High-pressure phases of nitrogen

Remarkable stability of γ - N 2 and its prevalence in the nitrogen phase diagram

Solid nitrogen exhibits a panoply of phenomena ranging from complex molecular crystalline configurations to polymerization and closing band gap at higher densities. Among the elemental molecular solids, nitrogen stands apart for having phases, which can only be stabilized following particular pressure-temperature pathways, indicative of metastability and kinetic barriers. Here, through the combination of Raman spectroscopy and dynamic compression techniques, we find that the appearance of the whole nitrogen phase diagram is determined by the P-T paths taken below 2 GPa. We reveal the existence of the path- and phase-dependent triple point between the β - N 2 , δ loc - N 2 and γ - or ϵ - N 2 . We further show that the β - N 2 towards γ - N 2 path below the triple point, that evades δ ( δ loc )- N 2 , results in the formation of γ - N 2 , which in turn becomes a dominant phase. We then demonstrate, that the β - N 2 through δ ( δ loc )- N 2 above the triple point path leads to the formation of ϵ - N 2 and the "well-established" phase diagram. An additional pathway, which by-passes the rotationally inhibited modifications δ ( δ loc )- N 2 , via rapid compression is found to produce γ - N 2 at higher temperatures. We argue that the pathway and phase sensitive triple point and the compression rate dependent phase formation challenge our understanding of this archetypal dense molecular solid.

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.

Stability, Chemical Bonding, and Electron Lone Pair Localization in AsN at High Pressure by Density Functional Theory Calculations

The covalent bonding framework of crystalline single-bonded cubic AsN, recently synthesized under high pressure and high temperature conditions in a laser-heated diamond anvil cell, is here studied by means of density functional theory calculations and compared to single crystal X-ray diffraction data. The precise localization of the nonbonding electron lone pairs and the determination of their distances and orientations are related to the presence of characteristic structural motifs and space regions of the unit cell dominated by repulsive electronic interactions, with the relative orientation of the electron lone pairs playing a key role in minimizing the energy of the structure. We find that the vibrational modes associated with the expression of the lone pairs are strongly localized, an observation that may have implications for the thermal conductivity of the compound. The results indicate the thermodynamic stability of the experimentally observed structure of AsN above ∼17 GPa, provide a detailed insight into the nature of the chemical bonding network underlying the formation of this compound, and open new perspectives to the design and high pressure synthesis of new pnictogen-based advanced materials for potential applications of energetic and technological relevance.

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.

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.

Novel superhard semiconducting structures of C8B2N2 predicted using the first-principles approach

Using first-principles calculations, we predicted three novel superhard semiconducting structures of C8B2N2 with a space group of P3m1. We investigated their mechanical properties and electronic structures up to 100 GPa. These three structures were successfully derived by substituting carbon (C) atoms with isoelectronic boron (B) and nitrogen (N) atoms in the P3m1 phase, which is the most stable structure of BCN and exhibits exceptional mechanical properties. Our results indicated that these structures had superior energy over previously reported t-C8B2N2, achieved by replacing C atoms in the diamond supercell with B and N atoms. To ensure their stable existence, we thoroughly examined their mechanical and dynamical stabilities, and we found that their hardness values reached 82.4, 83.1, and 82.0 GPa, which were considerably higher than that of t-C8B2N2 and even surpassing the hardness of c-BN. Calculations of the electron localization function revealed that the stronger carbon-carbon covalent bonds made them much harder than t-C8B2N2. Additionally, our further calculations of band structures revealed that these materials had indirect bandgaps of 4.164, 4.692, and 3.582 eV. These findings suggest that these materials have the potential to be used as superhard semiconductors, potentially surpassing conventional superhard 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.

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

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

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.

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

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

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.

Polymeric Hydronitrogen N4H: A Promising High-Energy-Density Material and High-Temperature Superconductor

Solid nitrogen-rich compounds are potential high-energy-density materials (HEDMs). The enormous challenge in this area is to synthesize and stabilize these energetic materials at moderate pressure and better under near-ambient conditions. Here, we perform an extensive theoretical study on hydronitrogens by the reverse design method considering both energies and energy densities. Four hydronitrogens with different stoichiometries, that is, N4H, N3H, N2H, and NH, are found to be stable at pressures of about 80-300 GPa and metastable with pressure releasing to ambient pressure. The energy densities of these hydronitrogens are about 5.6-6.5 kJ/g and 1.3-1.5 times larger than that of trinitrotoluene (TNT). Most importantly, the Pbam phase of the N4H compound is an excellent high-temperature superconductor with a Tc of 37.7 K at 72 GPa. The present findings enrich new phases of hydronitrogens under high pressure and characterize their structural and energetic properties and superconductivity, which offer crucial insights for further design and synthesis of exceptional materials with high energy density and high-temperature superconductivity.

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.

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.

Formation of twelve-fold iodine coordination at high pressure

Halogen compounds have been studied widely due to their unique hypercoordinated and hypervalent features. Generally, in halogen compounds, the maximal coordination number of halogens is smaller than eight. Here, based on the particle swarm optimization method and first-principles calculations, we report an exotically icosahedral cage-like hypercoordinated IN6 compound composed of N6 rings and an unusual iodine-nitrogen covalent bond network. To the best of our knowledge, this is the first halogen compound showing twelve-fold coordination of halogen. High pressure and the presence of N6 rings reduce the energy level of the 5d orbitals of iodine, making them part of the valence orbital. Highly symmetrical covalent bonding networks contribute to the formation of twelve-fold iodine hypercoordination. Moreover, our theoretical analysis suggests that a halogen element with a lower atomic number has a weaker propensity for valence expansion in halogen nitrides.

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...

Semiconductors with Chiral Crystal Structure in Group IVB Transition Metal Pernitrides

Group IVB transition metal (TM) nitrides rarely exhibit semiconductor phase, except for TM3N4 (TM = Ti, Zr, and Hf) compounds. In this study, using the ab-initio calculations based on density...

Crystal structure of silver pentazolates AgN5 and AgN6

Silver pentazolate, a high energy density compound containing the cyclo-N₅^- anion, has recently been synthesized under ambient conditions. However, due to high sensitivity to irradiation, its crystal structure has not been determined. In this work, silver-nitrogen crystalline compounds under ambient conditions and at high pressures, up to 100 GPa, are predicted and characterized by performing first-principles evolutionary crystal structure searching with variable stoichiometry. It is found that newly discovered AgN₅ and AgN₆ are the only thermodynamically stable silver-nitrogen compounds at pressures between 42 and 80 GPa. In contrast to AgN₅, the pentazolate AgN₆ compound contains N₂ diatomic molecules in addition to cyclo-N₅^-. These AgN₅ and AgN₆ crystals are metastable under ambient conditions with positive formation enthalpies of 54.95 kJ mol^-1 and 46.24 kJ mol^-1, respectively. The underlying cause of the stability of cyclo-N₅^- silver pentazolates is the enhanced aromaticity enabled by the charge transfer from silver atoms to nitrogen rings. To aid in the experimental identification of these materials, calculated Raman spectra are reported at ambient pressure: the frequencies of N₅^- vibrational modes of AgN₅ are in good agreement with those measured in the experiment.

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.

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.

Polymerization of Nitrogen in Nitrogen-Fluorine Compounds under Pressure

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

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.

Modeling the α- and β-resorcinol phase boundary via combination of density functional theory and density functional tight-binding

The ability to predict not only what organic crystal structures might occur but also the thermodynamic conditions under which they are the most stable would be extremely useful for discovering and designing new organic materials. The present study takes a step in that direction by predicting the temperature- and pressure-dependent phase boundary between the α and β polymorphs of resorcinol using density functional theory (DFT) and the quasi-harmonic approximation. To circumvent the major computational bottleneck associated with computing a well-converged phonon density of states via the supercell approach, a recently developed approximation is employed, which combines a supercell phonon density of states from dispersion-corrected third-order density functional tight binding [DFTB3-D3(BJ)] with frequency corrections derived from a smaller B86bPBE-XDM functional DFT phonon calculation on the crystallographic unit cell. This mixed DFT/DFTB quasi-harmonic approach predicts the lattice constants and unit cell volumes to within 1%-2% at lower pressures. It predicts the thermodynamic phase boundary in almost perfect agreement with the experiment, although this excellent agreement does reflect fortuitous cancellation of errors between the enthalpy and entropy of transition.

Tungsten Hexanitride with Single-Bonded Armchairlike Hexazine Structure at High Pressure

WN_{6} phase discovered at 126-165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group R3[over ¯]m which is consistent with the WN_{6} structure with armchairlike hexazine (N_{6}) rings, while strong A_{1g} Raman mode confirms its N─N single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent N_{6} rings, and repulsions between N_{6} rings that make WN_{6} ultrastiff and tough. The WN_{6} phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

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.

Crystalline Structures and Energetic Properties of Lithium Pentazolate under Ambient Conditions

Recently, it has been reported that high-pressure synthesized lithium pentazolates could be quenched down to ambient conditions. However, the crystalline structures of LiN5 under ambient conditions are still ambiguous. In this work, the structures of LiN5 compound were directly explored at atmospheric pressure by using a new constrain structure search method. By using this method, three new allotropes were confirmed, and they show lower energy than the previous reported LiN5 phases. Both their thermodynamic and dynamic stability were confirmed through formation enthalpies, phonon spectrum, and ab initio molecular dynamics simulations under ambient conditions. Moreover, these three allotropes show similar formation enthalpies and properties, which suggests that it is hard to obtain a single LiN5 phase, which is well consistent with the experimental phenomenon. Furthermore, because of their low formation energy, all of them possess low energy density when they directly decompose to Li3N and nitrogen (0.52 kJ/g). Instead, the decomposed energy could be further improved to 3.78 kJ/g when they decompose under an oxygen-rich environment.

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.

Prediction of new thermodynamically stable ZnN2O3 at high pressure

Pressure has become a useful parameter to prepare novel functional materials. Considering the excellent performance of ZnO and Zn₃N₂ and the formation of strong Zn-O, Zn-N, and N-O bonds in the known compounds, we explored potential Zn-N-O ternary compounds with interesting properties. With the aid of first-principles swarm-intelligence search calculations, we identified a hitherto unknown ZnN₂O₃ ternary compound with a symmetry of P2₁. Its remarkable feature is that N pairs interconnect the distorted Zn-centered decahedrons, in which the Zn atom forms bonds with one N and six O atoms. The compression of ZnO + NO₂ + N₂ might be an easy way to synthesize ZnN₂O₃. Electronic property calculations disclose that ZnN₂O₃ is a wide band gap semiconductor with a gap value of 3.48 eV, which is larger than those of ZnO and Zn₃N₂. Moreover, the high-pressure phase diagram of Zn-N binary compounds was explored with a wide range of chemical compositions. Two metallic N-rich zinc nitrides (e.g., ZnN₂ and ZnN₄) are proposed, containing intriguing N₂ dimers and zigzag N chains. ZnN₂ exhibits superconducting properties, and becomes the first example of superconductor in zinc nitrides. Our current results unravel the unusual stoichiometry of Zn-N-O compounds and provide further insight into the diverse electronic properties of zinc nitrides under high pressure.

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.

Stabilization of the High-Energy-Density CuN5 Salts under Ambient Conditions by a Ligand Effect

A series of excellent works have demonstrated that high-nitrogen-content metal pentazolate (cyclo-N5 -) compounds could be stabilized by high pressure. However, under ambient conditions, low stability precludes their synthesis and application in the field of high-energy-density material. In this work, by using a constrained structure search method, we predicted two new structures as P212121-CuN5 and P21/c-CuN5 containing cyclo-N5 - with strong N-N and Cu-N bonds. In both structures, cyclo-N5 - form four coordination with the Cu+ ligand, which increases the structural stability by lowering the disturbance to the aromaticity of cyclo-N5 -. The calculated results show that the P212121-CuN5 and P21/c-CuN5 structures exhibit high dynamic and thermal stability up to 400 K, indicating that they can be stabilized under ambient conditions. The decomposing energy of P212121-CuN5 and P21/c-CuN5 can reach up to 2.40 and 2.42 kJ/g, respectively. Strikingly, the detonation velocity and the pressure of P212121-CuN5 is predicted to be up to 10.42 km/s and 617.46 kbar, respectively, indicating that they are promising high-energy candidates in the field of explosive combustion.

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.

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.

Direct Reaction between Copper and Nitrogen at High Pressures and Temperatures

Transition-metal nitrides have applications in a range of technological fields. Recent experiments have shown that new nitrogen-bearing compounds can be accessed through a combination of high temperatures and pressures, revealing a richer chemistry than was previously assumed. Here, we show that at pressures above 50 GPa and temperatures greater than 1500 K  elemental copper reacts with nitrogen, forming copper diazenide (CuN₂). Through a combination of synchrotron X-ray diffraction and first-principles calculations we have explored the stability and electronic structure of CuN₂. We find that the novel compound remains stable down to 25 GPa before decomposing to its constituent elements. Electronic structure calculations show that CuN₂ is metallic and exhibits partially filled N₂ antibonding orbitals, leading to an ambiguous electronic structure between Cu⁺/Cu²⁺. This leads to weak Cu-N bonds and the lowest bulk modulus observed for any transition-metal nitride.

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.

Prediction of Thermodynamically Stable Compounds of the Sc-N System under High Pressure

Materials under high pressure often exhibit unusual physical and chemical behaviors. We investigated the Sc-N system under high pressure in the range of 0-110 GPa using variable-composition methodology implemented in Universal Structure Predictor: Evolutionary Xtallograpgy (USPEX) in conjunction with Vienna Ab Initio Simulation Package (VASP). The calculation led to prediction of new thermodynamically stable compounds, Sc4N3, Sc8N7, ScN3, and ScN5, and also phase transition of ScN5 from triclinic to monoclinic at 104 GPa. These results are important to understand the structure of Sc-N compounds under high pressure and their elastic and electronic properties.

The polymerization of nitrogen in Li2N2 at high pressures

The polymerization of nitrogen can be used as high energy density materials. The crystal structures of Li₂N₂ at high pressures are explored by using the first-principles method combined with evolutionary algorithm. The phase transitions Pmmm → Immm → Pnma → Cmcm-1 → I4₁/acd are predicted in the pressure range of 0–300 GPa. Enthalpy calculations reveal that the tetragonal phase I4₁/acd containing the spiral nitrogen chains is stable above 242 GPa, indicating that the polymerization of nitrogen is realized in Li₂N₂ under pressure.