Theoretical Studies Of New Transition Metal Nitrogen-rich Structures And Confinement Of Cage-like N₁₀ Polynitrogen

High energy density materials are a highly active research field.It is significant to find efficient and environmentally friendly high energy materials.Polymeric nitrogen contains a large number of N-N single or double bonds,with high energy density due to the large energy difference between single bond,double bond(N-N:167 k J/g;N=N:419 k J/g),and triple bond(945 k J/g).Its energy density is 2-3 times as much as that of the TNT.Besides,the decomposition product of polymeric nitrogen is nitrogen.Therefore,polymeric nitrogen is a new type of environment-friendly high energy density material.It can be used in modern industries,such as propellants,explosives,and energy storage.From current studies,the synthesis conditions of polymeric nitrogen are harsh(>110 GPa,>2000K).Besides,the decomposition studies show that they can only be preserved at high pressure(>42 GPa).Therefore,the main challenges in its application are reducing the synthesis pressures and improving the dynamic stability.At present,one way to enhance the stability of polymeric nitrogen and reduce its synthesis pressure is introducing metal elements,which can provide“chemical precompression”for nitrogen to reduce the synthesis pressure and provide electronics to strengthen its stability.Up to now,some novel polymeric nitrogen structures have been predicted in the metal nitrides,for example,ring structures,chain structures,and layered structures.The energy density of metal nitrides is closely related to the nitrogen content.Nitrogen-rich metal nitride is an efficient energy carrier.Therefore,it is worth to improve nitrogen content and explore new type nitrogen-rich materials with high energy density and high stability.Another way to strengthen the stability of polymeric nitrogen at ambient conditions is to confine it into nanotubes or layered materials with hollow structures,using confinement interaction to stabilize polymeric nitrogen.At present,the confined method is still in its infancy.Only two metastable structures,N₈and A7,have been reported to be stabilized to ambient conditions.The paper will pay attention to two important points.Firstly,using CALYPSO structure prediction software,the new type Co-N and Y-N polynitrides were predicted,and the stability mechanism and energy properties were explored.Secondly,the stability of cage-like N₁₀ was studied by using the confined method and its stability mechanism was revealed.The research results of this paper are as follows:1.Using CALYPSO structure search software,the structure searching for CoN_xsystem(x=1/4,1/3,1/2,1,2,3,4,6,8,10)under the pressure of 0,20,50,100,150GPa have been applied.Five high-pressure stable phases(Pnnm-Co₂N,Pmn2₁-Co₂N,Pmna-CoN,Pnnm-CoN₂,P-1-CoN₄)and two metastable phases(P-31c-CoN₈ and P-1-CoN₁₀)were proposed.The calculation results enrich the high-pressure phase diagram of Co-N system.The novel layer-shaped N-structure and band-shaped N-structure in P-31c-CoN₈ and P-1-CoN₁₀,respectively,were first reported,which provide a new idea for pressure-inducing novel nitrogen-rich structures.The energy property and stability analysis reveal that CoN₈(6.14 k J/g)and CoN₁₀(5.18 k J/g)with high energy density can stabilized at ambient conditions.In addition,the electron properties and bonding analysis revealed that charge transfer between Co and N atoms plays an important role in its stability.This work expands the research field of metal nitrides and provides a theoretical reference for the experimental study of Co-N systems by analyzing the structural characteristics,stability,electronic structure properties,bonding characteristics,and vibration modes of Co-N systems in detail.2.Using CALYPSO structure search software,the structure searching of the Y-N system with different stoichiometry is performed at 0,20,50,100,and 150 GPa.Five novel phases(P4/nmm-YN,C2/m-YN₄,P-1-II-YN₄,P-1-YN₆,and P31c-YN₈)were proposed and the high-pressure phase diagram of Y-N was enriched.The stable pressure ranges are determined by the enthalpy difference analysis.The energy densities of P-1-II-YN₄,P-1-YN₆,and P31c-YN₈ are 1.98,2.35,and 3.77 k J/g,respectively.The detonation velocities(V_d)are 11.25,12.76 and 15.53 km/s,respectively,which are almost twice as much as that of TNT(6.9 km/s).Therefore,these three structures are candidates of energy materials.Ab initio molecular dynamics simulation(AIMD)demonstrated that P-1-II-YN₄ and P-1-YN₆ could be preserve at ambient conditions.The electron structure-property analysis shows that the stability mechanism of Y-N structure is the weak interaction between Y and N due to the charge transfer.Moreover,the Bader charge analysis shows that the charge transfer between Y and N increased as the pressure decreased.Y atoms have the advantage in stabilizing polymeric nitrogen under low pressure or even ambient conditions.This study has provided a new insight of understanding the mechanism of metal stabilizing polymeric nitrogen at low pressure.The vibration modes of P-1-II-YN₄,P-1-YN₆,and P31c-YN₈ were performed to guide further experimental synthesis.3.Using the first-principles method,carbon nanotubes and boron nitride nanotubes with different diameters are selected as confined model materials to perform the stability analysis of N₁₀@NTs system.As a result,(8,0)carbon nanotubeis an ideal material to confined N₁₀.AIMD simulation results reveal that N₁₀ can be preserved at ambient conditions and the decomposition temperature is 1100 K,which is milder than that of other one-dimensional confined systems(1400 K and 5000 K).At the same time,the nitrogen content and the energy density(3.26 k J/g)of the confined system(N₁₀@CNT)is higher than that of other one-dimensional confined systems.Electron properties and bonding analysis show that the stability mechanism of N₁₀ is forming covalent bonds with carbon nanotubes.Different from the charge transfer effect in other systems,it is the first time to report that covalent bonds stabilize the polymeric nitrogen in the confined system,which provides a new idea for stabilizing the polymeric nitrogen and extends the research field of confinement polymeric nitrogen.