High Pressure And High Temperature Study Of Metal Polynitrdies Compounds

Nitrogen is the most abundant element in the earth’s atmosphere,and it is easy to form stable diatomic molecules under ambient temperature and pressure conditions.Due to the huge energy difference between nitrogen triple bond and nitrogen single/double bond,polymeric nitrogen structure formed by nitrogen single/double bond will release a lot of energy when it is decomposed into environmentally pollution-free nitrogen,which is a new generation of environmentally friendly materials with high energy density,and has a wide application background.High pressure can change the distance between atoms,regulate the electronic structure and bonding state,and is an effective means to obtain new materials and new properties that are difficult to exist under ambient pressure.In recent years,a variety of pure nitrogen polymeric nitrogen materials have been synthesized and stabilized by high pressure technology,but these harsh temperature and pressure conditions seriously limit the further exploration and application of polymeric nitrogen.The introduction of metal elements into the pure polymeric nitrogen system will be beneficial to the synthesis and stability of polynitrogen.The metal atoms can produce chemical precompression effect on the polymeric nitrogen structure,which makes the synthetic pressure significantly lower than that required to obtain the pure nitrogen polymeric nitrogen structure by compressing pure nitrogen molecules.At the same time,metal atoms can further regulate the electronic structure of polymeric nitrogen and enhance its stability through charge transfer.More and more nitrogen-rich metal compounds have been reported in theoretical studies,in which it is predicted that the compounds contain a variety of novel polynitrogen structures,including zero-dimensional all nitrogen ring,short chain structure,one-dimensional infinite chain structure,two-dimensional layered polymeric nitrogen structure and three-dimensional network polymeric nitrogen structure.Nitrogen-rich compounds with a higher proportion of nitrogen are easy to form unique two-dimensional and three-dimensional polymeric nitrogen structures,which is conducive to the improvement of their energy density,and has become a research focus in recent years.At present,the synthesis and stabilization pathways of novel layered and cage-like polymeric nitrogen structures need to be further explored.At the same time,the novel electronic structure and topological properties of the polymeric nitrogen structure also need to be studied.In order to solve the above problems,the diamond anvil cell device combined with laser heating technology was used to carry out the experimental work of structural phase transformation and chemical transformation of potassium azide,sodium azide,yttrium metal and molecular nitrogen under high temperature and pressure,and the synthesis path of new nitrogen-rich materials was studied in detail.Combined with theoretical simulation,the crystal structure,electronic structure and stability of new nitrogen-rich compounds were systematically studied by using experimental techniques such as high pressure in situ Raman scattering spectroscopy and high pressure in situ X-ray diffraction.The research results are as follows:1.The formation of polynitrides structure induced by HTHP of NaN₃in nitrogen-rich environment and the stability of the new compounds were studied.Binary metal pentazolate with space group Cm phase were obtained by laser heating experiments with NaN₃and N₂under 36 GPa,2100 K.The average bond length of N-N bond in cyclo-N5^-is between single bond and double bond,which is a potential high energy density material.A convincing fingerprint of Cm-NaN5 was obtained by the measurement of high pressure in situ Raman scattering spectra combined with theoretical calculations.The pressure release at room temperature indicates that Cm-NaN₅may exist as an amorphous state when the pressure is lower than 17 GPa.The pressure release at low temperature shows that Cm-NaN₅can be capture under ambient pressure at the temperature of 25 K.2.The formation of polynitrides structure induced by HTHP of KN₃in nitrogen-rich environment and the structure,properties and stability of the new compounds were studied.By using KN₃and N₂under HTHP of 80-90 GPa and2100-2300 K,the layered structure was synthesized for the first time and stabilized on K₂N₁₆,a supernitrides compound.The structure of planar layer is constructed by N₁₈ring as the basic unit,and its average bond length is close to N-N single bond,which has high energy density.Bader charge analysis showed that the potassium atomic layer stabilizes this unique polymeric nitrogen layer structure by charge transfer to the nitrogen layer.Further analysis of the electronic band structure of K₂N₁₆shows that it is a topological semimetal with multiple Dirac points on the Fermi surface,and hots high order Dirac fermions with cubic dispersion,which are contributed by fully sp²-hybirdized polymeric nitrogen layers arrange in P6/mcc symmetry.Stability studies of K₂N₁₆show that K₂N₁₆remains stable above 50 GPa and transforms to KN₅with further release of pressure.3.The formation of polynitrides structure induced by HTHP of metal Y in nitrogen-rich environment and the structure and properties of the new compouds were studied.P-1-YN₆with one-dimension infinite nitrogen chain structure was obtained by laser heating experiment of Y and N₂at 85 GPa.By analyzing its bond formation,the average bond length of infinite nitrogen chain in this structure is 1.302,which is close to the N-N single bond,so the synthesized P-1-YN₆may be a potential high energy density material.The laser heating experiment of Y and N₂mixture at 100 GPa verified the P6₂22-Y₂N₁₁with double helix chain structure obtained by Laniel et al at100 GPa.P-4-YN₉,a supernitrides material with a cage diamond-like structure,was successfully synthesized by laser heating of Y and N₂mixtures at 120 GPa.This is the first experimental synthesis of metal-stabilized polynitrides with a cage structure,in which the cage-like structure is composed of all single bonds,and its energy density is as high as 5.36 k J/g.It is by far the highest energy density of any metal polynitrides.The energy density of the P-4-YN₉structure after removing the metal Y atom is as high as 13.17 k J/g,which exceeds the current experimental synthesis of pure nitrogen polymeric nitrogen structures（cg-N,LP-N,HLP-N and BP-N）.