High-Pressure And High-Temperature Syntheses And Stabilities Studies Of Novel Metal Polynitrogen Compounds

Nitrogen is the most abundant element in the Earth’s atmosphere,which stabilizes with the strongest homoatomic triple bonds.The polymeric nitrogen structures composed of low-order covalent bonded nitrogen atoms can release enormous energy upon decomposition into nitrogen molecules.Thus,the polynitrogen compounds have been regarded as a new generation of high energy density materials（HEDMs）,possessing great application potential in the explosives and propellants.Various polymeric nitrogen structures have been discovered theoretically,including N₅ and N₆rings,1D infinite polymeric nitrogen chains,and 2D layered polymeric nitrogen structures.Through coordinating with metal elements,these polymeric nitrogen structures are suggested to be stabilized in novel polynitrogen compounds with various stoichiometries and structures under much milder conditions.Pressure can modulate the structures,bonding forms and physical and chemical properties of materials,causing the structural transitions and chemical reactions which cannot be achieved at ambient conditions.The high-pressure strategy has been proved to be an effective route to obtain new materials,discover new structures and explore new properties.In recent years,a series of novel polynitrogen compounds have been successfully synthesized through laser heating metal or metal azide buried in solid nitrogen under high-pressure conditions.However,most of them become unstable upon decompression,limiting their practical applications under ambient conditions.Thus,the synthesis of novel polynitrogen compounds with high energy content and high stability is still an important issue in the studies of HEDMs.Cyclo-N₅^-is the most widely-studied layered polymeric nitrogen structure for its unique all-nitrogen ring structure and good thermal stability from aromaticity.Various binary pentazolate compounds with higher cyclo-N₅^-content have been synthesized through high-pressure and high-temperature experiments.However,their stabilities can be further improved.Introducing transition metal elements with richer valence state is beneficial to obtain novel polynitrogen compounds with more novel polymeric nitrogen structures.In this paper,lithium azide,sodium azide and mixture of scandium and nitrogen have been selected as precursors.The syntheses and stabilities of novel polynitrogen compounds have been studied through laser heating DAC system and liquid-nitrogen cycling cryogenic system.Combined with in-situ high-pressure synchrotron XRD and Raman spectral measurements,the structures of novel polynitrogen compounds and the corresponding products after pressure-induced structural transitions and chemical transformations have been systematacially characterized and analyzed.The research results are as follows:1.We studied the chemical transformation of lithium azide under high-pressure and high-temperature conditions and the structures and stabilities of the new laser-heated products.Two novel Li-N compounds,P2₁/c-LiN₅ and P-1-LiN₂,have been synthesized by directly compressing and laser heating LiN₃ at～60 GPa and 2000 K for the first time.The P2₁/c-LiN₅ structure contains cyclo-N₅^-,while P-1-LiN₂ structure contains infinite polyacetylene-like nitrogen chains.Both of them have N-N bond lengths intermediate between single and double bond lengths,indicating the energetic nature.Upon decompression,the structural transition from P2₁/c-LiN₅ to P2₁/m-LiN₅occurs at 15.6 GPa,and cyclo-N₅^-remains stable.Meanwhile,the polyacetylene-like nitrogen chains of P-1-LiN₂ decompose into N₂^-units of P6₃/mmc-LiN₂.As the energetic polymeric nitrogen structures with similar bond lengths,cyclo-N₅^-presents higher stability than polyacetylene-like nitrogen chains upon decompression,which should be contributed by the aromaticity of cyclo-N₅^-.At room temperature,LiN₅ can be traced down to 5.7 GPa,and LiN₂ can be traced down to ambient conditions.At low-temperature of 80 K,the pressure-produced LiN₅ can be recovered to ambient pressure.2.We studied the high-pressure and high-temperature syntheses,the pressure-induced structural transition and low-temperature recovery of sodium pentazolate compounds.At 60 GPa,laser heating Na N₃ produces a mixture of Pmn2₁-Na N₅ and Pm-Na₂N₅ compounds.These two sodium pentazolate compounds contain cyclo-N₅^-and N₅^2-anions respectively and can be regarded as the potential high energy density materials.Upon decompression,the pressure-induced structural transition from Pmn2₁-Na N₅ to Cm-Na N₅ has been firstly observed in the pressure range of 14-23 GPa.The Cm-Na N₅ still contains cyclo-N₅^-,which can be traced down to 4.7 GPa at room temperature.Raman spectral measurements under low-temperature conditions indicates Na N₅ can be recovered to ambient pressure at 160 K.The Pm-Na₂N₅ is suggested to decompose into the P4/mmm-Na N₂ and Na N₃ during the decompression.3.We studied the high-pressure and high-temperature syntheses of new Sc-N compounds.Fm-3m-Sc N has been synthesized at 50 GPa by laser heating scandium and nitrogen.P-1-Sc N₅has been obtained by laser heating scandium and nitrogen at 60GPa with Fm-3m-Sc N as a by-product.The novel P-1-Sc N₅ compound contains armchair-like N₁₀^6-chains.At 80 GPa,laser heating scandium and nitrogen results in the formation of P2₁/c-Sc N₅ and Fm-3m-Sc N.The P2₁/c-Sc N₅ is a novel compound possessing 2D layered polymeric nitrogen structures.The average N-N bond lengths of both two new polymeric nitrogen structures are between single and double bond lengths,indicating the energetic nature.Decompression experiments indicate that the P-1-Sc N₅can be traced down to 35 GPa,and P2₁/c-Sc N₅ can be traced down to 10 GPa at room temperature.