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

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

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 ScNx 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.

High nitrogen polymer in silver polynitrides.

The Ag-N system is enriched by proposing four polymeric structures in first-principles calculations at high-pressure. The proposedP-1-AgN2andP21/c-AgN2not only break the new record in the polymeric configuration of MN2compound, but also in the energy density. In the MN2stoichiometric ratio, the infinite armchair-antiarmchair chain and the layered polymer structure are reported for the first time inP-1-AgN2andP21/c-AgN2, indicating that the highly polymerized structure can also be formed in nitrides with low nitrogen content at high pressure. The mechanism of structure evolution behaviors of AgN2under pressure is revealed by the analysis of atomic orbital hybridization. Meanwhile, the high energy densities are obtained inP-1-AgN2(1.76 kJ g-1) andP21/c-AgN2(2.01 kJ g-1) compared to all the reported MN2nitrides. In addition, the infinite band-shaped structure and infinite curve layer structure are obtained inP-1-AgN5andC2/m-AgN6, respectively. The energy densities ofP-1-AgN5(3.82 kJ g-1) andC2/m-AgN6(4.51 kJ g-1) are comparable to or larger than that of TNT (4.3 kJ g-1). Moreover,P-1-AgN2,P21/c-AgN2,P-1-AgN5, andC2/m-AgN6exhibit excellent explosive performance in terms of detonation velocity and detonation pressure.

Pressure-Dependent Structural and Band Gap Tuning of Semiconductor Copper(I) Thiocyanate (CuSCN).

Copper(I) thiocyanate (CuSCN) is a p-type semiconductor with exceptional properties for optoelectronic devices such as solar cells, thin-film transistors , organic light-emitting diodes, etc. Understanding the structure-optical property relationships in CuSCN is critical for its optoelectronic applications. Herein, high-pressure techniques combined with theoretical calculations are used to thoroughly investigate the structural and optical changes of CuSCN upon compression. Under high pressure, CuSCN exhibits a progressive decrease of the band gap with different rates, which is relevant to the ß to α phase transition in CuSCN and the subsequent amorphization through polymerization. UV-vis spectra measurements reveal a reduction in band gap from 3.4 to 1.3 eV upon decompression to ambient conditions. Such transitions could be attributed to the pressure-induced rotation of CuNS3 tetrahedron and bond length shrinkage. The severe distortion of the polyhedral units prompts breakdown of the structure and thus the amorphization, which is quenchable to ambient conditions. Our study demonstrates that high pressure can be utilized to adjust the structure and optical characteristics of CuSCN compound, potentially extending the material's uses in optoelectronic devices.

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

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

High-Pressure Synthesis and Stability Enhancement of Lithium Pentazolate.

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

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.

Molecular insertion regulates the donor-acceptor interactions in cocrystals for the design of piezochromic luminescent materials.

Developing a universal strategy to design piezochromic luminescent materials with desirable properties remains challenging. Here, we report that insertion of a non-emissive molecule into a donor (perylene) and acceptor (1,2,4,5-tetracyanobezene) binary cocrystal can realize fine manipulation of intermolecular interactions between perylene and 1,2,4,5-tetracyanobezene (TCNB) for desirable piezochromic luminescent properties. A continuous pressure-induced emission enhancement up to 3 GPa and a blue shift from 655 to 619 nm have been observed in perylene-TCNB cocrystals upon THF insertion, in contrast to the red-shifted and quenched emission observed when compressing perylene-TCNB cocrystals and other cocrystals reported earlier. By combining experiment with theory, it is further revealed that the inserted non-emissive THF forms blue-shifting hydrogen bonds with neighboring TCNB molecules and promote a conformation change of perylene molecules upon compression, causing the blue-shifted and enhanced emission. This strategy remains valid when inserting other molecules as non-emissive component into perylene-TCNB cocrystals for abnormal piezochromic luminescent behaviors.

Pressure-Induced Electronic and Structural Transition in Nodal-Line Semimetal ZrSiSe.

The nodal-line semimetals have recently gained attention as a promising material due to their exotic electronic structure and properties. Here, we investigated the structural evolution and physical properties of nodal-line semimetal ZrSiSe under pressure via experiments and theoretical calculations. An isostructural electronic transition is observed at ∼6 GPa. Upon further compression, the original tetragonal phase starts to transform into an orthorhombic phase at ∼13 GPa and the two phases coexist until the maximal experimental pressure. By analysis of the electronic band structure, we suggest that the significant changes in the Fermi surface contribute to the occurrence of the isostructural electronic transition. The results provide a new insight into the structure and properties of ZrSiSe.

New Cadmium-Nitrogen Compounds at High Pressures.

A systematic high-pressure study of the CdNx (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.

Risk Assessment of Commercial dangerous -goods truck drivers using geo-location data: A case study in China.

The primary objective of this study is to understand the relationship between driving risk of commercial dangerous-goods truck (CDT) and exposure factors and find a way to evaluate the risk of specific transportation environment, such as specific transportation route. Due to increasing transportation demand and potential threat to public, commercial dangerous goods transportation (CDGT) has drawn attention from decision makers and researchers within governmental and non-governmental safety organization. However, there are few studies focusing on driving risk assessment of commercial dangerous-goods truck by environmental factors. In this paper we employ survival analysis methods to analyze the impact of risk exposure factors on non-accident mileage of commercial dangerous-good truck and assess risk level of specific driving environment. Using raw location data from six transportation companies in China, we derive a set of 17 risk exposure factors that we use for model parameters estimation. The survival model and hazard model were estimated using the Weibull distribution as the baseline distribution. The results show that four factors - weather, traffic flow, travel time and average velocity have a significant impact on the non-accident mileage of driver in this company, and the assessment results of survival function and hazard function are robust to the different levels of testing data. The employment time has some effect on the results but does not result in a significant difference in most cases, and the task stability has little impact on the results. The findings of this study should be useful for decision makers and transportation companies to better risk assessment of CDT.

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.

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.

Two-dimensional Penta-BP5 Sheets: High-stability, Strain-tunable Electronic Structure and Excellent Mechanical Properties.

Two-dimensional (2D) crystals exhibit unique and exceptional properties and show promise for various applications. In this work, we systematically studied the structures of a 2D boronphosphide (BP) monolayer with different stoichiometric ratios (BPx, x = 1, 2, 3, 4, 5, 6 and 7) and observed that each compound had a stable 2D structure with metallic or semiconducting electronic properties. Surprisingly, for the BP5 compounds, we discovered a rare penta-graphene-like 2D structure with a tetragonal lattice. This monolayer was a semiconductor with a quasi-direct band gap of 2.68 eV. More importantly, investigation of the strain effect revealed that small uniaxial strain can trigger the band gap of the penta-BP5 monolayer to transition from a quasi-direct to direct band gap, whereas moderate biaxial strain can cause the penta-BP5 to transform from a semiconductor into a metal, indicating the great potential of this material for nanoelectronic device applications based on strain-engineering techniques. The wide and tuneable band gap of monolayer penta-BP5 makes it more advantageous for high-frequency-response optoelectronic materials than the currently popular 2D systems, such as transition metal dichalcogenides and black phosphorus. These unique structural and electronic properties of 2D BP sheets make them promising for many potential applications in future nanodevices.