Synthesis of magnesium-nitrogen salts of polynitrogen anions

N18 ring: A building block for constructing 1D and 2D polymeric nitrogen frameworks

Exploring the fundamental building block is essential for constructing functional materials with extended topological configurations. In the polymeric nitrogen system, no fundamental building blocks have been reported so far. Here, we successfully synthesize the buckled 1-dimensional (1D) band-shaped and 2-dimensional (2D) layered polymeric nitrogen frameworks with N18 ring as a fundamental building block for the first time. Furthermore, the dimensions of the polymeric nitrogen frameworks can be regulated by pressure conditions. Bader charge analyses indicate that the charge transfer from the La atom to the low-order bonded nitrogen atom plays a crucial role in stabilizing these two low-dimensional polymeric frameworks. Both LaN16 and LaN8 are promising high-energy-density materials (HEDMs). This study reveals that the N18 ring can serve as a fundamental building block, analogous to thesix-membered ring in carbon-based materials, enabling the construction of novel polymeric nitrogen materials with extended frameworks.

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.

Mechanism of Pressure-Modulated Self-Trapped Exciton Emission in Cs2TeCl6 Double Perovskite

Pressure-modulated self-trapped exciton (STE) emission mechanism in all-inorganic lead-free metal halide double perovskites characterized by large Stokes-shifted broadband emission, has attracted much attention across various fields such as optics, optoelectronics, and biomedical sciences. Here, by employing the all-inorganic lead-free metal halide double perovskite Cs2TeCl6 as a paradigm, the authors elucidate that the performance of STE emission can be modulated by pressure, attributable to the pressure-induced evolution of the electronic state (ES). Two ES transitions happen at pressures of 1.6 and 5.8 GPa, sequentially. The electronic behaviors of Cs2TeCl6 can be jointly modulated by both pressure and ES transitions. When the pressure reaches 1.6 GPa, the Huang-Rhys factor S, indicative of the strength of electron-phonon coupling, attains an optimum value of ≈12.0, correlating with the pressure-induced photoluminescence (PL) intensity of Cs2TeCl6 is 4.8-fold that of its PL intensity under ambient pressure. Through analyzing the pressure-dependent STE dynamic behavioral changes, the authors have revealed the microphysical mechanism underlying the pressure-modulated enhancement and quenching of STE emission in Cs2TeCl6.

Theoretical Prediction and Comprehensive Characterization of an All-Nitrogenatomic Ring, Cyclo[18]Nitrogen (N18)

The cyclic molecule cyclo[18]carbon composed of 18 carbon atoms has been observed in condensed phase experiment in recent years and has attracted great attention. Through state-of-art quantum chemistry calculation, this study found that 18 nitrogen atoms can also form a macrocyclic system, cyclo[18]nitrogen (N18), though its lifetime is very short at room temperature and can only exist for a relatively long time at very low temperatures. We comprehensively theoretically studied properties of N18, including geometric configurations, thermal decomposition mechanism and rate, molecular dynamics behavior, energetic properties, vibrational and electronic spectra. We also discussed in depth the electronic structure of N18, including nature of the N-N bonds, lone-pairs, charge distribution characteristics, electronic delocalization, and aromaticity. This work is not only the first exploration of the macrocyclic N18 molecule, but also the first time to systematically examine a very long-chain substance fully composed of nitrogen atoms in isolated state.

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.

Genesis of Polynitrogen Compounds Employing Silicon Substituted cyclo[18]carbon: A DFT Investigation

Activation of molecular N2 and its catalytic ability to form NH3 using C17Si has been already reported. This current study reports the formation of exclusive polynitrogen clusters (N4 and N5) on the C17Si ring. The clusters are generated using N2 and N3 respectively. Physical and chemical property analyses of the clusters show that the N5 cluster exhibits greater stability than N4. The former is seen to experience reduced molecular strain compared to the latter owing to its co-planar geometry. The thermodynamic calculations of the systems further show that the formation of the N5 cluster is spontaneous compared to N4 on the C17Si ring.

Geometries and stabilities of chromium doped nitrogen clusters: mass spectrometry and density functional theory studies

Metal-doped nitrogen clusters serve as effective models for elucidating the geometries and electronic properties of nitrogen-rich compounds at the molecular scale. Herein, we have conducted a systematic study of VIB-group metal chromium (Cr) doped nitrogen clusters through a combination of mass spectrometry techniques and density functional theory (DFT) calculations. The laser ablation is employed to generate CrNn+ clusters. The results reveal that CrN8+ cluster exhibits the highest signal intensity in mass spectrometry. The photodissociation experiments with 266 nm photons confirm that the chromium heteroazide clusters are composed of chromium ions and N2 molecules. Further structural searches and electronic structure calculations indicate that the cationic CrN8+ cluster possesses an X shaped geometry with D2 symmetry and exhibits robust stability. Molecular orbital and chemical bonding analyses demonstrate the existence of strong interactions between Cr+ cation and N2 ligands. The present findings enrich the geometries of metal doped nitrogen clusters and provide valuable guidance for the rational design and synthesis of novel transition metal nitrides.

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.

Predicted High-Energy Density MN8 Containing Anionic 18-Crown-6 Ring-Based Polynitrogen Monolayers Acting as Cryptand

In this study, we investigate the potential of the 18-crown-6-like two-dimensional (2D)-N8 structure to accommodate electrons from metals without compromising its covalent nitrogen network. Employing the crystal structure prediction enhanced by evolutionary algorithm and density functional theory methodology, we successfully predicted the existence of 16 layered M@2D-N8 complexes from a total of 39 MN8 systems investigated at 100 GPa (M = s-block Na-Cs, Be-Ba and d-block Ag, Au, Cd, Hg, Hf, W, and Y). Among those, there are 13 quenchable M@2D-N8 compounds that are dynamically stable at 1 atm. Orbital interactions and bonding analysis show that 2D-N8 presents a flat localized π* band that can accommodate one or two electrons without breaking the 2D covalent nitrogen network. Depending on the metal-to-polynitrogen charge transfer (formally, 1-4 electrons), these N-rich phases are semiconducting or metallic under ambient conditions. Ab initio molecular dynamics simulations show that K(I)@2D-N8 and Ca(II)@2D-N8 are thermally stable up to 600 K, while the Hf(IV)@2D-N8 compound is thermally not viable at 400 K because of the weakening of the N═N bonds due to a strong four-electron reduction. These metal 18-crown-6 ring-based polynitrogen compounds, as expected due to their high nitrogen content (eight nitrogen atoms per metal), could potentially serve as new high-energy density materials.

Ultrafast Dynamics Across Pressure-Induced Electronic State Transitions, Fluorescence Quenching, and Bandgap Evolution in CsPbBr3 Quantum Dots

This work investigates the impact of pressure on the structural, optical properties, and electronic structure of CsPbBr3 quantum dots (QDs) using steady-state photoluminescence, steady-state absorption, and femtosecond transient absorption spectroscopy, reaching a maximum pressure of 3.38 GPa. The experimental results indicate that CsPbBr3 QDs undergo electronic state (ES) transitions from ES-I to ES-II and ES-II to ES-III at 0.38 and 1.08 GPa, respectively. Intriguingly, a mixed state of ES-II and ES-III is observed within the pressure range of 1.08-1.68 GPa. The pressure-induced fluorescence quenching in ES-II is attributed to enhanced defect trapping and reduced radiative recombination. Above 1.68 GPa, fluorescence vanishes entirely, attributed to the complete phase transformation from ES-II to ES-III in which radiative recombination becomes non-existent. Notably, owing to stronger quantum confinement effects, CsPbBr3 QDs exhibit an impressive bandgap tuning range of 0.497 eV from 0 to 2.08 GPa, outperforming nanocrystals by 1.4 times and bulk counterparts by 11.3 times. Furthermore, this work analyzes various carrier dynamics processes in the pressure-induced bandgap evolution and electron state transitions, and systematically studies the microphysical mechanisms of optical properties in CsPbBr3 QDs under pressure, offering insights for optimizing optical properties and designing novel materials.

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.

Nitrogen-rich Ce-N compounds under high pressure

Four high-pressure N-rich compounds (Pmn21-CeN7, Amm2-CeN9, P1̄-CeN10, and P1̄-II-CeN10) are proposed using first-principles calculations. Novel polymeric units (a heart shaped layered structure, chain-like N8 rings, and two new banded structures) in four cerium nitrides are reported for the first time in this study. The analyses of electronic structures and bonding properties show that the charge transfer between Ce and N atoms promotes the formation of the Ce-N ionic bond and N-N covalent bond, which play an important role in stabilizing the nitrogen skeleton. Four new phases possess high energy densities (3.24-3.86 kJ g-1), indicating that they are favorable high-energy density materials. Moreover, P1̄-CeN10 possesses ultra-incompressibility along the [1 0 0] direction. Finally, infrared and Raman spectra are analyzed to provide guidance for experimental synthesis.

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.

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.

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.

Ultrafast dynamics under high-pressure

High-pressure is a mechanical method to regulate the structure and internal interaction of materials. Therefore, observation of properties' change can be realized in a relatively pure environment. Furthermore, high-pressure affects the delocalization of wavefunction among materials' atoms and thus their dynamics process. Dynamics results are essential data for understanding the physical and chemical characteristics, which is valuable for materials application and development. Ultrafast spectroscopy is a powerful tool to investigate dynamics process and becoming a necessary characterization method for materials investigation. The combination of high-pressure with ultrafast spectroscopy in the nanocosecond∼femtosecond scale enables us to investigate the influence of the enhanced interaction between particles on the physical and chemical properties of materials, such as energy transfer, charge transfer, Auger recombination, etc. Base on this point of view, this review summarizes recent progress in the ultrafast dynamics under high-pressure for various materials, in which new phenomena and new mechanisms are observed. In this review, we describe in detail the principles ofin situhigh pressure ultrafast dynamics probing technology and its field of application. On this basis, the progress of the study of dynamic processes under high-pressure in different material systems is summarized. An outlook onin situhigh-pressure ultrafast dynamics research is also provided.

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.

Revealing Phosphorus Nitrides up to the Megabar Regime: Synthesis of α'-P3 N5, δ-P3 N5 and PN2

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 XN6 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-P3 N5 and PN2 were found to form at 72 and 134 GPa, respectively, and both feature dense 3D networks of the so far elusive PN6 units. The two compounds are ultra-incompressible, having a bulk modulus of K0 =322 GPa for δ-P3 N5 and 339 GPa for PN2 . Upon decompression below 7 GPa, δ-P3 N5 undergoes a transformation into a novel α'-P3 N5 solid, stable at ambient conditions, that has a unique structure type based on PN4 tetrahedra. The formation of α'-P3 N5 underlines that a phase space otherwise inaccessible can be explored through materials formed under high pressure.

Synthesis of rare-earth metal compounds through enhanced reactivity of alkali halides at high pressures

Chemical stability of the alkali halides NaCl and KCl has allowed for their use as inert media in high-pressure high-temperature experiments. Here we demonstrate the unexpected reactivity of the halides with metals (Y, Dy, and Re) and iron oxide (FeO) in a laser-heated diamond anvil cell, thus providing a synthetic route for halogen-containing binary and ternary compounds. So far unknown chlorides, Y2Cl and DyCl, and chloride carbides, Y2ClC and Dy2ClC, were synthesized at ~40 GPa and 2000 K and their structures were solved and refined using in situ single-crystal synchrotron X-ray diffraction. Also, FeCl2 with the HP-PdF2-type structure, previously reported at 108 GPa, was synthesized at ~160 GPa and 2100 K. The results of our ab initio calculations fully support experimental findings and reveal the electronic structure and chemical bonding in these compounds.

The New High-Pressure Phases of Nitrogen-Rich Ag-N Compounds

The high-pressure phase diagram of Ag-N compounds is enriched by proposing three stable high-pressure phases (P4/mmm-AgN2, P1-AgN7 and P-1-AgN7) and two metastable high-pressure phases (P-1-AgN4 and P-1-AgN8). The novel N7 rings and N20 rings are firstly found in the folded layer structure of P-1-AgN7. The electronic structure properties of predicted five structures are studied by the calculations of the band structure and DOS. The analyses of ELF and Bader charge show that the strong N-N covalent bond interaction and the weak Ag-N ionic bond interaction constitute the stable mechanism of Ag-N compounds. The charge transfer between the Ag and N atoms plays an important role for the structural stability. Moreover, the P-1-AgN7 and P-1-AgN8 with the high-energy density and excellent detonation properties are potential candidates for new high-energy density species.

High-Pressure Synthesis and Stability Enhancement of Lithium Pentazolate

The pentazolate anion, cyclo-N₅^-, has received extensive attention as a new generation of energetic species for explosive or propulsion applications. Binary pentazolate compounds have been obtained under high-pressure conditions and their stability enhancement is crucial for obtaining more competitive high energy density materials (HEDMs). Here, we report the synthesis of a new solid phase of lithium pentazolate (space group P2₁/c) through the chemical transformation of pure lithium azide under high-pressure and high-temperature conditions. Upon decompression, the structural transition from P2₁/c-LiN₅ to P2₁/m-LiN₅ at ∼15.6 GPa was observed for the first time. Cyclo-N₅^- can be traced down to ∼5.7 GPa at room temperature and recovered to ambient pressure under a low-temperature condition (80 K). Our results reveal the enhancement of pentazolate anion stability with the increasing content of metal cations and demonstrate that low temperature is an effective route for the recovery of the pentazolate anion.

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.

Testing the performance of secondary anvils shaped with focused ion beam from the single-crystal diamond for use in double-stage diamond anvil cells

The success of high-pressure research relies on the inventive design of pressure-generating instruments and materials used for their construction. In this study, the anvils of conical frustum or disk shapes with flat or modified culet profiles (toroidal or beveled) were prepared by milling an Ia-type diamond plate made of a (100)-oriented single crystal using the focused ion beam. Raman spectroscopy and synchrotron x-ray diffraction were applied to evaluate the efficiency of the anvils for pressure multiplication in different modes of operation: as single indenters forced against the primary anvil in diamond anvil cells (DACs) or as pairs of anvils forced together in double-stage DACs (dsDACs). All types of secondary anvils performed well up to about 250 GPa. The pressure multiplication factor of single indenters appeared to be insignificantly dependent on the shape of the anvils and their culets' profiles. The enhanced pressure multiplication factor found for pairs of toroidally shaped secondary anvils makes this design very promising for ultrahigh-pressure experiments in dsDACs.

Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters

Highly selective separation materials that recover total ammonia nitrogen (i.e., ammonia plus ammonium, or TAN) from wastewaters as a pure product can supplement energy-intensive ammonia production and incentivize pollution mitigation. We recently demonstrated that commercial acrylate cation exchange polymer resins loaded with transition metal cations, or metal-loaded ligand exchangers, can recover TAN from wastewater with high selectivity (TAN/K+ equilibrium selectivity of 10.1) via metal-ammine bond formation. However, the TAN adsorption efficiency required further improvement (35%), and the optimal concentration and pH ranges were limited by both low ammonia fractions and an insufficiently strong resin carboxylate-metal bond that caused metal elution. To overcome these deficiencies, we used a zinc-acrylate ligand exchange resin and a tertiary amine acrylic weak base resin (pH buffer resin) together to achieve resin-mediated pH control for optimal adsorption conditions. The high buffer capacity around pH 9 facilitated gains in the adsorbed TAN per ligand resin mass that enhanced the TAN adsorption efficiency to greater than 90%, and constrained zinc elution (below 0.01% up to 1 M TAN) because of decreased ammonia competition for zinc-carboxylate bonds. During TAN recovery, resin-mediated pH buffering facilitated recovery of greater than 99% of adsorbed TAN with 0.2% zinc elution, holding the pH low enough to favor ammonium but high enough to prevent carboxylate protonation. For selective ion separation, solid phase buffers outperform aqueous buffers because the initial solution pH, the buffering capacity, and the ion purity can be independently controlled. Finally, because preserving the resin-zinc bond is crucial to sustained ligand exchange performance, the properties of an ideal ligand resin functional group were investigated to improve the properties beyond those of carboxylate. Ultimately, ligand exchange adsorbents combined with solid pH buffers can advance the selective recovery of nitrogen and potentially other solutes from wastewaters.

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.

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.

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.

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.

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.

Nitro-sonium nitrate (NO+NO3 -) structure solution using in situ single-crystal X-ray diffraction in a diamond anvil cell

At high pressures, autoionization - along with polymerization and metallization - is one of the responses of simple molecular systems to a rise in electron density. Nitro-sonium nitrate (NO+NO3 -), known for this property, has attracted a large interest in recent decades and was reported to be synthesized at high pressure and high temperature from a variety of nitro-gen-oxygen precursors, such as N2O4, N2O and N2-O2 mixtures. However, its structure has not been determined unambiguously. Here, we present the first structure solution and refinement for nitro-sonium nitrate on the basis of single-crystal X-ray diffraction at 7.0 and 37.0 GPa. The structure model (P21/m space group) contains the triple-bonded NO+ cation and the NO3 - sp 2-trigonal planar anion. Remarkably, crystal-chemical considerations and accompanying density-functional-theory calculations show that the oxygen atom of the NO+ unit is positively charged - a rare occurrence when in the presence of a less-electronegative element.

Pressure-induced stability and polymeric nitrogen in alkaline earth metal N-rich nitrides (XN6, X = Ca, Sr and Ba): a first-principles study

The Fddd-SrN₆ structure can transform into P1̄-SrN₆, and polymerized to infinite nitrogen chain structures at P = 22 GPa. For BaN₆, the Fmmm-BaN₆ structure can transform into C2/m-BaN₆, and polymerized to N₆ ring network structure at P = 110 GPa.

First principles investigation of electron correlation and Lifshitz transition within iron polynitrides

Metal poly-nitrogen compounds are gaining great interests as potential high energy density materials. Several iron polynitrides have been recently synthesized and investigated under high pressure (2018Nature Communications92756). In this work the electron correlations within these iron poly-nitrogen compounds were self-consistently determined, benchmarked with those obtained from linear response approach. Along with the increase of the concentration of nitrogen, the Coulomb interaction strengths show a monotonic decrease, where FeN and FeN₂are antiferromagnetic and the others are ferromagnetic. Then the electron correlation is studied along with the pressure, where the electrons are more delocalized as pressure becomes higher. One electronic topological transition was found for FeN₂, owing to a breaking of symmetry of spin and a transition of magnetism induced by a structural change. The band structure, densities of states, Fermi surface and absorption spectra were calculated and discussed.

Bipentazole (N10): A Low-Energy Molecular Nitrogen Allotrope with High Intrinsic Stability

In this Letter, we report a crystal structure prediction and characterization of a molecular nitrogen allotrope N₁₀ (bipentazole) using state-of-the-art computational methods. To date, in the form of a P2₁ space group crystal, this allotrope is the most stable predicted form of nitrogen, other than N₂, in the pressure range 0-42 GPa. Its metastability at ambient conditions was justified using phonon dispersion and mechanical properties calculations as well as ab initio molecular dynamics simulations. Due to a high intrinsic stability caused by aromaticity, bipentazole may appear to be the first nitrogen allotrope stable enough for a large-scale synthesis at ambient conditions. The calculations of propulsive characteristics revealed that bipentazole is an excellent "green" energetic material. A potential strategy for the synthesis of this compound is offered and rationalized. The unique electronic structure of bipentazole makes it a strongly electrophilic all-nitrogen reagent, which can exhibit unusual chemistry.

High-Pressure Polymeric Nitrogen Allotrope with the Black Phosphorus Structure

Studies of polynitrogen phases are of great interest for fundamental science and for the design of novel high energy density materials. Laser heating of pure nitrogen at 140 GPa in a diamond anvil cell led to the synthesis of a polymeric nitrogen allotrope with the black phosphorus structure, bp-N. The structure was identified in situ using synchrotron single-crystal x-ray diffraction and further studied by Raman spectroscopy and density functional theory calculations. The discovery of bp-N brings nitrogen in line with heavier pnictogen elements, resolves incongruities regarding polymeric nitrogen phases and provides insights into polynitrogen arrangements at extreme densities.