Theoretical Structure Prediction And Experimental Synthesis Of New Ag-N Compounds At High Pressures

Polymeric nitrogen is an environment-friendly high energy density material（HEDM）,which has extensive applications including engery storage,aerospace and new energy,etc.However,great challenges still remain in synthesis of polymeric nitrogen.Firstly,the synthesis condition of polymeric nitrogen is extremely harsh,needing pressure larger than 100 GPa and temperature higher than 2000 K.Secondly,polymeric nitrogen can not be quenched to ambient conditions,just stabilizing at more than 42 GPa,due to the low kinetic stability of N-N single and double bonds.Therefore,how to reduce the synthesis condition of polymeric nitrogen and improve its stability is a fundamental issue.Recently,many researches have been revealed introducing metal element（M）into molecular nitrogen is an effective approach for reducing its synthesis pressure and enhancing the stability of polymeric nitrogen.It is because metal element can provide electrons into the bonding environment of nitrogen,inducing a“chemical precompression”effect,which may promote the dissociation of N≡N triple bond,thereby,reducing the synthesis pressure towards the novel polymeric structure.Meanwhile,electrons transfer from metal element to nitrogen structure can intensively enhance the covalent bond interaction of N-N bond,and besides,form an M-N ionic bond weak interaction.Both of the two interactions can strongly strengthen its stability of new polynitrogen structure in M-N system.Based on above strategy,theoretical structure prediction and experimental synthesis of new Ag-N compounds at high pressures were studied systematically.In theory,structure predictions in Ag-N system were performed at different pressures by using CALYPSO（Crystal structure Ana LYsis by Particle Swarm Optimization）code,combined with first-principles calculations.A group of new polymeric structures have been uncovered under high pressures and ones with high stability and high energy density were screened out.In experiment,a high-pressure in-situ laser heating system was designed and built for generating high pressures and high temperatures.Structural phase transitions of Ag-N system at high pressure and high temperature conditions were studied.Combined with theoretical predictions,crystal structures,synthesis conditions and stability properties for these novel high-pressure structures were explored.Our findings provide a new insight into nitrogen-based HEDMs preparation and stabilization.These research results of this work are as follows:1.Using CALYPSO structure search software,the structure searching for Ag-N system under different pressures of 50 GPa,100 GPa,150 GPa has been performed.Two novel stable polymeric structures（P1-AgN₇ and P-1-AgN₇）and two metastable phases（P-1-AgN₄ and P-1-AgN₈）were proposed,respectively.Importantly,a novel folded layer-shaped N-structure compositing by N₇ and N₂₀ rings was firstly reported,which exists in P-1-AgN₇.In addition,a new high-pressure phase P4/mmm-AgN₂ was also released and the P-1-AgN₃ phase,obtained in previous work,was certificated in this work,which validates the reliability of our predicting method adopted here.While elevating pressures up to 200 GPa and 300 GPa,several other new polynitrogen structures were predicted,including armchair-antiarmchair chains（P-1-AgN₂）,layers（P2₁/c-AgN₂）,one-dimensional infinite bands（P-1-AgN₅）and two-dimensional layers（C2/m-AgN₆.）.The analysis of high-pressure structural evaluations of AgN₂ revealed that orbital hybridization conversing of N（from sp to sp² to sp³）plays a crucial role in arising of the transformations in N-structures from clusters into chains,even into layers.The stable pressure ranges of Ag-N structures are determined by the enthalpy difference analysis,the phonon spectrum and elastic constant etc.The analysis of the electron properties and bonding shows the stability mechanism of Ag-N structures is charge transfer and the weak interaction between Ag and N atoms.The energy property analysis reveals that several Ag-N structures（P-1-AgN₅,C2/m-AgN₆,P-1-AgN₇ and P-1-AgN₈）exhibit high energy density,which is comparable with TNT.And it is interest that their explosion characteristics are superior to TNT,thus far,making them ideal candidates as HEDMs.Noticeably,the energy densities of P-1-AgN₂ and P2₁/c-AgN₂ are 1.76 kJ/g and 2.01 kJ/g,respectively,higher than other MN₂ compounds known today.This theoretical prediction enriches the high-pressure phase diagram of Ag-N system.Furthermore,this study provides several promising candidats for synthesis polymeric nitrogen in experiment.2.According to the technical requirements of scientific researches in experiment,several new high-pressure techniques have been developed in this work.Firstly,a high pressure in-situ laser heating experimental system was designed and built.And the correlative experimental techniques were developed,including sealing technique,heat absorption and adiabatic technique,etc.Based on the above techniques,high pressure and high temperature conditions were successfully achieved in experiment.Subsequently,a portable pressure-calibrating system based on the“ruby pressure scale”was built and well matched with diamond anvil cell（DAC）.This system radically solved tow technical issues.One is the microscopic focusing of ruby ball pressure-calibrator（φ<10μm）,which is marked in micron-scale high-pressure cavity.The other is the miniaturizing of optical path.In this system,pressure calibration in the range of 0-130 GPa was successfully realized by using 405 nm laser as exciting line.Therefore,it is realistic that pressure-calibrating can completely get rid of some comprehensive spectral systems,such as Raman spectrum and PL spectrum,etc.Meanwhile,several sets of high-pressure experimental systems have been built and upgraded,and a series of new high-pressure techniques have been developed.Above all,these innovations and improvements can well satisfy the subsequent experimental researches in this paper and some other important explorations at high-pressure and high-temperature conditions as well.3.The high-pressure phase transition in Ag-N system was investigated by in-situ laser heating technology,high-pressure X-ray diffraction（XRD）and Raman scattering,combined with first-principles structural predictions.It is noticed that a novel infinite chain-shaped polymeric structure（P-1-AgN₄）is firstly synthesized through heating metallic silver and molecular nitrogen at 25 GPa.This structure was proposed in our predictions.While decompression,P-1-AgN₄can be quenched to 15 GPa,then,recovered to P4/mmm-AgN₂ and N₂,which confirms by our theoretical exploration as well.After released to 12 GPa,P4/mmm-AgN₂ is decomposed into metallic silver and molecular nitrogen.This work strongly confirmed our theoretical predictions in experiment.More importantly,these results motivate our interest in hunting for novel HEDMs in future.