High Pressure Studies On The Phase Transition And The Polymerization Of Sodium Azide

Under high pressure,the interaction between the nitrogen molecules will be increasing.When the interaction between the nitrogen molecules and the internal interactiom in the molecule can be compared,the nitrogen molecules will be dissociated by pressure and form a new material with the single bond,which is polymerized nitrogen.Due to the large energy difference between the single bond(159 kJ / mol)and the triple bond(946 kJ / mol),the polymerized nitrogen will release a large amount of energy when it is depolymerized to nitrogen.It ' s of significant interest as high energy density materials(HEDM)for explosive or propulsion applications.Compared with the nitrogen molecules,it is easier to decompose to reform the polymeric phase for azide compounds with the double bond.Therefore the high pressure studies on the phase transition is a hotspot in the field of condensed physics.Sodium azide as the typical azide compound,its pressure-induced behavior and high pressure phase crystal structure is not very clear yet.According to these matter,we did study in the phase transition of NaN3,obtained the following results:First,we investigated detailed the vibration of NaN3 by in-situ Raman spectroscopy under high pressure up to 87.37 GPa.It was found that hexagonal ?-Na N3 began to transform into monoclinic high pressure phase I(?-NaN3)at 2.20 GPa.When the pressure loaded to 13.65 GPa,the HP phase II(?-NaN3)appeared and the phase transformation is completed at 19.58 GPa.Up to 19.58 GPa,the ?-NaN3 changes to the HP phase III(?-NaN3),and the transition is completed at 33.45 GPa.As the pressure arises to 61.45 GPa,the ?-NaN3 disappeared and the the HP phase IV(?-NaN3)comes out.In order to verify the stability of the ?-NaN3,we carried out the Raman spectroscopy in the pressure releasing process.On decompression from thehighest pressures,the ?-NaN3 remained at pressures down to 25.10 GPa,then the ?-NaN3 is decomposed and forms a new vibration mode,which further proving that the phase change is irreversible.Next,we characterized systematically the high pressure structure of sodium azide by using the in-situ synchrotron XRD combined with the CALYPSO structure prediction method.The highest pressure in the experiment was 123 GPa.In the results,at ambient condition,the structure of sodium azide belongs to be hexagonal system,when pressure increase to 4.0GPa it transforms to monoclinic phase(high pressure phase ?).As the pressure increased to 12.9GPa,the phase ?-NaN3 occurs.When pressure loaded to 21.7GPa,the phase ?-NaN3 comes out,and the phase ?-NaN3 appeared at 74.5GPa.Combined with the theoretical calculations,the crystal structures of phase ?-NaN3,phase ?-NaN3 and phase ?-NaN3 were proposed for the first time: phase ?-NaN3 exhibited I4/mcm structure,phase ?-NaN3 crystallized in Cmcm structure and phase ?-NaN3 polymerized in Immm structure,respectively.Remarkably,the polymeric structure of phase ?-NaN3 was suggested which consisted of pseudo-benzene N6 hexagon parallel to the ab plane layer.Through the study in stability,we found the polymeric structure could be preserved on decopression to 27.82 GPa but transform to another new phase,which means the irreversibility.That result consists with the result of the in-situ Raman spectroscopy.In this paper,we explored the high-pressure phase transition of NaN3,and obtained the pressure-induced phase transformation mechanism of NaN3.Combined experiment with the calculation,we firstly proposed the crystal structure of NaN3high-pressure phases and its internal bonds.