The Study Of New Polymeric Nitrogen Structures And The Stability Of Confined Polymeric Nitrogen Under High Pressure

Polymeric nitrogen is a new kind of network structure.Due to its high energy density and the fact that the decomposition products have no pollution to the environment,the synthesis of polymeric nitrogen has attracted extensive attention.Up to now,a large number of polymeric nitrogen structures have been proposed in theory,such as cubic network structure（cg-N）,the helical tunnel P2₁2₁2₁ structure,chain polymeric structure（N₆,N₈）,N₁₀ cage structure and so on.Three structures of polymeric nitrogen have been confirmed experimentally:cg-N（>110 GPa,2000 K）,LP-N（150 GPa,3000 K）and HLP-N（244 GPa,laser-heating）.These results have greatly stimulated people’s enthusiasm for the research of polymeric nitrogen.However,it is noteworthy that the synthesis conditions of polymeric nitrogen are extremely stringent,which need to meet both ultra-high pressures and temperature（100 GPa,>1000 K）.The polymeric nitrogen that was synthesized experimentally is high-pressure metastable phase,which can not exist at room temperature and pressure conditions.These problems seriously restrict the application of polymeric nitrogen in practice.In order to solve the above scientific problems,people try to look for polymeric nitrogen in other nitrogenous systems.In view of the above research problems,the high-pressure structural phase transition of typical nitrogen-containing systems which include azide system and nano-confinment system has been studied by using CALYPSO and other theoretical methods.The following conclusions have been drawn.1.Silver azide（AgN₃）is selected as the precursor for the synthesis of polymeric nitrogen.The structural evolution behavior of AgN₃ is systematically studied by combining particle swarm optimization（CALYPSO）with density functional theory in the pressure range of 0-150 GPa.It is found that the AgN₃ stuctures undergoe two structural phase transitions in the whole pressure range,when the pressure reaches 4.7GPa,AgN₃ stucture transforms from cubic phase（Ibam）to tetragonal phase（I4/mcm）,and when the pressure continues to increase to 24 GPa,a new P-1 phase is formed.It is noteworthy that nitrogen atoms are bonded in the form of N=N double bonds（SP²hybridization）with a chain-like polymeric structure in the P-1 phase,which is the first time that the existence of polymeric structure is found in the high-pressure phase of AgN₃;electronic structure studies show that the P-1 phase is metal phase;kinetics studies show that the polymeric structure can exist stably at atmospheric pressure.Our research work enriches people’s understanding of phase transition and polymerization of AgN₃ under high pressure,and provides important theoretical guidance for synthesis of polynitrogen under high pressure.2.We propose a novel three-dimensional confinement structure:A7@graphene.Using the theoretical method based on the first principle,A7@graphene system was studied in depth.Studies have shown that A7 polynitrogen layer can exist stably at room temperature and pressure due to the confinment effect of graphene sheet layer.Further research reveals that the physical mechanism of A7@graphene stability is the charge transfer between host and guest.At the same time,duo to there is only a small amount of charge transfer between graphene matrix and A7 polynitrogen layer;the A7@graphene system can release energy at mild conditions（450 K）.The nitrogen content of A7@graphene system is 53.84%,and the corresponding energy density is5.2 KJ/g,which is higher than the energy density of TNT（4.2 KJ/g）.A7@graphene system has better detonation velocity and pressure than traditional explosives（TNT,HMX）.A7@graphene is a new high energy density material with great application prospects.Our research results provide an important theoretical reference for the storage and utilization of polymeric nitrogen in environmental conditions.