Studies On High-pressure Phase Transitions Of Energetic Organic Azides

With the increasing demanding of energy,researchers focus on the discovery of energetic materials.As one of the high energy density materials,single-bonded polymeric nitrogen is being widely studied.Azides are considered to be the ideal precursors for the formation of single-bond polymeric nitrogen under high pressure due to the intrinsic properties of their energetic azide groups.Importantly,the high-pressure studies of azides also involve several important problems,such as structural phase transitions and intermolecular interactions,which lead to a hot topic on the research of azides under high pressure.In this thesis,trimethylsilane azide（TMSiN₃）,diphenylphosphoryl azide（DPPA）,and 4-azidobenzoic acid（4-ABA）were investigated by in-situ Raman scattering and synchrotron X-ray diffraction measurements in diamond anvil cells（DACs）.The high-pressure phase transition behaviors of three organic azides and the variations of their asymmetric and linear azide groups with pressure were studied.The main contents are summarized as follows:Firstly,TMSiN₃was studied by in situ Raman scattering and synchrotron X-ray diffraction measurements in DACs to investigate the changes of its organic groups under high pressure.The Raman spectra showed that phase transitions occurred at0.13 and 7.91 GPa.The synchrotron X-ray diffraction results showed that TMSiN₃remained in the liquid state during the phase transitions,which indicates that two liquid-liquid phase transitions occurred in TMSiN₃.In the pressure range of 0.13-2.62GPa,the distortion of the Si-C₃bond and the shortening of the bond between the azide group and the silicon atom led to the first phase transition.The second phase transition at 7.91 GPa was attributed to the rotation of the CH₃group.With further compression,the azide group became increasingly asymmetric and decomposed at33.83 GPa.The decomposition pressure of azide group in TMSiN₃is lower than that of the azide ions in inorganic azides,which is beneficial to the nitrogen polymerization at a relatively low pressure.Secondly,DPPA was studied by in situ Raman scattering and synchrotron X-ray diffraction measurements in DACs to investigate its structural changes.The analyses of Raman spectra showed that liquid DPPA changed from phase I to phase II at 0.42GPa,and went to phase III at 1.31 GPa.The first phase transition was caused by the change of molecular conformation,and the second phase transition was attributed to the deformation of the benzene rings.The Synchrotron X-ray diffraction results showed that DPPA remained in a liquid state during the phase transitions,indicating that the two phase transitions of DPPA are liquid-liquid phase transitions.The azide group became increasingly asymmetric under pressure and further decomposed at11.61 GPa.Finally,4-ABA was studied by in situ Raman scattering and synchrotron X-ray diffraction measurements in DACs to investigate its structural changes.The analyses of Raman spectra revealed that 4-ABA underwent a phase transition in the pressure range of 0.81-1.33 GPa and a conformational change at 6.31 GPa,which was consistent with the synchrotron X-ray diffraction results.The phase transition was caused by the distortion of the benzene ring,and the variation of hydrogen bonds led to the conformational change.Above 11.21 GPa,the Bragg peak of（112）shifted to a low angle,indicating the increase of interplanar distance of 4-ABA.The characteristics of the azide group in 4-ABA might be helpful for the hybridization of electron orbitals between adjacent azide groups under high pressure.