The High Pressure Study Of Alkali Polynitrogen Compounds

Polynitrogen compounds usually contain singly bonded or doubly bonded nitrogen atoms,which are different from the traditional nitrides.The bond energy of triple-bonded molecular nitrogen is as high as 954 k J/mol,which is much more than those of doubly bonded（418 k J/mol）and singly bonded（160 k J/mol）nitrogen atoms.This huge energy difference makes polynitrogen compounds release huge energy when they decompose into pure nitrogen.Therefore,polynitrogen compounds are considered to have great potential for high energy density materials（HEDMs）which have broad application in the field of national defense and aerospace.As the most representative polynitrogen compounds,the alkali pentazolate have the advantages of high nitrogen content and high stability.At present,the synthesis of alkali pentazolate has become a hot topic and has attracted much attention from researches.High pressure can effectively control the distance between atoms（molecules）,affect the interaction and bonding between atoms（molecules）,and then change the structure and properties of substances.It is an important technical means to obtain new structures and new materials.The high pressure study of crystal structure transiton and chemical transformation of polynitrogen compounds can deepen our understanding of the structure evolution and synthesis mechanism of polynitrogen materials under high pressure and also provide new ideas for the construction of polynitrogen compounds with new structures and special energitic characteristics.In this paper,the diamond anvil cell technique combined with high pressure in-situ Raman scattering spectroscopy and synchrotron radiation XRD methods are used to analyze the phase transition and chemical transformation of sodium azide,lithium azide and pure nitrogen,and lithium and pure nitrogen under high pressure.The synthesis of polynitrogen compounds was systematically studied,and precursor affection on the structure and physical properties of the synthesized polynitrogen compounds was discussed.The synthetic routes and stability of the polynitrogen compounds were studied by the laser heating technology,high pressure in-situ low temperature Raman spectroscopy and theoretical calculations.The main conclusions obtained in this paper are as follows:1.Alkali azides are powerful candidate for the high pressure sythesis of HEDMs.We have studied the crystal structure evolution and chemical transformation of NaN₃under high pressure by in-situ Raman spectroscopy and synchrotron X-ray diffraction methods up to 57.9 GPa.The initial rhombohedral NaN₃ transforms into the monoclinic C2/m-NaN₃ at 0.6?GPa,which is in agreement with previous studies.Monoclinic NaN₃ transforms into the tetragonal I4/mcm-NaN₃ at 15.5 GPa.With further compression,in the pressure range of 19.6-21.7 GPa,both the Raman spectra and XRD diffraction patterns indicate the chemical transformation from NaN₃ to new kinds of Na-N compounds with appearance of experimental signals which cannot be attributed to NaN₃.It is very likely that a partial chemical transformation from NaN₃to NaN₅ occurs at 19.6 GPa.The NaN₅ is not stable at ambient conditions upon decompression.For the first time,we found the transformation of pressure-induced NaN₃ into a new polynitrogen compound NaN₅ by high pressure in-situ Raman spectroscopy and synchrotron X-ray diffraction methods.NaN₅ has high density（3.34g/cm³）and high nitrogen content（75.23%）,and the bond lengths in N₅ˉare between single and double bonds.Our study indicates that NaN₅ can be formed by cold-compressing NaN₃,providing a potential route for the synthesis of binary alkali pentazolate compound.2.Searching for polynitrogen compounds has attracted great attention due to their potential applications as high energy density materials.We have carried out the synthesis study of new structured lithium pentazolate（Li N₅）compound by compressing lithium azide and molecular nitrogen under 41.1 GPa and 1800 K.The formation of Li N₅ with the crystal structure P2₁/m-Li N₅ was identified according to the appearance of the vibrational modes of N₅ˉrings by in-situ Raman spectra and synchrotron X-ray diffraction measurements under high pressure.During the decompression process,Li N₅ remains stabled down to 18.5 GPa,while LiN₂ remains stabled down to 8.7 GPa.The bond lengths in N₅ˉare between single-and double-bond lengths.Through high pressure in-situ low temperature Raman spectroscopy measurements,P2₁/m-Li N₅ can be stabilized to ambient pressure in 190K.For the first time,P2₁/m-Li N₅ which has been predicted under high pressure,was synthesized by laser heating DAC technology.P2₁/m-Li N₅ has quite different thermodynamic stability with that of the P2-Li N₅which has been synthesized in previous study.This study indicates that the precursor can effectively tune the crystal structure of pentazolate,providing us an alternative route to synthesize a polynitrogen compound with a novel structure and deepens the understanding of the crystal structure and stability of Li N₅ under different temperature and pressure conditions.3.Synthesis of Li-N polynitrogen compounds has been carried out by laser heating compressed Li and N₂ at 65.8 GPa and 2000 K.High pressure in-situ Raman scattering spectroscopy and synchrotron X-ray diffraction methods were used to characterize the crystal structure and chemical composition of the formation products.The experimental results show that the products after laser heating Li and N₂ under high pressure are P4₂/mmc-Li N and P-1-LiN₂,which are different from the P2-Li N₅reported in previous study.The P4₂/mmc-Li N and P-1-LiN₂contain dumbbells-shaped N₂^2-ions and infinite zigzag nitrogen chains with N₄^2-as repeating units,respectively.Decompression experiments show that P4₂/mmc-Li N and P-1-LiN₂ remain stable down to 35.8 GPa.According to theoretical calculation,it can be concluded that there is an ionic bond interaction between the lithium atom and the nitrogen chain in LiN₂.