High-pressure Synthesis And Stabilization Of Novel Polynitrogen Structures Based On The Confinement Effect

Nitrogen molecule,in which the nitrogen atoms are connected by one of the strongest triple bonds in nature,is the most abundant component in Earth’s atmosphere.In the polymeric nitrogen structures,the nitrogen atoms are connected by single or double bonds,which enable the releasing of huge amounts of energy with decomposition into nitrogen molecules.Therefore,polymeric nitrogen allotropes and nitrogen-rich compounds containing polymeric nitrogen structures are considered as a new type of high energy density materials（HEDMs）,and have important application prospects in the fields of explosive initiation and rocket propulsion.Pressure can directly reduce the distance between nitrogen molecules,induce the delocalization of bonding electrons in nitrogen molecules,and then form the polymeric nitrogen structure.At present,the high-pressure synthesis technology is the most important route to obtain the polymeric nitrogen allotropes.Using nitrogen as precursor material,a series of polymeric nitrogen allotropes（cg-N,LP-N,HLP-N,bp-N）with high energy density have been synthesized under extreme high pressure（>110 GPa）and high temperature（>2000 K）conditions.However,these polymeric nitrogen materials cannot be captured to ambient conditions due to the low stability,which prevents their application as HEDMs.In the past two decades,people have been exploring new nitrogen-rich HEDMs with higher stability.Theoretical studies have predicted that the stability of polymeric nitrogen structures can be significantly enhanced by confined in one-dimensional or two-dimensional nanospace,which is expected to achieve the capture at ambient conditions.In addition,by using the idea of"crystal lattice confinement",metal coordination are introduce into the polymeric nitrogen structures,which can provide"chemical prepressing"resulting in the the enhancement of stability in the polynitrogen structure,the reduction of the temperature and pressure conditions for synthesis of the novel energitic nitrogen-rich compounds.Therefore,the confinement strategy can effectively improve the structural stability of polymeric nitrogen,and provide a new way to explore and expand the region of nitrogen-rich HEDM.At present,studies about the stabilization of polymeric nitrogen structures based nano-confinment are still in the theoretical stage,and the experimental route for realizing the confinement of polymeric nitrogen structures are still needed to be explored.In addtion,the theoretically predictied cage-like polymeric nitrogen structure has not yet been obtained experimentally.In this paper,based on the nano-confinement and crystal lattice confinement routes,respectively,the stabilization of Na N₅and polymeric nitrogen structures were studied by using the diamond anvil cell combined with laser heating technique,and the low-dimensional boron nitride nanotubes（BNNTs）were used as the confinement templates.The synthesis route of new La-N compounds was explored under high-temperature and high-pressure（HTHP）conditions.The main conclusions of this paper are as follows:1.With BNNTs as the confinement template,Na N₃@BNNTs was successfully prepared by freeze-drying method.The high-presurre experimental studies of Na N₃@BNNTs at room temperature and high–temperature（laser heating）were carried out,respectively.At room temperature,Na N₃confined in BNNTs experiences two structural phase transitions under compression:fromβ-Na N₃（R-3m-Na N₃）toα-Na N₃（C2/m-Na N₃）at 1.06 GPa,and fromα-Na N₃（C2/m-Na N₃）to I4/mcm-Na N₃（γ-Na N₃）at 16.1 GPa.In pressure-range of 30-35 GPa,partialγ-Na N₃begins to transiform into Cm-Na N₅and Cmmm-Na N₂.Up to 111.6 GPa,the confinedγ-Na N₃has completely transformed into Cm-Na N₅and Cmmm-Na N₂.Upon decompression,Cm-Na N₅transforms into theγ-Na N₃,andγ-Na N₃transforming intoα-Na N₃subsequently.In the HTHP experiments,Pmn2₁-Na N₅and Pm-Na₂N₅were synthesized by laser heating the confined sample at 50.8 GPa.Upon decompression,the confined Pmn2₁-Na N₅transformation into P2/c-Na N₅at 14 GPa,and Na N₅can be recovered to ambition conditions.TEM images and electron energy loss spectroscopy（EELS）show that the BN exists in the sp³-hybridized amorphous form in recovered sample,while the Na N₅cannot be captured under the ambient conditions and the BN remained in the sp²-hybridized hexagonal structure in recovered sample without laser heating.2.By confining molecular nitrogen with varied content in BNNTs,two diffenerent N₂@BNNTs confined system were prepared.The N₂@BNNTs was studied under HPHT condtions.The Raman behavior of the confined nitrogen in the high nitrogen content N₂@BNNTs is different from that of unconfined N₂.After laser heating at 123 GPa,N₂in the sample cavity is transformed into cg-N structure,and decomposition occurs at about 40 GPa.Under ambient conditions,the confined N₂is suggested to be stabilized in the high-presure liquid phase.In the low nitrogen content N₂@BNNTs,the confined N₂transforms into new polymeric nitrogen structure,which possesses N=N double bonds with different bond lengths close to the those in the N₃^-anion and N₄⁺clusters,respectively,after laser-heating in the pressure range of122-150 GPa.This polymeric nitrogen structure can be stabilized down to 25 GPa.3.New La-N compounds,in which La was indced to stabilize the new polymeri nitrogen structures,were explored under HPHT conditons.The P^—4-La N₉and Imm2-La₃N₅were synthesized at 16-26.8 GPa by laser heating the lanthanum and molecular nitrogen mixed samples.In thev P^—4-La N₉,all the N atoms have been polymerized into the form of N₁₀cage which is successfully synthesized for the first time.The theoretical energy density of P^—4-La N₉structure is 4.36 k J/g and comparable to TNT.The N₁₀cage structure remain stable with pressure down to 6.2 GPa.The P^—1-La N₁₆and Fm3m-La N were synthesized at the pressure-range of 30-50 GPa by laser heating the lanthanum and molecular nitrogen mixed samples.The N atom in P^—1-La N₁₆structure form the folded N₁₈ring which constitute an infinite banded polymeric nitrogen structure.The theoretical energy density of P^—1-La N₁₆is 5.21 k J/g.The banded polymeric nitrogen structure remian stable with pressure down to 14 GPa.