Novel high pressure structures of polymeric nitrogen

Ambient-Condition Recoverable Polymeric N10 Discovered from the Predicted Zr-N Compounds

Polynitrogen has been widely studied recently as a rising star of high energy density materials. Here, we performed a systematic study of the Zr-N compounds in the N-rich region by the first-principles method. The high-pressure phase diagram of the Zr-N system is enriched by proposing five new compounds. ZrN10 with the infinitely extended band shaped structure is first reported. The band-like polynitrogen of ZrN10 decomposes into a more stable chain-like polynitrogen structure under the influence of temperature. Additionally, the novel honeycomb-like band-shaped N10 structure hcb-N10 has been discovered by removing the Zr atoms. The absence of the -4 oxidation state in the N10 unit prompts its further polymerization, which makes hcb-N10 possess dynamical and thermal stability in ambient conditions. hcb-N10 is a semiconductor with a bandgap of 2.97 eV due to highly localized electrons. Both chain-ZrN10 and hcb-N10 represent potential candidates for HEDMs with outstanding energy and explosive performance.

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.

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.

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.

Novel nitrogen-rich lanthanum nitrides induced by the ligand effect under pressure

N-rich La-N compounds have been studied by first-principles calculations in this work. We identified nine unknown polynitrides, which reveals that the miraculous ligand effect of La plays an important role in the electronic properties and hybridization of nitrogen atoms. Unique tri-coordination atoms with alternate sp2 and sp3 hybridizations are formed in the N18 ring of LaN8. The ligand effect of La induces a novel 1-D chain-like N10 cage polymeric structure in LaN10 and stabilizes it under mild pressure (25 GPa). Moreover, the ligand effect of the introduced La atom on the N10 cage has been clarified by the analysis of the structural evolution behavior from I4̄3m-N10 to Imm2-LaN10. In addition, Imm2-LaN10 with excellent energy (4.56 kJ g-1) and explosive performance (Vd = 16.88 km s-1, Vp = 1887.53 kbar) is a good energetic material candidate.

Prediction of metastable phase of the Sc-N system in the N-rich region under high pressure

The prediction of the high-pressure structure of the ScN_x system enriches the phase diagram of the Sc-N system: three metastable phase structures (P1̄-ScN8, P1-ScN9 and P1-ScN11) are proposed in the N-rich region. These structures have novel polymeric nitrogen structures, and enrich the structural types of polymeric nitrogen under pressure. Interestingly, the P1-ScN11 phase can be quenched to ambient conditions, and release energy at a relatively mild temperature of 800 K. The larger charge transfer plays an important role in the structural stability by inducing the Sc-N ionic bond interaction and N-N covalent bond interaction. The prominent energy properties of P1̄-ScN8, P1-ScN9 and P1-ScN11 make them potential candidates in the application of propellants and explosives.

Study of polymerization of high-pressure nitrogen by ab initio molecular dynamics

We study properties of nitrogen at high pressure and temperature (100-120 GPa, 2000-3000 K) where molecular and polymeric phases compete both in solid and liquid phase. We employ ab initio MD simulations with the SCAN functional and study the pressure-induced polymerization in liquid nitrogen for system sizes up to 288 atoms in order to reduce finite-size effects. The transition is studied upon both compression and decompression, and at 3000 K, it is found to take place between 110 and 115 GPa, coming close to experimental data. We also simulate the molecular crystalline phase close to the melting line and analyze its structure. We show that the molecular crystal in this regime is highly disordered, in particular, due to pronounced orientational and also translational disorder of the molecules. Its short-range order and vibrational density of states are very close to those of the molecular liquid revealing that the system likely represents a plastic crystal with high entropy.

Novel high-pressure phases of nitrogen-rich Y-N compounds

Five new high-pressure phases (I4̄3d-Y₄N₃, R3c-Y₂N₃, P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈) are proposed by the crystal structure prediction. A series of polynitrogen forms were achieved in the nitrogen-rich Y-N compounds, including diatomic N₂, an isolated N₈ chain, an infinite N chain with an N₆ unit, and an infinite N layer with bent N₁₈ rings. The high energy densities of P1̄-II-YN₄ (1.98 kJ g^-1), P1̄-YN₆ (2.35 kJ g^-1), and P31c-YN₈ (3.77 kJ g^-1) make them potential high energy density materials. More importantly, P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈ exhibit excellent explosive performance, with detonation pressures 4-8 times that of TNT (19 GPa) and detonation velocities 1-2 times that of TNT (6.90 km s^-1). The electronic structure and bonding properties show that the high stability of Y-N compounds originates from the strong N-N covalent bond and the weak Y-N ionic bond interaction. The increase in the transferred charge quantity as the pressure decreased is more conducive to stabilizing the polymeric nitrogen structure, which leads to the metastable properties of P1̄-II-YN₄ and P1̄-YN₆ under ambient conditions. Finally, the infrared (IR) spectra of P1̄-II-YN₄, P1̄-YN₆, and P31c-YN₈ are calculated to provide a reference in experimental synthesis.

Prediction of erbium-nitrogen compounds as high-performance high-energy-density materials

To explore high-energy-density materials, intense attention has been focused on how to stabilize the N-N bond in nitrogen-rich compounds. Here, we report several stable phases of erbium-nitrogen compounds ErN_xas high-energy-density materials. Specifically, the phase diagrams of stable high-pressure structuresImmm-ErN₂,C2-ErN₃,P1--ErN₄, andP1--ErN₆, are theoretically studied by combining first-principles calculation with particle swarm optimization algorithm. In these erbium-nitrogen compounds, the N-N bonds are stabilized as diatomic quasi-molecule N₂, helical-like nitrogen chains, armchair nitrogen chains, and armchair-anti-armchair nitrogen chains, respectively. Among them, theP1--ErN₆harbors excellent stability at high thermal up to 1000 K. More importantly, theP1--ErN₆has outstanding explosive performance with high-energy-density of 1.30 kJ g^-1, detonation velocity of 10.87 km s^-1, and detonation pressure of 812.98 kbar, which shows its promising application prospect as high-energy-density materials.

Ambient-Pressure Recoverable Polynitrogen Solids Assembled by Pentazolate Rings with High Energy Density

Crystal structure predictions and first-principles calculations were used to predict three polynitrogen solids (aP8-N, aP12-N, and oP24-N) that possess competitive enthalpies as compared to the synthesized open-chain N₈ phase at pressures in the range of 0-60 GPa. aP8-N, aP12-N, and oP24-N contain edge-shared, N₂-linked, and N-bridged pentazolate rings and form molecular N₈, molecular N₁₂, and quasi-one-dimensional N_∞ ribbons, respectively. The calculations of formation enthalpies show that the three polynitrogen solids can be synthesized by compressing cyclo-N₅ salts in hydrogen-saturated environments. Molecular simulations suggest that the three polynitrogen solids have the ability of quench recoverability under ambient conditions once being synthesized at high pressure. With estimated energy densities in the range of 5.6-6.5 kJ/g, these three polynitrogen phases show notable promise for applications as high-energy-density materials.

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.

Theoretical methods for structural phase transitions in elemental solids at extreme conditions: statics and dynamics

In recent years, theoretical studies have moved from a traditionally supporting role to a more proactive role in the research of phase transitions at high pressures. In many cases, theoretical prediction leads the experimental exploration. This is largely owing to the rapid progress of computer power and theoretical methods, particularly the structure prediction methods tailored for high-pressure applications. This review introduces commonly used structure searching techniques based on static and dynamic approaches, their applicability in studying phase transitions at high pressure, and new developments made toward predicting complex crystalline phases. Successful landmark studies for each method are discussed, with an emphasis on elemental solids and their behaviors under high pressure. The review concludes with a perspective on outstanding challenges and opportunities in the field.

Low pressure metastable single-bonded solid nitrogen phases

Within the framework of the density functional theory, the possibility of the formation of single-bonded solid atomic nitrogen phases as a result of adiabatic compression of molecular and cluster nitrogen structures at zero temperature has been studied. It has been demonstrated that nitrogen clusters N₈(C_2v)-B, which are theoretically predicted as one of the promising candidates for high energy density materials, can transform under compression into a solid atomic phase with crystal lattice symmetry P2₁. The P2₁ phase is dynamically stable under decompression to zero pressure. It is shown that the ε-N₂ molecular phase transforms under compression into a solid atomic phase with R3̄c symmetry, and retains a vibrationally stable crystal structure when the pressure is reduced to 30 GPa, transforming into a stable cluster form at lower pressures. The atoms in the P2₁ and R3̄c solid atomic phases are linked by single bonds; therefore, these structures can store a large amount of energy ≈1.4 eV per atom. A detailed comparison of the properties of new P2₁ and R3̄c solid atomic phases with other nitrogen crystal structures that are dynamically stable at low pressures has been carried out.

Molecular and Electronic Structures of Neutral Polynitrogens: Review on the Theory and Experiment in 21st Century

The data on the existence and physicochemical characteristics of uncharged single element chemical compounds formed by nitrogen atoms and containing more than two nuclides of this element (from N₄ to N₁₂₀, oligomeric and polymeric polynitrogens) have been systematized and generalized. It has been noticed that these data have a predominantly predictive character and were obtained mainly using quantum chemical calculations of various levels (HF, DFT, MP, CCSD etc.). The possibility of the practical application of these single element compounds has been considered. The review mainly covers articles published in the last 25 years. The bibliography contains 128 references.

Pressure-stabilized polymerization of nitrogen in manganese nitrides at ambient and high pressures

Two stable high-pressure phases (C2/m-MnN₄ and P1̄-MnN₄) and four metastable phases (P4/mmm-MnN₄, P1̄-MnN₅, C2/m-MnN₆ and P1̄-MnN₈) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N₂, the armchair chain and S-like chain, the N₄ ring and N₂₂ ring are firstly reported in the P4/mmm-MnN₄ and P1̄-MnN₅ phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. Ab initio molecular dynamics simulations show that P1̄-MnN₄ and P1̄-MnN₅ can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P1̄-MnN₄ (2000 K) and P1̄-MnN₅ (3000 K). Interestingly, P4/mmm-MnN₄ with the N₄ ring exhibits outstanding mechanical properties, including high incompressibility, high hardness, uniform strength in the 2-D direction and excellent ductility. Strong N-N covalent bond and weak Mn-N ionic bond interactions are observed in the predicted Mn-N compounds, and the charge transfer between the Mn and N atoms provides an important contribution to the stabilization of polymeric N-structures. All the proposed structures are metallic phases. Our results provide a deep understanding of the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of these new manganese polynitrides in future.

Pressure-stabilized high-energy-density material YN10

Polynitrogen compounds have been intensively studied for potential applications as high energy density materials, especially in energy and military fields. Here, using the swarm intelligence algorithm in combination with first-principles calculations, we systematically explored the variable stoichiometries of yttrium-nitrogen compounds on the nitrogen-rich regime at high pressure, where a new stable phase of YN₁₀adoptingI4/msymmetry was discovered at the pressure of 35 GPa and showed metallic character from the analysis of electronic properties. In YN₁₀, all the nitrogen atoms weresp²-hybridized in the form of N₅ring. Furthermore, the gravimetric and volumetric energy densities were estimated to be 3.05 kJ g^-1and 9.27 kJ cm^-1respectively. Particularly, the calculated detonation velocity and pressure of YN₁₀(12.0 km s^-1, 82.7 GPa) was higher than that of TNT (6.9 km s^-1, 19.0 GPa) and HMX (9.1 km s^-1, 39.3 GPa), making it a potential candidate as a high-energy-density material.

Materials by design at high pressures

Pressure, a fundamental thermodynamic variable, can generate two essential effects on materials. First, pressure can create new high-pressure phases via modification of the potential energy surface. Second, pressure can produce new compounds with unconventional stoichiometries via modification of the compositional landscape. These new phases or compounds often exhibit exotic physical and chemical properties that are inaccessible at ambient pressure. Recent studies have established a broad scope for developing materials with specific desired properties under high pressure. Crystal structure prediction methods and first-principles calculations can be used to design materials and thus guide subsequent synthesis plans prior to any experimental work. A key example is the recent theory-initiated discovery of the record-breaking high-temperature superhydride superconductors H3S and LaH10 with critical temperatures of 200 K and 260 K, respectively. This work summarizes and discusses recent progress in the theory-oriented discovery of new materials under high pressure, including hydrogen-rich superconductors, high-energy-density materials, inorganic electrides, and noble gas compounds. The discovery of the considered compounds involved substantial theoretical contributions. We address future challenges facing the design of materials at high pressure and provide perspectives on research directions with significant potential for future discoveries.

A novel all-nitrogen molecular crystal N16 as a promising high-energy density material

All-nitrogen solids, if successfully synthesized, are ideal high energy density materials because they store a great amount of energy and produce only harmless N2 gas upon decomposition. Currently, the only...

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.

Energy landscapes of perfect and defective solids: from structure prediction to ion conduction

The energy landscape concept is increasingly valuable in understanding and unifying the structural, thermodynamic and dynamic properties of inorganic solids. We present a range of examples which include (i) structure prediction of new bulk phases including carbon nitrides, phosphorus carbides, LiMgF3 and low-density, ultra-flexible polymorphs of B2O3, (ii) prediction of graphene and related forms of ZnO, ZnS and other compounds which crystallise in the bulk with the wurtzite structure, (iii) solid solutions, (iv) understanding grossly non-stoichiometric oxides including the superionic phases of δ-Bi2O3 and BIMEVOX and the consequences for the mechanisms of ion transport in these fast ion conductors. In general, examination of the energy landscapes of disordered materials highlights the importance of local structural environments, rather than sole consideration of the average structure.

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.

New Two-Dimensional Wide Band Gap Hydrocarbon Insulator by Hydrogenation of a Biphenylene Sheet

Based on first-principles calculations, the ground state configuration (Cmma-CH) of a hydrogenated biphenylene sheet ( Science 2021, 372, 852) is carefully identified from hundreds of possible candidates generated by RG² code ( Phys. Rev. B. 2018, 97, 014104). Cmma-CH contains four inequivalent benzene molecules in its crystalline cell due to its Cmma symmetry. Hydrogen atoms bond to carbon atoms in each benzene with a boat-like (DDUDDU) up/down sequence and reversed boat-1 (UUDUUD) sequence in adjacent benzene rings. Cmma-CH is energetically less stable than the proposed allotropes of hydrogenated graphene, but the formation energy for hydrogenating a biphenylene sheet is remarkably lower than that for hydrogenating graphene to graphane. Our results of mechanical and dynamical stability also confirm that Cmma-CH is a stable 2D hydrocarbon, which is expected to be realized experimentally. Especially, biphenylene undergoes a transition from normal metal to a wide band gap insulator (4.645 eV) by hydrogenation to Cmma-CH, which has potential applications in nanodevices at elevated temperatures and high voltages.

A novel high-pressure phase of ScN5with higher stability predicted from first-principles calculations

For binary compounds of Sc-N, the stable structures and stoichiometries were studied from ambient condition to high pressure of 100 GPa, adopting CALYPSO method. The newly predictedP2₁/c-ScN₅compound was more energetically stable under high pressureP= 62 GPa comparing with the three previously reported phases ofP1-ScN₅,Cm-ScN₅andC2/m-ScN₅. Furthermore, the high-pressure phase ofP2₁/c-ScN₅was dynamically stable at ambient condition, so the ambient-pressure recovery is possible. In this paper, the study suggested that the energetic polynitrides can be obtained in transition metal nitrides under high pressure. And we identified one novel 3D extended puckered poly-nitrogen network in theP2₁/c-ScN₅structure, which is similar to theC2/m-ScN₅. The decomposition ofP2₁/c-ScN₅to ScN and N₂under ambient pressure was estimated to release 5.02 eV energy per formula unit (f.u.), corresponding to 4.19 kJ g^-1in energy density, which was expected to be highly exothermic. The present results can conduce to obtain more polynitrogen forms and theoretically encourages experimental discovery in these promising materials.

Pressure-driven electronic phase transition in the high-pressure phase of nitrogen-rich 1H-tetrazoles

High-energy-density materials (HEDMs) require new design rules collected from experimental and theoretical results and a proposed mechanism. One of the targeted systems is the nitrogen-rich compounds as precursors for possible polymeric nitrogen or its counterpart in a reasonable pressure range. 1H-tetrazole (CH₂N₄) with hydrogen bonds was studied under pressure by both diffraction and spectroscopy techniques. The observed crystal structure phase transition and hydrogen bond-assisted electronic structure anomaly were confirmed by first-principles calculation. The rearrangement of the hydrogen bonds under pressure elucidates the bonding interactions of the nitrogen-rich system in local 3D chemical environments, allowing the discovery and design of a feasible materials system to make new-generation high-energy materials.

Combined high pressure in situ spectra with first-principles calculations, a possible hydrogen-bond assisted phase transition was proposed in tetrazole.

New Cadmium-Nitrogen Compounds at High Pressures

A systematic high-pressure study of the CdN_x (x = 2, 3, 4, 5, and 6) system is performed by using the first-principles calculation method in combination with the particle swarm optimization algorithm. We proposed four stable high-pressure phases (P4mbm-CdN₂, Cmmm-CdN₄, I4̅2d-CdN₄, and C2/c-CdN₅) and one metastable high-pressure phase (C2/m-CdN₆), for which the structural frames are composed of a diatomic quasi-molecule N₂, standard armchair N-chain, S-type bent armchair N-chain, zigzag-antizigzag N-chain, and N₁₄ network structure. Among them, the novel zigzag-antizigzag N-chain and N₁₄ network structure are reported for the first time. More importantly, Cmmm-CdN₄ and C2/m-CdN₆ possess high stability under ambient conditions, which may be quenched to ambient conditions once they are synthesized at high-pressure conditions. The high decomposition energy barrier (1.14 eV) results in a high decomposition temperature (2500 K) of Cmmm-CdN₄, while a low decomposition energy barrier (0.19 eV) results in a mild decomposition temperature (500 K) of C2/m-CdN₆. The high energy density and outstanding explosive performance make Cmmm-CdN₄, I4̅2d-CdN₄, C2/c-CdN₅, and C2/m-CdN₆ potential high-energy materials. The electronic structure analyses show that these predicted high-pressure structures are all metallic phases, and the N-N and Cd-N bonds are the strong covalent and ionic bond interactions, respectively. The charge transfer from the Cd atom plays an important role in the stability of the proposed structures.

The nature of the intermolecular interaction in (H2X)2 (X = O, S, Se)

Hydrogen bonds (HBs) are crucial non-covalent interactions in chemistry. Recently, the occurrence of an HB in (H2S)2 has been reported (Arunan et al., Angew. Chem., Int. Ed., 2018, 57, 15199), challenging the textbook view of H2S dimers as mere van der Waals clusters. We herein try to shed light on the nature of the intermolecular interactions in the H2O, H2S, and H2Se dimers via correlated electronic structure calculations, Symmetry Adapted Perturbation Theory (SAPT) and Quantum Chemical Topology (QCT). Although (H2S)2 and (H2Se)2 meet some of the criteria for the occurrence of an HB, potential energy curves as well as SAPT and QCT analyses indicate that the nature of the interaction in (H2O)2 is substantially different (e.g. more anisotropic) from that in (H2S)2 and (H2Se)2. QCT reveals that the HB in (H2O)2 includes substantial covalent, dispersion and electrostatic contributions, while the last-mentioned component plays only a minor role in (H2S)2 and (H2Se)2. The major contributions to the interactions of the dimers of H2S and H2Se are covalency and dispersion as revealed by the exchange-correlation components of QCT energy partitions. The picture yielded by SAPT is somewhat different but compatible with that offered by QCT. Overall, our results indicate that neither (H2S)2 nor (H2Se)2 are hydrogen-bonded systems, showing how the nature of intermolecular contacts involving hydrogen atoms evolves in a group down the periodic table.

Phase relations, and mechanical and electronic properties of nickel borides, carbides, and nitrides from ab initio calculations

Based on density functional theory and the crystal structure prediction methods, USPEX and AIRSS, stable intermediate compounds in the Ni–X (X = B, C, and N) systems and their structures were determined in the pressure range of 0–400 GPa. It was found that in the Ni–B system, in addition to the known ambient-pressure phases, the new nickel boride, Ni₂B₃-Immm, stabilizes above 202 GPa. In the Ni–C system, Ni₃C-Pnma was shown to be the only stable nickel carbide which stabilizes above 53 GPa. In the Ni–N system, four new phases, Ni₆N-R3̄, Ni₃N-Cmcm, Ni₇N₃-Pbca, and NiN₂-Pa3̄, were predicted. For the new predicted phases enriched by a light-element, Ni₂B₃-Immm and NiN₂-Pa3̄, mechanical and electronic properties have been studied.

Based on density functional theory and the crystal structure prediction methods, USPEX and AIRSS, stable intermediate compounds in the Ni–X (X = B, C, and N) systems and their structures were determined in the pressure range of 0–400 GPa.

Properties of Alkaline-Earth-Metal Polynitrogen Ternary Materials at High Pressure

We report the formation of cubic and tetragonal BaSrN3 at 100 GPa using an ab initio structure search method. Pressure ramping to 0 GPa reveals a reaction pressure threshold of 4.92 and 7.23 GPa for the cubic and tetragonal BaSrN3, respectively. The cubic phase is stabilized by coulombic interaction between the ions. Meanwhile, tetragonal BaSrN3 is stabilized through an expansion of the d-orbital in Ba and Sr atoms that is compensated by delocalization of π-electrons in N through reduction of π overlap. Elastic properties analysis suggests that both phases are mechanically stable. The structures also have high melting points as predicted using an empirical model, and all imaginary modes vanishes at about 2000 K. These results have significant implication for the design of cleaner and environmentally friendly high energy density materials.

Nitrogen in black phosphorus structure

Group V elements in crystal structure isostructural to black phosphorus with unique puckered two-dimensional layers exhibit exciting physical and chemical phenomena. However, as the first element of group V, nitrogen has never been found in the black phosphorus structure. Here, we report the synthesis of the black phosphorus-structured nitrogen at 146 GPa and 2200 K. Metastable black phosphorus-structured nitrogen was retained after quenching it to room temperature under compression and characterized in situ during decompression to 48 GPa, using synchrotron x-ray diffraction and Raman spectroscopy. We show that the original molecular nitrogen is transformed into extended single-bonded structure through gauche and trans conformations. Raman spectroscopy shows that black phosphorus-structured nitrogen is strongly anisotropic and exhibits high Raman intensities in two A _g normal modes. Synthesis of black phosphorus-structured nitrogen provides a firm base for exploring new type of high-energy-density nitrogen and a new direction of two-dimensional nitrogen.

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.

Novel All-Nitrogen Molecular Crystals of Aromatic N10

Nitrogen has unique bonding ability to form single, double, and triple bonds, similar to that of carbon. However, a molecular crystal formed by an aromatic polynitrogen similar to a carbon system has not been found yet. Herein, a new form of stable all-nitrogen molecular crystals consisting of only bispentazole N10 molecules with exceedingly high energy density is predicted. The crystal structures and the conformation of N10 molecules are strongly correlated, both depending on the applied external pressure. These molecular crystals can be recovered upon the release of the pressure. The first-principles molecular dynamics simulations reveal that these all-nitrogen materials decompose at temperatures much higher than room temperature. The decompositions always start from breaking off N2 molecules from the nitrogen ring and can release a large amount of energy. These new polynitrogens are aromatic and are more stable than all the other polynitrogen crystals reported previously, providing a new green strategy to get all-nitrogen, nonpolluting high energy density materials without introducing any metal or other guest stabilizer.

Amorphous polymerization of nitrogen in compressed cupric azide

Metal azides have attracted increasing attention as precursors for synthesizing polymeric nitrogen. In this article, we report the amorphous polymerization of nitrogen by compressing cupric azide. The ab initio molecular dynamics simulations show that crystalline cupric azide transforms into a disordered network composed of singly bonded nitrogen at a hydrostatic pressure of 40 GPa and room temperature. The transformation manifests the formation of a π delocalization along the disordered Cu-N network, thus resulting in a semiconductor-metal transition. The estimated heat of formation of the amorphous polymeric nitrogen system is comparable to conventional high-energy-density materials. The amorphization provides an alternative route to the polymerization of nitrogen under moderate conditions.

Formation mechanism of insensitive tellurium hexanitride with armchair-like cyclo-N6 anions

The lower decomposition barriers of cyclo-N₆ anions hinder their application as high-energy-density materials. Here, first-principles calculations and molecular dynamics simulations reveal that enhancing the covalent component of the interaction between cyclo-N₆ anions and cations can effectively improve the stability of cyclo-N₆ anions. Taking tellurium hexanitride as a representative, the exotic armchair-like N₆ anions of tellurium hexanitride exhibit resistance towards electronic attack and gain extra stability through the formation of covalent bonds with the surrounding elemental tellurium under high pressures. These covalent bonds effectively improve the chemical barrier and insensitivity of tellurium hexanitride during blasting, which prevents the decomposition of solid cyclo-N₆ salts into molecular nitrogen. Furthermore, the high-pressure induced covalent bonds between cyclo-N₆ anions and tellurium enable the high bulk modulus, remarkable detonation performance, and high-temperature thermodynamic stability of tellurium hexanitride.

Pressure-stabilized polymerization of nitrogen in alkaline-earth-metal strontium nitrides

High-pressure technology can help us to obtain excellent materials. We have explored alkaline-earth-metal strontium nitrides under different pressures, theoretically. A variety of stable Sr-N structures were predicted by the structure searching method using CALYPSO code. Six new stoichiometries, SrN, Sr₂N₃, SrN₂, SrN₃, SrN₄, and SrN₅, were predicted. And our calculation proved that all these compounds were stable existing under ambient pressure up to 100 GPa. A rich variety of poly-nitrogen forms appeared in the newly predicted SrN_x compounds, including four nitrogen polymerization forms: ranging from N₂, N₃, N₄, and N₅ molecules, to zig-zag nitrogen chains and extended chains connected by puckered "N₆" rings. Significantly, the 1D extended polymeric chain of puckered "N₆" rings was firstly identified in the P1[combining macron]-SrN₃ structure at 60 GPa. Another N-rich C2/c-SrN₄ was stable only under the relatively high-pressure of 20 GPa, but this phase can be quenched under atmospheric pressure. The N-rich phase SrN₅ maintained structural stability when the pressure reached 50-70 GPa. The delocalization of π electrons from N atoms was the principal cause for its metallicity in SrN₅. In this paper, our calculated results indicated that the energetic poly-nitrides in alkaline-earth-metal nitrides can be obtained by the high-pressure method.

Unraveling the Mechanism of cyclo-N5- Production through Selective C-N Bond Cleavage of Arylpentazole with Ferrous Bisglycinate and m-Chloroperbenzonic Acid: A Theoretical Perspective

Very recently, the bulk synthesis of cyclo-N₅^- from arylpentazole through the treatment with m-chloroperbenzonic acid (m-CPBA) and ferrous bisglycinate ([Fe(Gly)₂]) (Zhang, C., et al. Science 2017, 355, 374) has greatly promoted the application of pentazolate anion as a novel high-performance energetic material. Yet the mechanism for this reaction is still unexplored. Herein we perform mechanistic studies on the selective C-N bond cleavage in arylpentazole by using density functional theory methods. The direct C-N bond activation by m-CPBA was computed to be kinetically inaccessible. Instead, the oxidation of [Fe(Gly)₂] by m-CPBA is much favorable, which leads to the generation of a high-valent iron(IV)-oxo product. The Fe(IV)-oxo intermediate has been examined by UV-vis absorption spectra experiments and further verified by excited-state calculations. It is found that the Fe(IV)-oxo serves as the key intermediate for the C-N bond activation of arylpentazole and the cyclo-N₅^- generation. Our calculations clarified the key mechanistic details of the cyclo-N₅^- generation, and the factors that affect the production yield are further discussed.

Strategy for chemically riveting catenated nitrogen chains

Catenated nitrogen chains (CNCs) attract considerable attention because of their structural novelty, their high energy, and their potential as precursors for all-nitrogen materials. However, pure CNCs are unstable under ambient conditions, limiting their on-site or practical applications. Chemical riveting of a CNC involves the maintenance of the structure of a CNC and placing the CNC in a special chemical structure as a moiety of an entire molecule to stabilize it. Other structures, except from the CNC, are named rivets. In this study, the molecular geometry, bond order, natural bond orbital charge, and frontier orbital among the pure CNCs (N_x), N_x^-, N_x⁺, and N_xH_y (x = 2-11) and CNCs riveted and implemented experimentally in several former studies are systematically analyzed and compared. The lone-pair repulsion between two neighboring N atoms is one of the primary factors for the low molecular stability of the pure CNCs and their derivatives. The riveted CNCs with rings are usually more stable than those with open chains. Moreover, the standard deviation of the bond orders or the bond lengths can be employed as an indicator for molecular stability, i.e., a lower standard deviation suggests higher molecular stability. Hence, electron-donating groups, groups with electronegativity within those of H and N atoms, as well as atoms or groups with empty orbitals are proposed to trap the lone pairs of CNCs as rivets to stabilize the CNCs. This strategy is expected to facilitate the creation of stable CNC-contained compounds with high structural novelty or high energy contents. Graphical abstractA strategy is proposed for chemically riveting instable CNCs to transform them into more stable ones.

Beyond molecular nitrogen: revelation of two ambient-pressure metastable single- and double-bonded nitrogen allotropes built from three-membered rings

In this study, we present a crystal structure prediction and characterization for two novel single- and double-bonded nitrogen allotropes (denoted as CubN and DobN) bearing three-membered nitrogen cycles. These materials are ambient-pressure metastable robust solids with strong cohesive energies. Due to the presence of strained nitrogen cycles, these allotropes are high-lying phases, which retain high energy densities at least up to 200 GPa, when CubN becomes lower in energy than ζ-N2. The studied allotropes are indirect wide-bandgap semiconductors. A detailed spectral characterization of these materials is also presented herein. Due to their extremely high heats of formation and crystal densities, CubN and DobN demonstrate excellent detonation properties that are comparable to those of previously reported phases (cg-N and TrigN). Propulsive properties (including specific impulse and characteristic velocity) of CubN and DobN as solid monopropellants were also estimated. Thus, if synthesized, the present nitrogen allotropes will have great potential for practical applications and achieving fundamental knowledge about nitrogen as a chemical element.

Predictions on High-Power Trivalent Metal Pentazolate Salts

High-energy-density materials (HEDMs) have been intensively studied for their significance in fundamental sciences and practical applications. Here, using the molecular crystal structure search method based on first-principles calculations, we have predicted a series of metastable energetic trivalent metal pentazolate salts MN₁₅ (M= Al, Ga, Sc, and Y). These compounds have high energy densities, with the highest nitrogen content among the studied nitrides so far. Pentazolate N₅^- molecules stack up face-to-face and form wave-like patterns in the C222₁ and Cc symmetries. The strong covalent bonding and very weak noncovalent interactions with nonbonded overlaps coexist in these ionic-like structures. We find MN₁₅ molecular structures are mechanically stable up to high temperature (∼1000 K) and ambient pressure. More importantly, these trivalent metal pentazolate salts have high detonation pressure (∼80 GPa) and velocity (∼12 km/s). Their detonation pressures exceeding that of TNT and HMX make them good candidates for high-brisance green energetic materials.

Armchair shaped polymeric nitrogen N8 chains confined in h-BN matrix at ambient conditions: stability and vibration analysis

A new hybrid material comprising of armchair shaped polymeric nitrogen chains (N₈) encapsulated in h-BN matrix is proposed and studied through ab initio calculations. Interestingly, the theoretical results demonstrate that N₈ chains, confined in h-BN matrix, are effectively stabilized at ambient pressure and room temperature. Moreover, N₈ chains can dissociate and release energy at a much milder temperature of 600 K. The confined polymer N₈ unit needs to absorb 0.68 eV energy to span the decomposition energy barrier before decomposing. Further research shows that the charge transfer between N₈ chain and h-BN layer is the stabilizing mechanism of this new hybrid material. And the low dissociation temperature is due to a much smaller amount of charge transfer compared to other confined systems in previous reports. The IR and Raman vibrational analyses suggest that host–guest interactions in the hybrid material influence the vibration modes of both the confined N₈ chain and h-BN layer.

Armchair shaped polymeric nitrogen N8 chains confined in h-BN matrix is stable at ambient conditions.

Metallic and anti-metallic properties of strongly covalently bonded energetic AlN5 nitrides

High pressure can stimulate numerous novel physical effects which are not observed under ambient conditions, such as the electronic redistribution and delocalization phenomenon in strongly covalently bonded nitrides. Through first principles simulations, we report a new N-rich aluminum nitride AlN5, which crystallizes with the space group P1[combining macron] at 20 GPa and then transforms into the I4[combining macron]2d phase at 60 GPa. We have identified and proved the delocalization effects of π electrons in the strongly covalent Lewis poly-nitrogen structure via the one-dimensional particle in a box mechanism, which contributes to the metallization and stability of the system. This implies that not all strongly covalently bonded systems with highly localized electrons exhibit nonmetallic properties in III-V main group nitrides. Furthermore, pressure results in the hybridization configuration mutation from sp2 in the P1[combining macron] phase to a mixture of sp2 and sp3 hybridization in the I4[combining macron]2d phase, which leads to phase transition from metal to insulator. With increasing pressure, the band gap increases abnormally, exhibiting anti-metallization induced by the strong hybridization. Interestingly, the P1[combining macron] and I4[combining macron]2d structures are simultaneously accompanied by a high energy density and hardness, which enable them to have a greater ability to resist elasticity, plastic deformation and external force destruction in potential applications. Their energy density and hardness are up to 3.29 kJ g-1 and 15.2 GPa in the P1[combining macron] phase but especially 6.14 kJ g-1 and 31.7 GPa in the I4[combining macron]2d phase.

Hexagonal Layered Polymeric Nitrogen Phase Synthesized near 250 GPa

The nitrogen triple bond dissociates in the 100 GPa pressure range and a rich variety of single-bonded polymeric nitrogen structures unique to this element have been predicted up to the terapascal pressure range. The nonmolecular cubic-gauche (cg-N) structure was first observed above 110 GPa, coupled to high temperature (>2000  K) to overcome the kinetic barrier. A mixture of cg-N with a layered phase was afterwards reported between 120 and 180 GPa. Here, by laser heating pure nitrogen from 180 GPa, a sole crystalline phase is characterized above 240 GPa while an amorphous transparent phase is obtained at pressures below. X-ray diffraction and Raman vibrational data reveal a tetragonal lattice (P4_{2}bc) that matches the predicted hexagonal layered polymeric nitrogen (HLP-N) structure. Density-functional theory calculations which include the thermal and dispersive interaction contributions are performed to discuss the stability of the HLP-N structure.

Origin of the considerably high thermal stability of cyclo-N5− containing salts at ambient conditions

Ionization, conjugation, hydrogen bonding, coordination bonding and π–π stacking consolidate the cyclo-N₅⁻ caged in salt crystals.

Unusually complex phase of dense nitrogen at extreme conditions

Nitrogen exhibits an exceptional polymorphism under extreme conditions, making it unique amongst the elemental diatomics and a valuable testing system for experiment-theory comparison. Despite attracting considerable attention, the structures of many high-pressure nitrogen phases still require unambiguous determination. Here, we report the structure of the elusive high-pressure high-temperature polymorph ι–N₂ at 56 GPa and ambient temperature, determined by single crystal X-ray diffraction, and investigate its properties using ab initio simulations. We find that ι–N₂ is characterised by an extraordinarily large unit cell containing 48 N₂ molecules. Geometry optimisation favours the experimentally determined structure and density functional theory calculations find ι–N₂ to have the lowest enthalpy of the molecular nitrogen polymorphs that exist between 30 and 60 GPa. The results demonstrate that very complex structures, similar to those previously only observed in metallic elements, can become energetically favourable in molecular systems at extreme pressures and temperatures.

Nitrogen has a complex phase diagram with rich polymorphism, which is challenging to characterize due to the extreme conditions and uncertain stability ranges needed to do so. Here the authors resolve one of the most elusive phases of this model system, reporting a crystalline structure with unusual complexity.

Polymeric Nitrogen A7 Layers Stabilized in the Confinement of a Multilayer BN Matrix at Ambient Conditions

Polymeric nitrogen, as a potential high-energy-density material (HEDM), has attracted many theoretical calculations and predictions for its potential applications, such as energy storage, propellants and explosives. Searching for an effective method to stabilize polymeric nitrogen in ambient conditions of temperature and pressure has become a hot topic. Herein, we propose a new hybrid material where polymeric nitrogen layers are intercalated in a multilayer BN matrix forming a three-dimensional structure, by means of ab initio density functional theory. It is demonstrated polymeric nitrogen layers can be stable at ambient conditions and can release tremendous energy just above 500 K, more gentle and controllable. Further calculations reveal the new hybrid material exhibits a much smaller charge transfer than that of previous reports, which not only stabilizes polymeric nitrogen layer at ambient conditions, but also favours energy releasing at milder conditions. It is also very exciting that, the weight ratio of polymeric nitrogen in new material is up to 53.84%, approximately three times higher than previous one-dimensional hybrid materials. The energy density (5.4 KJ/g) also indicates it is a promising HEDMs candidate. Our findings provide a new insight into nitrogen-based HEDMs capture and storage.

Metallization and molecular dissociation of dense fluid nitrogen

Diatomic nitrogen is an archetypal molecular system known for its exceptional stability and complex behavior at high pressures and temperatures, including rich solid polymorphism, formation of energetic states, and an insulator-to-metal transformation coupled to a change in chemical bonding. However, the thermobaric conditions of the fluid molecular–polymer phase boundary and associated metallization have not been experimentally established. Here, by applying dynamic laser heating of compressed nitrogen and using fast optical spectroscopy to study electronic properties, we observe a transformation from insulating (molecular) to conducting dense fluid nitrogen at temperatures that decrease with pressure and establish that metallization, and presumably fluid polymerization, occurs above 125 GPa at 2500 K. Our observations create a better understanding of the interplay between molecular dissociation, melting, and metallization revealing features that are common in simple molecular systems.

Nitrogen is a model system still presenting unknown behaviors at the pressures and temperatures typical of deep planets’ interiors. Here the authors explore, by pulsed laser heating in a diamond anvil cell and optical measurements, the metallization and non-molecular states of nitrogen in a previously unexplored domain above 1 Mbar and at 2000-7000K.

Route to high-energy density polymeric nitrogen t-N via He-N compounds

Polymeric nitrogen, stabilized by compressing pure molecular nitrogen, has yet to be recovered to ambient conditions, precluding its application as a high-energy density material. Here we suggest a route for synthesis of a tetragonal polymeric nitrogen, denoted t-N, via He-N compounds at high pressures. Using first-principles calculations with structure searching, we predict a class of nitrides with stoichiometry HeN₄ that are energetically stable (relative to a mixture of solid He and N₂) above 8.5 GPa. At high pressure, HeN₄ comprises a polymeric channel-like nitrogen framework filled with linearly arranged helium atoms. The nitrogen framework persists to ambient pressure on decompression after removal of helium, forming pure polymeric nitrogen, t-N. t-N is dynamically and mechanically stable at ambient pressure with an estimated energy density of ~11.31 kJ/g, marking it out as a remarkable high-energy density material. This expands the known polymeric forms of nitrogen and indicates a route to its synthesis.

Structural phase transition and bonding properties of high-pressure polymeric CaN3

Two new monoclinic P1̄-CaN₃ and C2/m-CaN₃ are predicted to become energetically stable under low pressure. For the first time, we identify one novel phase featuring charged “N₆” chain in the P1̄-CaN₃ structure.