Prediction of new phases of nitrogen at high pressure from first-principles simulations

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.

Sodium catalytic phenylpentazole cracking: a theoretical study

This work presents an in-depth investigation into the cracking reaction mechanism of phenylpentazole (C6H5N5) under the catalytic influence of sodium metal, utilizing density functional theory. The geometries of the reactants, transition states, intermediates, and products are meticulously optimized employing the GGA/PW91/DNP level of theory. Also, a rigorous analysis is undertaken, encompassing various key factors including configuration parameters, Mulliken charges, densities of states, and reaction energies. Three distinct reaction pathways are comprehensively examined, shedding light on the intricate details and intricacies of each pathway. The results show that a remarkable outcome in which the activation energy of the C6H5N5 cracking reaction releases N2, facilitated by catalytic metal Na, reveals a strikingly reduced value of a mere 5.2 kcal mol-1 compared to the previously reported activation energies ranging from 20 to 30 kcal mol-1. Evidently, this significantly lowered barrier can be readily surpassed at typical room temperatures, exhibiting practical applicability. Notably, the alkali metal Na effectively serves as a catalyst, successfully diminishing the activation energy required for N2 production through the pyrolysis of pentazole compounds. This breakthrough discovery provides a theoretical basis for experimental research on the low-temperature cracking of pentazole compounds. It also offers valuable insights for the development and application of new high energy density materials, contributing to the creation of a green and low-carbon circular economic system.

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.

Structurally Constrained Evolutionary Algorithm for the Discovery and Design of Metastable Phases

Metastable materials are abundant in nature and technology, showcasing remarkable properties that inspire innovative materials design. However, traditional crystal structure prediction methods, which rely solely on energetic factors to determine a structure's fitness, are not suitable for predicting the vast number of potentially synthesizable phases that represent a local minimum corresponding to a state in thermodynamic equilibrium. Here, we present a new approach for the prediction of metastable phases with specific structural features and interface this method with the XtalOpt evolutionary algorithm. Our method relies on structural features that include the local crystalline order (e.g, the coordination number or chemical environment), and symmetry (e.g, Bravais lattice and space group) to filter the breeding pool of an evolutionary crystal structure search. The effectiveness of this approach is benchmarked on three known metastable systems: XeN8, with a two-dimensional polymeric nitrogen sublattice, brookite TiO2, and a high pressure BaH4 phase, which was recently characterized. Additionally, a newly predicted metastable melaminate salt, P1̅ WC3N6, was found to possess an energy that is lower than that of two phases proposed in a recent computational study. The method presented here could help in identifying the structures of compounds that have already been synthesized, and in developing new synthesis targets with desired properties.

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.

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.

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.

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.

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.

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.

Hydrogen Bond and π-π Stacking Interaction: Stabilization Mechanism of Two Metal Cyclo-N5 --Containing Energetic Materials

In recent years, cyclo-N₅ ^- has attracted extensive attention because all-nitrogen high-energy-density materials (HEDMs) have been expected to reach a TNT equivalent of over 3.0. However, for cyclo-N₅ ^--containing HEDMs, the stabilization mechanism has remained enigmatic. In this study, two typical cyclo-N₅ ^--containing metal hydrates, [Na(H₂O)(N₅)]·2H₂O (Na-cyclo-N₅ ^-) and [Mg(H₂O)₆(N₅)₂]·4H₂O (Mg-cyclo-N₅ ^-), are selected to gain insights into the factors affecting their stability by the first-principles method. Both binding/lattice energy calculations and density of states analysis show that Mg-cyclo-N₅ ^- is more stable than Na-cyclo-N₅ ^-. Hydrogen bonding is the main stabilization mechanism for stabilizing crystals and cyclo-N₅ ^-. Two types of hydrogen bonds, O-H···O and O-H···N, are clarified, which construct a 3D hydrogen bond network in Mg-cyclo-N₅ ^- and an intralayer 2D hydrogen bond network in Na-cyclo-N₅ ^-. Moreover, nonuniform stress causes distortion of cyclo-N₅ ^-. Comparing the two samples, the distortion degree of cyclo-N₅ ^- is higher in Na-cyclo-N₅ ^-, which indicates that cyclo-N₅ ^- decomposition is easier. These findings will enhance the future prospects for the design and synthesis of cyclo-N₅ ^--containing HEDMs.

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.

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.

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.

Energetics of polymeric carbon monoxide

The transformation of carbon monoxide (CO) from a molecular liquid to a polymeric solid under isothermal compression at room temperature is investigated using first principles theory. We report structural and thermodynamic properties from ambient density up to 2.45 g/cc obtained using density functional theory molecular dynamics simulations, including hybrid exchange corrections. The theoretical results are compared with newly obtained polymeric CO samples, synthesized in a large volume press. The explosive performance of polymeric CO is predicted and discussed. Under most favorable assumptions, it is found to be comparable to trinitrotoluene.

Pressure-induced novel nitrogen-rich aluminum nitrides: AlN6, Al2N7 and AlN7 with polymeric nitrogen chains and rings

Pressure-induced non-molecular phases of polymeric nitrogen have potential applications in the field of energetic materials. Here, through a structural search method combined with first-principles calculations, we have predicted four novel nitrogen-rich aluminum nitrides C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7. Nitrogen atoms polymerize into infinite chains in C2/m-AlN6, C2-Al2N7 and P1-AlN7 structures and form N3 chains and N4 rings in Cm-Al2N7. C2/m-AlN6 is stable in the pressure range from 30 to 80 GPa and Cm-Al2N7, C2-Al2N7 and P1-AlN7 are metastable in the pressure ranges of 35-65, 65-80 and 41-80 GPa, respectively. The present study predicts that C2/m-AlN6 has a superconducting transition temperature of 18.9 K at 0 GPa and can be quenched and recovered under ambient conditions. The energy densities of C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7 compounds are expected to be 4.80, 4.59, 5.46 and 5.59 kJ g-1, respectively, due to their high nitrogen content, indicating that they have potential to be high-energy density materials. The calculated Vickers hardness of C2/m-AlN6, Cm-Al2N7, Cm-Al2N7 and P1-AlN7 is 43.86, 39.32, 63.96 and 33.58 GPa, respectively, showing that the novel nitrogen-rich aluminum nitrides are potential superhard materials as well. This study may encourage further experimental exploration of high N content superhard or high-energy density nitrides.

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.

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.

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.

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.

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.

High energetic polymeric nitrogen sheet confined in a graphene matrix

Polymeric nitrogen, as a potential high-energy-density material (HEDM), has many applications, such as in energy storage systems, explosives and propellants. Nowadays it is very urgent to find a suitable method to stabilize polymeric nitrogen at ambient conditions. Herein, we present a new hybrid structure where polymeric nitrogen sheets are sandwiched between graphene sheets in the form of a three-dimensional crystal. According to ab initio molecular dynamics (AIMD) calculations and phonon spectrum calculations, it is demonstrated that polymeric nitrogen sheets are stable at ambient pressure and temperature. The hybrid material has a higher nitrogen content (the weight ratio of nitrogen is up to 53.84%), and the corresponding energy density is 5.2 kJ g-1. The hybrid material (A7@graphene system) has a satisfactory energy density, detonation velocity and detonation pressure. Importantly, the hybrid material can be preserved up to 450 K, and above this temperature, the polymeric nitrogen sheets break up into polymeric nitrogen chains or nitrogen gases and release tremendous energy. Further calculations reveal that small charge transfer between the polymeric nitrogen sheets and graphene sheets creates a weak electrostatic attraction compared with other hybrid materials, which is just good for the stabilization of the polymeric nitrogen sheets at ambient conditions, and favors energy release in a gentle way. The proposed confinement hybrid material which has a high energy density and a gentle energy release temperature, provides a highly promising method for the capture and application of polymeric nitrogen in a controllable way.

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.

Intermolecular Interactions in Highly Disordered, Confined Dense N2

Molecular nitrogen is a benchmark system for condensed matter and, in particular, for looking at universal properties of strongly confined dense systems. We conducted Raman and X-ray diffraction measurements on a dense and disordered form of molecular nitrogen subnanoconfined in a noncatalytic pure SiO₂ zeolite under pressure, up to 50 GPa. In this form, N₂-N₂ interactions and, consequently, distances are found to be very close to those of bulk N₂ and intramolecular interactions progressively weaken upon increasing pressure. Surprisingly, the filled zeolite is still crystalline at 50 GPa with silicon in tetrahedral coordination by oxygen, which is a record pressure for this type of coordination among all the known forms of silica. We have thus found a rationale for the polymerization of a number molecules occurring in the microchannels of noncatalytic zeolites under pressure, where the pressure threshold is found to be very similar to that observed in bulk samples.

Alkaline-earth metal (Mg) polynitrides at high pressure as possible high-energy materials

The P1̄-MgN₃ and P1̄-MgN₄ are predicted to become energetically stable under pressure, suggesting that it may be prepared by high-pressure synthesis. P1̄-MgN₃ and P1̄-MgN₄ are expected to release an enormously large amount of energy (2.83 and 2.01 kJ g⁻¹). The present study encourages experimental exploration of these promising materials in the future.

Super high-energy density single-bonded trigonal nitrogen allotrope—a chemical twin of the cubic gauche form of nitrogen

A new ambient-pressure metastable single-bonded nitrogen allotrope was predicted using reliable theoretical methods. The predicted allotrope has a number of similarities with the experimentally detected cubic gauche nitrogen allotrope.

High-pressure studies with x-rays using diamond anvil cells

Pressure profoundly alters all states of matter. The symbiotic development of ultrahigh-pressure diamond anvil cells, to compress samples to sustainable multi-megabar pressures; and synchrotron x-ray techniques, to probe materials' properties in situ, has enabled the exploration of rich high-pressure (HP) science. In this article, we first introduce the essential concept of diamond anvil cell technology, together with recent developments and its integration with other extreme environments. We then provide an overview of the latest developments in HP synchrotron techniques, their applications, and current problems, followed by a discussion of HP scientific studies using x-rays in the key multidisciplinary fields. These HP studies include: HP x-ray emission spectroscopy, which provides information on the filled electronic states of HP samples; HP x-ray Raman spectroscopy, which probes the HP chemical bonding changes of light elements; HP electronic inelastic x-ray scattering spectroscopy, which accesses high energy electronic phenomena, including electronic band structure, Fermi surface, excitons, plasmons, and their dispersions; HP resonant inelastic x-ray scattering spectroscopy, which probes shallow core excitations, multiplet structures, and spin-resolved electronic structure; HP nuclear resonant x-ray spectroscopy, which provides phonon densities of state and time-resolved Mössbauer information; HP x-ray imaging, which provides information on hierarchical structures, dynamic processes, and internal strains; HP x-ray diffraction, which determines the fundamental structures and densities of single-crystal, polycrystalline, nanocrystalline, and non-crystalline materials; and HP radial x-ray diffraction, which yields deviatoric, elastic and rheological information. Integrating these tools with hydrostatic or uniaxial pressure media, laser and resistive heating, and cryogenic cooling, has enabled investigations of the structural, vibrational, electronic, and magnetic properties of materials over a wide range of pressure-temperature conditions.

Modeling Polymorphic Molecular Crystals with Electronic Structure Theory

Interest in molecular crystals has grown thanks to their relevance to pharmaceuticals, organic semiconductor materials, foods, and many other applications. Electronic structure methods have become an increasingly important tool for modeling molecular crystals and polymorphism. This article reviews electronic structure techniques used to model molecular crystals, including periodic density functional theory, periodic second-order Møller-Plesset perturbation theory, fragment-based electronic structure methods, and diffusion Monte Carlo. It also discusses the use of these models for predicting a variety of crystal properties that are relevant to the study of polymorphism, including lattice energies, structures, crystal structure prediction, polymorphism, phase diagrams, vibrational spectroscopies, and nuclear magnetic resonance spectroscopy. Finally, tools for analyzing crystal structures and intermolecular interactions are briefly discussed.

Exotic stable cesium polynitrides at high pressure

New polynitrides containing metastable forms of nitrogen are actively investigated as potential high-energy-density materials. Using a structure search method based on the CALYPSO methodology, we investigated the stable stoichiometries and structures of cesium polynitrides at high pressures. Along with the CsN3, we identified five new stoichiometric compounds (Cs3N, Cs2N, CsN, CsN2, and CsN5) with interesting structures that may be experimentally synthesizable at modest pressures (i.e., less than 50 GPa). Nitrogen species in the predicted structures have various structural forms ranging from single atom (N) to highly endothermic molecules (N2, N3, N4, N5, N6) and chains (N∞). Polymeric chains of nitrogen were found in the high-pressure C2/c phase of CsN2. This structure contains a substantially high content of single N-N bonds that exceeds the previously known nitrogen chains in pure forms, and also exhibit metastability at ambient conditions. We also identified a very interesting CsN crystal that contains novel N4(4-) anion. To our best knowledge, this is the first time a charged N4 species being reported. Results of the present study suggest that it is possible to obtain energetic polynitrogens in main-group nitrides under high pressure.

Nitrogen oxides under pressure: stability, ionization, polymerization, and superconductivity

Nitrogen oxides are textbook class of molecular compounds, with extensive industrial applications. Nitrogen and oxygen are also among the most abundant elements in the universe. We explore the N-O system at 0 K and up to 500 GPa though ab initio evolutionary simulations. Results show that two phase transformations of stable molecular NO₂ occur at 7 and 64 GPa, and followed by decomposition of NO₂ at 91 GPa. All of the NO⁺NO₃⁻ structures are found to be metastable at T = 0 K, so experimentally reported ionic NO⁺NO₃⁻ is either metastable or stabilized by temperature. N₂O₅ becomes stable at 9 GPa, and transforms from P-1 to C2/c structure at 51 GPa. NO becomes thermodynamically stable at 198 GPa. This polymeric phase is superconducting (T_c = 2.0 K) and contains a -N-N- backbone.

Nitrogen Backbone Oligomers

We found that nitrogen and hydrogen directly react at room temperature and pressures of ~35 GPa forming chains of single-bonded nitrogen atom with the rest of the bonds terminated with hydrogen atoms - as identified by IR absorption, Raman, X-ray diffraction experiments and theoretical calculations. At releasing pressures below ~10 GPa, the product transforms into hydrazine. Our findings might open a way for the practical synthesis of these extremely high energetic materials as the formation of nitrogen-hydrogen compounds is favorable already at pressures above 2 GPa according to the calculations.

Crystalline LiN5 Predicted from First-Principles as a Possible High-Energy Material

The search for stable polymeric nitrogen and polynitrogen compounds has attracted great attention due to their potential applications as high-energy-density materials. Here we report a theoretical prediction of an interesting LiN5 crystal through first-principles calculations and unbiased structure searching techniques. Theoretical calculations reveal that crystalline LiN5 is thermodynamically stable at pressures above 9.9 GPa, and remains metastable at ambient conditions. The metastability of LiN5 stems from the inherent stability of the N5(-) anions and strong anion-cation interactions. It is therefore possible to synthesize LiN5 by compressing solid LiN3 and N2 gas under high pressure and quench recover the product to ambient conditions. To the best of our knowledge, this is the first time that stable N5(-) anions are predicted in crystalline states. The weight ratio of nitrogen in LiN5 is nearly 91%, placing LiN5 as a promising high-energy material. The decomposition of LiN5 is expected to be highly exothermic, releasing an energy of approximately 2.72 kJ·g(-1). The present results open a new avenue to synthesize polynitrogen compounds and provide a key perspective toward the understanding of novel chemical bonding in nitrogen-rich compounds.

Transformation pathways in high-pressure solid nitrogen: from molecular N2 to polymeric cg-N

The transformation pathway in high-pressure solid nitrogen from N2 molecular state to polymeric cg-N phase was investigated by means of ab initio molecular dynamics and metadynamics simulations. In our study, we observed a transformation mechanism starting from molecular Immm phase that initiated with formation of trans-cis chains. These chains further connected within layers and formed a chain-planar state, which we describe as a mixture of two crystalline structures--trans-cis chain phase and planar phase, both with Pnma symmetry. This mixed state appeared in molecular dynamics performed at 120 GPa and 1500 K and in the metadynamics run at 110 GPa and 1500 K, where the chains continued to reorganize further and eventually formed cg-N. During separate simulations, we also found two new phases--molecular P2(1)/c and two-three-coordinated chain-like Cm. The transformation mechanism heading towards cg-N can be characterized as a progressive polymerization process passing through several intermediate states of variously connected trans-cis chains. In the final stage of the transformation chains in the layered form rearrange collectively and develop new intraplanar as well as interplanar bonds leading to the geometry of cg-N. Chains with alternating trans and cis conformation were found to be the key entity--structural pattern governing the dynamics of the simulated molecular-polymeric transformation in compressed nitrogen.

Pressure-induced symmetry-lowering transition in dense nitrogen to layered polymeric nitrogen (LP-N) with colossal Raman intensity

We present the discovery of a novel nitrogen phase synthesized using laser-heated diamond anvil cells at pressures between 120-180 GPa well above the stability field of cubic gauche (cg)-N. This new phase is characterized by its singly bonded, layered polymeric (LP) structure similar to the predicted Pba2 and two colossal Raman bands (at ∼1000 and 1300  cm^{-1} at 150 GPa), arising from two groups of highly polarized nitrogen atoms in the bulk and surface of the layer, respectively. The present result also provides a new constraint for the nitrogen phase diagram, highlighting an unusual symmetry-lowering 3D cg-N to 2D LP-N transition and thereby the enhanced electrostatic contribution to the stabilization of this densely packed LP-N (ρ=4.85  g/cm^{3} at 120 GPa).

Pressure-induced planar N6 rings in potassium azide

The first-principles method and the evolutionary algorithm are used to identify stable high pressure phases of potassium azide (KN₃). It has been verified that the stable phase with space group I4/mcm below 22 GPa, which is consistent with the experimental result, will transform into the C2/m phase with pressure increasing. These two phases are insulator with anions. A metallic phase with P6/mmm symmetry is preferred above 40 GPa, and the N atoms in this structure form six-membered rings which are important for understanding the pressure effect on anions and phase transitions of KN₃. Above the studied pressure (100 GPa), a polymerization of N₆ rings may be obtained as the result of the increasing compactness.

Stable all-nitrogen metallic salt at terapascal pressures

The phase diagram and equation of state of dense nitrogen are of interest in understanding the fundamental physics and chemistry under extreme conditions, including planetary processes, and in discovering new materials. We predict several stable phases of nitrogen at multi-TPa pressures, including a P4/nbm structure consisting of partially charged N(2)(δ+) pairs and N(5)(δ-) tetrahedra, which is stable in the range 2.5-6.8 TPa. This is followed by a modulated layered structure between 6.8 and 12.6 TPa, which also exhibits a significant charge transfer. The P4/nbm metallic nitrogen salt and the modulated structure are stable at high pressures and temperatures, and they exhibit strongly ionic features and charge density distortions, which is unexpected in an element under such extreme conditions and could represent a new class of nitrogen materials. The P-T phase diagram of nitrogen at TPa pressures is investigated using quasiharmonic phonon calculations and ab initio molecular dynamics simulations.

Hydrogenated K4 carbon: a new stable cubic gauche structure of carbon hydride

The structural and electronic properties of hydrogenated K(4) carbon as a new cubic gauche structure in I2(1)3 symmetry are investigated using first-principles calculations. The total energy for this carbon hydride (labeled by K(4)-CH) is 0.47 eV per CH unit lower than that of solid molecular cubane, suggesting its energetic stability. Based on the calculated phonon dispersion curves and electronic band structure obtained by hybrid density functional method, we find that K(4)-CH is dynamically stable and exhibits as an insulator with an indirect band gap of 6.07 eV, which is close to 6.10 eV of cubic gauche nitrogen (cg-N). To study the doping effect of nitrogen, we have also investigated N-doped K(4)-CH with a composition of C(4)H(4)N(4) in P2(1)3 symmetry. The phonon and electronic band structures show that it is dynamically stable and behaves as an insulator with an indirect band gap of 5.39 eV, smaller than that of both K(4)-CH and cg-N. These results broaden our understanding of the cubic gauche structure.

Cagelike diamondoid nitrogen at high pressures

Under high pressure, triply bonded molecular nitrogen dissociates into singly bonded polymeric nitrogen, a potential high-energy-density material. The discovery of stable high-pressure forms of polymeric nitrogen is of great interest. We report the striking stabilization of cagelike diamondoid nitrogen at high pressures predicted by first-principles structural searches. The diamondoid structure of polymeric nitrogen has not been seen in any other elements, and it adopts a highly symmetric body-centered cubic structure with lattice sites occupied by diamondoids, each of which consists of ten nitrogen atoms, forming a N(10) tetracyclic cage. Diamondoid nitrogen possesses a wide energy gap and is energetically most stable among all known polymeric structures above 263 GPa, a pressure that is accessible to a high-pressure experiment. Our findings represent a significant step toward the understanding of the behavior of solid nitrogen at extreme conditions.

High-pressure phases of hydrogen cyanide: formation of hydrogenated carbon nitride polymers and layers and their electronic properties

We have performed a set of first-principles simulations to consider the possible phase transitions in molecular crystals of HCN under high pressure. Our calculations reveal several transition paths from the orthorhombic phase to tetragonal and then to triclinic phases. The transitions from the orthorhombic to the tetragonal phases are of the second order, whereas those from the tetragonal to the triclinic phases turn out to be of the first-order type and characterized by an abrupt decrease in volume. Our calculations show that, by adjustment of the temperature and pressure of the HCN molecular crystal, novel layered and polymeric crystals with insulating, semiconducting or metallic properties can be found. Based on our simulation results, two different crystal formation mechanisms are deduced. The stabilities of the predicted structures at ambient pressure are further assessed by performing phonon or MD simulations. In addition, the electron transport properties of the predicted polymers are obtained using the non-equilibrium Green's function technique combined with density functional theory. The results show that the polymers have metallic-like I-V characteristics.

Ab initio random structure searching

It is essential to know the arrangement of the atoms in a material in order to compute and understand its properties. Searching for stable structures of materials using first-principles electronic structure methods, such as density-functional-theory (DFT), is a rapidly growing field. Here we describe our simple, elegant and powerful approach to searching for structures with DFT, which we call ab initio random structure searching (AIRSS). Applications to discovering the structures of solids, point defects, surfaces, and clusters are reviewed. New results for iron clusters on graphene, silicon clusters, polymeric nitrogen, hydrogen-rich lithium hydrides, and boron are presented.