Phase Transition,Synthesis And Thermal Decomposition Of Ammonium Azide Under High Temperature And High Pressure

High-nitrogen-content energetic compounds containing multiple N-N bonds are an attractive alternative toward developing a new generation of environmentally friendly and more powerful energetic materials.Unfortunately,single-bonded cubic gauche phase of nitrogen（cg-N）,which is produced by the phase transition of N₂molecules under high pressure（P>110.0 GPa）and high-temperature（T>2000 K）,reverts to molecular N₂ below 40.0 GPa at 300 K.Searching polynitrogen materials,which are（meta）stable at ambient conditions,has been a subject of intense efforts in the recent past years.Ammonium azide（AA,NH₄N₃）has been considered as a promising precursor to poly-hydronitrogen.Computer simulations have predicted the polymerization of nitrogen in various forms in NH₄N₃ compressed to pressures in excess of 60.0 GPa.Recently,a theoretical work has predicted that crystals of NH₄N₅and N₅H,both featuring all-nitrogen cyclic pentazole ions（N₅^-）,could be synthesized at high pressure.However,the crystal structure of phase II,stable above 3.0 GPa,has remained elusive until now.At the same time,the polymerization reaction of NH₄N₃and N₂ and the thermal decomposition of NH₄N₃ under high temperature and high pressure are still research blanks.In this thesis,phase transition,synthesis and thermal decomposition of NH₄N₃ were systematically studied under high temperature and high pressure by combining X-Ray diffraction（XRD）,Raman spectroscopy measurements and first-principles calculations.The results of the study are as follows:1)We have determined the long-time sought crystal structure of NH₄N₃ phase II based on previous theoretical prediction results;phase II is a hydrogen（H）-bonded structure of ammonium and azide ions with monoclinic symmetry,space group P2/c.The experimental Raman spectra of NH₄N₃ were measured to 85.0 GPa and correlate very well with the predicted behavior for the vibrational modes of P2/c structure computed by DFPT.2)Our experiments also demonstrate the absence of phase transition in NH₄N₃-P2/c from 3.0 to 85.0 GPa,which is consistent with our DFT calculations.The latter predicts the transition to the hydronitrogen solid with 1D zigzag chains of nitrogen saturated by hydrogen at 102.6 GPa,while no pressure range of stability is found for the TTZ solid.This should motivate future investigations to extend the pressure range of experimental studies.This work thus reveals that NH₄N₃ is even more stable under pressure than previously thought,making the compression of pure NH₄N₃ a nonpractical way to obtain hydronitrogen solids.3)We used in situ high pressure and high-temperature Raman experimental technique to characterize the reaction of NH₄N₃ and N₂ system in the range of 0-30.0GPa and 300-700 K.At 20.0-25.0 GPa,650K,a polymeric nitrogen material was synthesized which contains N-N and N=N,named Phase A.Phase A is an orthogonal structure with Ima2 symmetry,and its Raman vibration mode is similar to that of N₉H in the theoretical prediction,and N₂ molecules are nested in the crystal structure.The phase A can exist stably to 7.0 GPa at room temperature.This work shows that the reaction of NH₄N₃ with N₂ under high pressure and high temperature is different from other metal azide compounds,and N₅^-ring structure cannot be synthesized.At the same time,we use in situ Raman experiment to characterize the phase transition of pure NH₄N₃ samples under high temperature and high pressure.At 20.0 GPa-620 K,NH₄N₃ will be converted into N₂ and a variety of NHy（y>1）compounds;this reflects that phase A can only be synthesized under NH₄N₃ in a nitrogen-rich environment.4)We used high pressure and high temperature in situ XRD technique to characterize the reaction of NH₄N₃ and N₂ system in the range of 0-30.0 GPa and300-700 K.Due to the decomposition effect of X-ray on azide compounds,the NH₄N₃and N₂ system will pass over the phase A and directly undergo a decomposition reaction at 25.0 GPa-610 K to produce N₂ and NH₃.At the same time,it is shown that the method of using NH₄N₃+N₂ to synthesize polymeric nitrogen by the in-situ XRD experiment is not feasible.5)We have detected the decomposition process of NH₄N₃ using high pressure and high temperature in situ Raman experiments.The experimental results show that NH₄N₃ will decompose into N₂ and NH₃ with a molar ratio of 1:1 at 3.5 GPa and 530K,without producing H₂.We found that if the sample NH₄N₃ is heated under lower pressure,the decomposition reaction will occur directly,and the NHy compound（phase B）cannot be produced.The result also reflects that there may be new polymerization reactions in the presence of NH₄N₃+N₂ at higher pressures.In these experiments,we have solved the technical problem of loading N₂ and NH₃ in the same cell and have provided a new way to study the phase diagram of N₂ and NH₃under high temperature and high pressure.6)Based on our experimental data,we give a rough phase diagram of NH₄N₃+N₂under high pressure and high temperature,which provides a reference for the future study of the polymerization reaction of NH₄N₃+N₂ under higher pressure and higher temperature.