Theoretical Studies On The Structures And Properties Of Novel Ce-N And Li-Ce-N Compounds Under High Pressure

Energy materials play an important role in daily life,industry,and military fields.Traditional energy materials are limited in practical application because of their poor density,environmental pollution,inadequate combustion and other defects.Polymeric nitrogen,with high energy density,high detonation performance,and environmental friendliness,is a potential new generation of high-energy material.It has potential application prospects in many fields such as energy storage,fuel,aviation,and explosives.A series of polynitrogen structures have been reported theoretically,including molecular crystal,N-chain,N-layer,N-cage,and network structures.In the experiment,cg-N,LP-N,HLP-N,and BP-N have been prepared under the conditions above 100 GPa and 2000 K by using high-temperature and high-pressure technology.It is found that four structures can only be stabilized above 42 GPa in the pressure release study.Obviously,the synthesis conditions of polynitrogen are extremely harsh and cannot be quenched to ambient conditions.Therefore,reducing the synthetic pressure of polynitrogen and improving its stability are the prerequisite and basis for its application.Introducing the coordination element（M）into polynitrogen is an effective method to solve the above problems.At present,most of the elements on the periodic table have been studied as coordination elements of nitrogen-rich compounds,including main group metals,transition metals,nonmetals,and inert elements.The effect of different coordination elements on polynitrogen is demonstrated vividly under high pressure.First,the effect of“chemical precompression”is used to dissociate the inert N≡N triple bond and reduce the synthetic pressure of polynitrogen.Second,the structural stability of polynitrogen is improved by means of charge transfer.Thirdly,the ligand effect of the coordination elements is used to induce rich polymeric structures.Therefore,finding suitable coordination elements and designing nitrogen-rich compounds with high nitrogen content and high stability have always been the core problems in the study of energy materials under high pressure.On the other hand,the energy properties can be reduced to a certain extent by introducing non-energetic coordination elements into nitrogen systems.The synthesis of Mxenes materials provides a new revelation,removing the coordination atoms in nitrogen-rich compounds,and further obtaining the pure polynitrogen with high stability and high energy density,which opens up a new research idea for the synthesis and application of polynitrogen.In this paper,cerium（Ce）,a lanthanide element with abundant valence electron structure,and lithium（Li）,a metal with the smallest mass,are selected as coordination elements to carry out studies on the structural design and properties of novel high energy density materials under high pressure.Firstly,using the flexible electron valence and strong ligand effect of Ce atom,a series of nitrogen-rich compounds with novel structures,moderate synthesis pressure,high stability and high-energy properties were obtained.In addition,the chemical stripping method was introduced into the nitrogen-rich system for the first time,the stripping of metal atoms was carried out,and a new pure polymeric nitrogen structure was successfully obtained.Finally,the metal Li was introduced into the Ce-N system,high-pressure research on the Li-Ce-N ternary system was performed,and a variety of new polymeric structures with high energy density were obtained.The main innovative results are summarized as follows:1.In the variable component structure prediction of Ce-N system in the range of 0-100 GPa,six new high-pressure stable phases（I4/mmm-Ce N₂,C2/m-Ce N₃,（?）-Ce N₃,（?）-Ce N₄,C2/c-Ce N₆,and（?）-Ce N₆）were obtained,which enriched the high-pressure phase diagram of Ce-N system.Among them,layered structure（?）-Ce N₆ with unique N₁₄ ring is firstly reported.Due to the strong ligand effect of cerium atom,（?）-Ce N₆has the lowest synthesis pressure（32 GPa）in layered ayered metal nitrides,and can be quenched to ambient conditions.Through the simulation of the Ce+3N₂→trans-Ce N₆→（?）-Ce N₆ reaction path,it is proved that Ce atoms can weaken the strength of N≡N bond,promote the transformation of N₂ to N-chain and then to N-layer,and clarify the formation mechanism of（?）-Ce N₆ under high pressure.In addition,（?）-Ce N₆ has high energy density and excellent detonation properties,making it a potential high-energy-density material.This study provides new insights into the physical mechanism by which metal atoms induce the transformation of nitrogen to polymeric structures under high pressure.2.Two high-pressure stable phases（I4₁/a-Ce N₄ and R（?）m-Ce N₆）and two metastable phases(P6mm-Ce N₁₄ and P6mm-Ce N₁₇)were obtained in the pressure range of 150-300 GPa,further updating the high-pressure phase diagram of the Ce-N system.Particularly,two new layered molecular sieve polymeric structures were first discovered in P6mm-Ce N₁₄ and P6mm-Ce N₁₇.P6mm-Ce N₁₄ possesses dynamical and mechanical stability at ambient pressure.The charge transfer between Ce and N atoms makes an important contribution to structural stability.The P6mm-Ce N₁₄ and P6mm-Ce N₁₇ have ultra-high energy densities（8.45 and 7.71 k J/g）.P6mm-Ce N₁₄ has the highest energy density and detonation performance among all metal nitrides.This study provides an important reference for designing new high-energy-density materials by increasing nitrogen content.3.The chemical stripping method was first applied to nitrogen-rich system,and the simulation study on stripping metal atoms from the nitrogen-rich structure to obtain polymeric nitrogen was carried out.At high pressure,a layered Ce-N compound（?）-Ce N₈ with crown-like N₁₈ ring was obtained using first-principles structural search.This structure has a moderate formation pressure（25.7 GPa）,and can be stabilized to ambient conditions.Drawing on the synthesis method of MXene materials,alkali anions were inserted into the interlayer of Ce N₈,and Ce atoms were successfully stripped from the Ce N₈ framework.A new crown-like polymeric structure cr-N was obtained,and can be quenched to at ambient conditions.In addition,cr-N exhibits excellent energy density（9.00 k J/g）,detonation pressure（88.46 GPa）,and detonation velocity（14.17 km/s）,making it have potential application prospect in the field of energy materials.This work provides a new research idea for the preparation of novel polymeric nitrogen structure.4.The least mass metal Li was introduced into the Ce-N system,and theoretical research on the structure search and properties of the Li-Ce-N system under high pressure was performed.Six new high-pressure phases(Cm-Li Ce N₄,C2/m-Li Ce N₆,P4/mmm-Li Ce N₈,R（?）m-Li Ce N₈,P3-Li Ce N₁₀,and C2-Li Ce N₁₂)were proposed.In R（?）m-Li Ce N₈ and C2-Li Ce N₁₂,a novel layered structure with folded N₁₈ ring and wavy spiral N-chain were first reported.The polymeric structure of P3-Li Ce N₁₀ is composed of independent N₁₀ cages.The dynamical and mechanical stability of C2/m-Li Ce N₆ and R（?）m-Li Ce N₈ can be maintained under ambient pressure.The Li and Ce atoms jointly transfer charge to N atoms to ensure structural stability.R（?）m-Li Ce N₈,P3-Li Ce N₁₀,and C2-Li Ce N₁₂ have high energy density and detonation performance.This work extends the metal nitrogen-rich compounds from a binary to a ternary system,which provided theoretical guidance for the study of new ternary nitrogen-rich compounds under high pressure.