Theoretical Study Of New Polynitrogen Structures At High Pressures And The Confinement Of Polymeric Nitrogen

With the huge development of society and the shortage of traditional energy materials,people are looking for new materials with high energy density to meet the increasing energy demand.As a potential high energy density material?HEDMs?,polymeric nitrogen has been widely explored by the scientific researchers.The nitrogen atom has five valence electrons?2s²2p³?which leads to rich bonding forms.Under high pressure,the distance between molecules?N₂?is reduced,the N?N triple bond is gradually dissociated and rebonds to a new polymeric nitrogen phase with single bond.Due to the huge energy difference stored in N-N single bond(160kJ·mol^-1),N=N double bond(418 kJ·mol^-1)and N?N triple bond(954 kJ·mol^-1),a large amount of energy will be released when polymeric nitrogen decomposes and reforms into molecular nitrogen.And most importantly,due to the fact that the dissociation product N₂ is environmentally friendly and pollution-free,polymeric nitrogen has become a promising high energy density material?HEDMs?with great expectations.At present,the research of polymeric nitrogen faces two major diffculties:one is that the synthesis conditions of polymeric nitrogen are extremely strict?millions of atmospheres of ultra-high pressure and thousands of K of high temperature environment,HPHT?and the other one is that polymeric nitrogen can only be stable under high pressure environment.Therefore,how to reduce the temperature and pressure conditions for the synthesis of polymeric nitrogen,to realize its controllable preparation under mild condition and to stablilize the polymeric nitrogen structure at room temperature and ambient pressure are the prerequisites and foundations for its application.Recently,a large number of studies have found that the metal coordination ions can produce a"chemical pre-compression"effect in nitrogen compounds.The introduction of metal ions can not only reduce the formation conditions of polynitrogen structures,but also improve the stability of the structure of polynitrogens.Therefore,the use of metal nitride compounds as a new type of nitrogen source material relpace the pure nitrogen provides an effective solution for the mild preparation of polynitrogen.On the other hand,some recent studies suggest that the use of confinement template with unique space can be an effective way to keep the polymeric nitrogen structure stable under ambient condition.The polymeric nitrogen structure is intercepted into a host template as a guest material,which can realize the capture of polymeric nitrogen structure at room temperature and pressure.This paper aims at the two scientific problems about the application of polymerized nitrogen.On the one hand,the introduction of metal coordination ions?Zn,V,Ca?into nitrogen are explored based on the density functional theory and the crystal structure search method.Theoretical research has obtained a series of novel,stable and metastable polynitrogen structures at high pressure,which not only enrich the high-pressure phase diagram of the binary polynitrogen structure,but also obtain some novel structures with high energy density and high stability.On the other hand,the layered A7 polymeric nitrogen structure is successfully stabilized in the boron nitride?h-BN?confinement template under ambient conditions,which provides theoretical guidance for the capture of polymeric nitrogen in experiments.1.Crystal structure prediction of Zn-N system at high pressure.Under the framework of density functional theory,the crystal structure search method is used to predict the new phase of Zn-N system with different stoichiometries under 20 GPa,50 GPa and 100 GPa.High pressure phases of three stable Zn₃N₂?Pbca,Pbcn,Pnma?,three stable ZnN₂?R-3m,Pnnm,P6₃/mcm?,one stable C2/m-ZnN₃ and two stable?P-1,Ibam?ZnN₄ structures are obtained by the calculations of formation enthalpy.A metastable phase of P-1-ZnN₆ with a higher stoichiometry is also obtained.The survival pressure range of the stable high pressure phases is determined by the enthalpy difference analysis,which greatly enriches the high pressure phase diagram of the Zn-N system.Band analysis shows that the predicted nine stable energetically phases and one metastable phase all exhibit metallic characteristics.The structural analysis shows that the two phases of the ZnN₄structure?P-1 and Ibam?possess infinite armchair nitrogen chain structures.The P-1-ZnN₆ structure adopts a new armchair nitrogen chain structure with the adjacent unit rollovers at 180°,which is bonded by the repeated armchair-antiarmchair N₆ unit.Further researches reveal that the energy densities of the P-1-ZnN₄,Ibam-ZnN₄ and P-1-ZnN₆ structures are 2 kJ/g,2.3 kJ/g and 2.72 kJ/g,respectively.Therefore,these polynitrogen phases are expected to become promising high energy density materials?HEDMs?.2.Crystal structure prediction of V-N system at high pressure.Based on the particle swarm optimization algorithm,we have theoretically predicted the stable crystal structures of V-N systems with different stoichiometries at0 GPa,20 GPa,50 GPa and 100 GPa.A series of vanadium nitride compounds with thermodynamic,dynamic and mechanical stability are obtained under high pressure for the first time.Finally,two stable high pressure phases of V₂N?Pbcn,Pnnm?are obtained.The enthalpy calculation shows that the phase transition of two new phases Pbcn-V₂N?Pnnm-V₂N occurs at 48.5 GPa and these two new phases both are metallic.At the same time,a P-6m2-VN structure with metallic characteristic is obtained which can remain stable in the range of 0-100 GPa.In addition,new I4/mcm-VN₂,P-1-VN₄,P4/mnc-VN₈ and Immm-VN₁₀ stable phases are also obtained.Except that the P-1-VN₄ phase is a semiconductor structure with an indirect bandgap about 0.95 eV,the other structures are all have the metallic characteristics.Among these stable phases,the P-1-VN₄,P4/mnc-VN₈ and Immm-VN₁₀ phases all possess infinite armchair polymeric nitrogen chain structure,in which N atoms are connected by nitrogen single bonds.In addition to the infinite nitrogen chains,the P-1-VN₄structure also contains short chains consisiting of N₄ units connected by N-N bonds.Further calculations show that the energy density of VN₄,VN₈ and VN₁₀ structures with polynitrogen configurations are 1.232 kJ/g,2.27 kJ/g and 2.82 kJ/g,respectively.Hence,the P4/mnc-VN₈ and Immm-VN₁₀ structures can be applied in the field of high energy density materials?HEDMs?in the future.3.Crystal structure prediction of Ca-N system at high pressure.Using the crystal structure search method based on density functional theory,the calcium nitride structures are predicted at 50 GPa and 100 GPa respectively.Finally,two kinds of new phases P2₁/c-Ca₅N₄ and P2₁/m-CaN₄ structures are obtained.The enthalpy calculation,phonon spectrum and elastic constant calculation results show that these two structures have dynamic stability and mechanical stability.Structural analysis shows that the nitrogen atoms in P2₁/c-Ca₅N₄ structure are bonded in the form of N-N single bonds and the interaction between N-Ca atoms are ionic bond.The electronic properties calculation finds that P2₁/c-Ca₅N₄ is a semiconductor material with a direct band gap of 1.447 eV.At the same time,the Raman vibration spectra and XRD diffraction patterns of P2₁/c-Ca₅N₄ structure are also presented,which provide theoretical guidance for the experimental synthesis of this structure.The P2₁/m-CaN₄ structure possesses a one-dimensional infinite armchair polymeric nitrogen chain with a high nitrogen content of 58.3%.In the meanwhile,the P2₁/m-CaN₄ structure presents metallic characteristic.The enthalpy difference analysis shows that the P2₁/m-CaN₄ structure in this work is more stable than the structure of P4₁2₁2-CaN₄ proposed before and the diatomic nitrogen phase P4/mbm-CaN₄ will transform to P2₁/m-CaN₄ polynitrogen phase at 18.3 GPa,instead of the former P4₁2₁2-CaN₄ phase at 19 GPa,which not only modifies the high-pressure phase diagram of the CaN₄,but also suggests a potentially high energy density material with lower polymerization pressure.4.Theoretical study on the stability of layered A7 polymeric nitrogen confined in 2D h-BN system.According to the characteristics of two-dimensional layered A7 polymeric nitrogen structure,the boron nitride?h-BN?sheet is selected as the confinement template to construct the A7@BNSs hybrid system.The stability and physical mechanism of the layered A7 structure are studied in detail based on the density functional theory combined with molecular dynamics simulation method and electronic property calculations.Phonon spectrum calculations and molecular dynamics results show that the polymeric nitrogen layers can be stable in A7@BNSs system at ambient conditions.The molecular dynamics simulation shows that when the temperature comes to 500 K,the A7 layers in A7@BNSs system begins to decompose.It is worth mentioning that the decomposition is a slow process.With the increasing of temperature,the polymeric A7 layers gradually decomposes in the form of chains.When the temperature increases to 1800 K,A7 layers are completely decomposed into molecular nitrogen.Therefore,the release of energy is a slow process,which is accompanied by the decomposition of the polymeric nitrogen.Energy analysis shows that the polymeric nitrogen content in A7@BNSs system as high as 53.84%and its energy density is 5.4 kJ/g,which is 1.29 times that of TNT.Further electronic calculations reveal the new A7@BNSs system exhibits a small amount of charge transfer,which not only stabilizes polymeric nitrogen layer at ambient conditions,but also can releas energy at a mild conditions.Therefore,a novel high energy density material with great application in the future is proposed through the confinement effect and an important solution is provided for the moderate and controllable release of the polymeric nitrogen energy.