pearson correlation analysis is performed, which reveals that the high-energy density of MN_x is related to the large N density, small M atomic radius, short M-N bond length, small MN_x ionicity, long N-N bond length and large M formal oxidation state. According to this correlation, H, Be, B, Al, Si and P elements have been proposed as the candidate impurities to synthesize high-energy density MN_x.

Stable nitrogen-rich scandium nitrides and their bonding features under ambient conditions

All-nitrogen salts have attracted extensive attention because of their unique chemical, physical properties, and potential applications as high-energy density materials. Using first-principles calculations and a particle swarm optimization structure search method, the pressure versus composition phase diagram of the Sc-N system is established. A new stable phase of C2/m-ScN₅ with the intriguing 2D N₁₀^6- is identified for the first time and we also found ScN₃ with the P1[combining macron] structure which has been reported before. Under ambient conditions, both of them have high kinetic and thermodynamic stability with high energy density (2.40 kJ g^-1 and 4.23 kJ g^-1 relative to ScN and N₂ gas). The analyses of chemical bonding pattern indicate that the nitrogen atoms in the N₆^6- chains are connected by covalent bonds with a combined σ and π bond character. We also give a possible high-pressure experimental route to P1[combining macron]-ScN₃ and C2/m-ScN₅ at modest pressure. We expect that our theoretical research could encourage experimental realization in the future and contribute to the understanding of the bonding features of nitrogen chains.

IrN4 and IrN7 as potential high-energy-density materials

Transition metal nitrides have attracted great interest due to their unique crystal structures and applications. Here, we predict two N-rich iridium nitrides (IrN₄ and IrN₇) under moderate pressure through first-principles swarm-intelligence structural searches. The two new compounds are composed of stable IrN₆ octahedrons and interlinked with high energy polynitrogens (planar N₄ or cyclo-N₅). Balanced structural robustness and energy content result in IrN₄ and IrN₇ being dynamically stable under ambient conditions and potentially as high energy density materials. The calculated energy densities for IrN₄ and IrN₇ are 1.3 kJ/g and 1.4 kJ/g, respectively, comparable to other transition metal nitrides. In addition, IrN₄ is predicted to have good tensile (40.2 GPa) and shear strengths (33.2 GPa), as well as adequate hardness (20 GPa). Moderate pressure for synthesis and ambient pressure recoverability encourage experimental realization of these two compounds in near future.

Preparation of iron(IV) nitridoferrate Ca4FeN4 through azide-mediated oxidation under high-pressure conditions

Transition metal nitrides are an important class of materials with applications as abrasives, semiconductors, superconductors, Li-ion conductors, and thermoelectrics. However, high oxidation states are difficult to attain as the oxidative potential of dinitrogen is limited by its high thermodynamic stability and chemical inertness. Here we present a versatile synthesis route using azide-mediated oxidation under pressure that is used to prepare the highly oxidised ternary nitride Ca₄FeN₄ containing Fe⁴⁺ ions. This nitridometallate features trigonal-planar [FeN₃]⁵⁻ anions with low-spin Fe⁴⁺ and antiferromagnetic ordering below a Neel temperature of 25 K, which are characterised by neutron diffraction, ⁵⁷Fe-Mössbauer and magnetisation measurements. Azide-mediated high-pressure synthesis opens a way to the discovery of highly oxidised nitrides.

High-valent metal nitrides are difficult to stabilise due to the high thermodynamic stability and chemical inertness of N₂. Here, the authors employ a large volume press to prepare an iron(IV) nitridoferrate Ca₄Fe^IVN₄ from Fe₂N and Ca₃N₂ via azide-mediated oxidation under high pressure conditions.

Nonlayered 2D ultrathin molybdenum nitride synthesized through the ammonolysis of 2D molybdenum dioxide

We report a new scalable strategy for the synthesis of nonlayered ultrathin two-dimensional (2D) molybdenum nitride (MoN) on a SiO₂/Si substrate by converting 2D molybdenum dioxide (MoO₂) through an ammonolysis process. The edge of MoN shows higher performance than that of the basal plane in both acidic and alkaline solutions.