Theoretical Design And Experimental Synthesis Of New Polymeric Nitrogen Structures At High Pressures

Polymeric nitrogen is a novel network structure formed by N-N bonds after the dissociation of diatomic nitrogen at high pressure.Due to the huge energy difference between N?N?946 KJ/mol?and N-N?159 KJ/mol?,the dissociation of the polymeric nitrogen will release a large amount of energy,which is more than five times that of the most energetic materials.Therefore,polymeric nitrogen is a promising HEDM,and has a wide range of applications in many areas.Up to now,two polymeric nitrogen structures have been successfully synthesized in experimental?cg-N and PL-N?.These experimental studies have greatly encouraged people to find new polymeric nitrogen in rich-nitrogen system and achieve the stability of polymeric nitrogen at ambient conditions.Based on these considerations above,we studied the synthesis and stability of polymeric nitrogen in different systems,including the azide compound system,N-H polymeric system,Na-N polymeric compounds system and nano-confined system by using high pressure method and theoretical and experimental method.1.High pressure experimental studies and theoretical calculations are performed to investigate the pressure induced phase transitions of NH₄N₃ up to 62.3 GPa.A helical N-H polymeric structure?P212121?formed by N-N bonds is obtained at room temperature and 15.8 GPa for the first time.Compared with cg-N and LP-N,the synthesis conditions of this P212121 are greatly reduced.This pressure can be generated by the commercial presses,which lays an important foundation for the synthesis of polymeric nitrogen on a macroscopic scale in the future.During the decompression process,P212121 structure can be maintained to about 4 GPa,which is due to the H atoms saturate skeleton defects and dangling,and greatly improve the stability of the polymeric structure.In the same time,we also determined the crystal structure of phase I.2.The structures of Na-N system under high pressure are studied by using CALYPSO software based on particle swarm optimization algorithm,and a series of novel structures?NaN₂,Na2N₂,NaN5?are proposed.More importantly,a high nitrogen content of NaN5 polymeric phase is found at 16.9 GPa,in which all N atoms are connected by N-N bonds forming a N5 ring structure.In addition to NaN5,another polymeric structure with zigzag configuration?Cmmm_II?is found above 51 GPa.Relative to the synthesis conditions of polymeric nitrogen in the pure N system,these pressures in our paper has a significant reduction,and there are abundant N-N bonds in the N5 ring and zigzag structures,indicating that these polymeric structures can be used as apromising candidate for HEDMs and have greatly practical application prospect.Moreover,the NaN3 begin to decompose into NaN₂ and NaN5 at above 50 GPa.3.The high pressure structure predictions of N-H system have been carried out by using the particle swarm optimization method,and three new structures are predicted at about 10 GPa,including C2/c,N8H8-ring and Pc structure.In these structures,all the N atoms are connected by N-N bonds and the dangling bonds are terminated by H atoms.Note that these structures found here have lower energy than the previously reported P21/m structure.These structures can be synthesized by N₂/H2 mixture at about 10 GPa,which is far lower than that of the synthesis conditions in the pure nitrogen system and this is because the introduction of H atoms effectively saturate the defects of N atoms and the dangling bonds.In particular,we find that the N8N8-ring structure still maintains metastability under ambient pressure,indicating that it might be quenched to ambientpressure if it is synthesized experimentally.At the same time,these polymeric structures have a very high energy density.4.The stabilities of polymeric nitrogen?N8 polymeric chain?confined in BNNTs are studied by using the first-principle calculation method.The results show that the N8 chain confined in?5,5?BNNT can be stable at ambient condition,which is due to the charge transfer between the BNNTs to N8 chain.For the other diameters of BNNTs,the N8 chain also can be stable with the application of little pressure.Compared with other hosting materials,BNNTs have excellent thermal stability,especially the strong nitrogen inertness under ultra high pressure and high temperature.Since the synthesis of polymeric nitrogen often requires high temperature and high pressure conditions,BNNT who have excellent physical and chemical properties make it possible to carry out the study on polymeric nitrogen using the confinement method.This study provides an important solution for obtainingpolymericnitrogen at ambient condition.