Theoretical Study On Structure And Properties Of Transition Metal Nitrogen-rich Compounds Under High Pressure

Traditional energy materials,such as coal,oil and natural gas,play an important role in daily life.However,with the continuous development of society,people have higher and higher requirements for the properties of energy materials.Traditional energy materials gradually expose their disadvantages and shortcomings,which are mainly reflected in two aspects:on the one hand,the energy density of traditional energy materials is limited and can not meet the demand for high energy density in some aspects,on the other hand,traditional energy materials will produce a large number of pollutants that damage the ecological environment.Therefore,people are eager to find new energy materials to replace these traditional energy materials in order to solve the problems of low energy density and environmental pollution.As a new high energy density material,polymeric nitrogen has attracted extensive attention.Polymeric nitrogen refers to the new polymeric structure formed by the dissociation of nitrogen(N₂)under high pressure.Firstly,polymeric nitrogen has the property of high energy density,and its energy mainly comes from the huge bond energy difference between the N?N and N=N/N-N bond.Secondly,the decomposition product of polymeric nitrogen is only nitrogen(N₂),which does not pollute the ecological environment.Therefore,polymeric nitrogen is an environment-friendly polymeric material with high energy density.At present,a large number of polymeric nitrogen structures with excellent properties have been reported in theory,and four kinds of polymeric nitrogen structures(cg-N?LP-N?HLP-N and BP-N)have been confirmed by experiments.These research results have greatly inspired people's enthusiasm for the research of polymeric nitrogen structures.At present,the application of polymeric nitrogen still faces two urgent problems to be solved:one is that the synthesis conditions of polymeric nitrogen are extremely harsh:it needs millions of atmospheres and thousands of K,the other is that the synthesized polymeric nitrogen can only be stable under high pressure.Therefore,reducing the synthesis conditions of polymeric nitrogen and improving its stability is a hot issue in the research of polymeric nitrogen at present.Recent studies have shown that the synthesis pressure of polymeric nitrogen structure can be greatly reduced and its structural stability can be improved by introducing coordination elements into the structure of polymeric nitrogen.The introduction of coordination elements provides an effective solution for the mild and controllable preparation and interception of polymeric nitrogen under ambient conditons.The formation pressure and stability of polymeric nitrogen structure formed after the introduction of coordination elements are closely related to the electronic valence state of coordination elements.Generally,the richer the electronic valence state of coordination elements,the more diversified the polymeric nitrogen structure formed,and the easier it is to find the polymeric nitrogen structure with excellent properties.In addition,the lower electronegativity of coordination elements,when combined with nitrogen atoms,it is easier to lose the outer valence electrons,which makes nitrogen change the original orbital hybridization mode and form a multi nitrogen polymeric structure with excellent properties.In this paper,transition metal elements Cd?Mn and Sc with rich electronic valence and relatively low electronegativity are selected as coordination elements to carry out theoretical research on high-pressure structure prediction and properties of transition metal nitrogen-rich compounds.Using the first-principles calculation method based on density functional theory and particle swarm optimization algorithm,the high-pressure structure prediction and properties of three transition metal nitride systems are studied.The main conclusions are as follows:1.Prediction of variable component structure and properties of Cd-N system under high pressureUsing the first-principles calculation method based on density functional theory and particle swarm optimization algorithm,the high-pressure variable component structure of Cd-N system is predicted,and a series of structures with novel structure and excellent properties are obtained:four high-pressure stable phase structures(P4mbm-CdN₂?Cmmm-CdN₄?I(?)d-CdN₄ and C2/c-CdN₅)and a metastable phase structure(C2/m-CdN₆).Through the analysis of structural characteristics,it is found that the nitrogen structural units in the P4mbm-CdN₂?Cmmm-CdN₄?I(?)d-CdN₄?C2/c-CdN₅ and C2/m-CdN₆ are diatomic N₂,standard armchair nitrogen chain,S-armchair nitrogen chain,Zigzag-antizigzag nitrogen chain and N₁₄ grid layered structure,respectively.Through the stability analysis,it is found that the structures of Cmmm-CdN₄ and C2/m-CdN₆ can be stabilized to ambient conditions,and the structures of Cmmm-CdN₄ and C2/m-CdN₆ have high decomposition temperatures(2000 K and 400 K)due to their high decomposition barrier.The analysis of energy characteristics shows that Cmmm-CdN₄?I(?)d-CdN₄?C2/c-CdN₅and C2/m-CdN₆have high energy density and excellent detonation pressure and velocity.The analysis of electronic structure characteristics shows that all the predicted structures are metal phases.The N-N bond and Cd-N bond in the polymeric structure exist in the form of strong covalent bond and weak ionic bond,respectively.The charge transfer between the Cd and N elements plays an important role in the stability of the structures.2.Study on high pressure new phase and property prediction of Mn-N systemThe first-principles calculation method based on density functional theory and particle swarm optimization algorithm are used to predict the high-pressure variable component structure of Mn-N system.A series of novel structures and excellent structures are proposed:two high-pressure stable phase structures(C2/m-Mn N₄?P (?)-Mn N₄)and four metastable phase structures(P4/mmm-Mn N₄?P (?)-Mn N₅?C2/m-Mn N₆ and P (?)-Mn N₈).Through the analysis of structural characteristics,it is found that the nitrogen structural units in the C2/m-Mn N₄?P (?)-Mn N₄?P4/mmm-Mn N₄?P (?)-Mn N₅?C2/m-Mn N₆ and P (?)-Mn N₈ structures are diatomic N₂,standard armchair-type nitrogen chain,N₄ ring,N₂₂ ring layered structure,S-type nitrogen chain and armchair-type nitrogen chain,respectively.The N₄ ring and N₂₂ring layered structure are reported for the first time.Through the analysis of structural mechanical properties,it is found that all the predicted structures are plastic materials with small Poisson's ratio,among which the P4/mmm-Mn N₄ phase structure is superhard structure with the strongest plasticity.Stability analysis shows that P (?)-Mn N₄ and P (?)-Mn N₅ structures can be stabilized to ambient conditions,and P (?)-Mn N₄ and P (?)-Mn N₅ structures have high decomposition temperature(2000 K,3000 K),mainly due to their large decomposition barrier.The analysis of energy properties shows that the predicted structure has high energy density and excellent detonation pressure and velocity.The analysis of electronic structure properties shows that the predicted structures are metal phases,the N-N bond is strong covalent bond,and the Mn-N is weak ionic bond.The charge transfer between the Mn and N elements plays an important role in the stability of the structures.The analysis of magnetic properties shows that C2/m-Mn N₄?P (?)-Mn N₄ and P (?)-Mn N₅ are ferromagnetic materials,while P4/mmm-Mn N₄?C2/m-Mn N₆ and P (?)-Mn N₈ are anti-ferromagnetic materials.3.Prediction of variable component structure of Sc-N system in nitrogen-rich region under high pressureThe first-principles calculation method based on density functional theory and particle swarm optimization algorithm are used to predict the high-pressure variable component structure of Sc-N system.In the nitrogen-rich region,a P (?)-Sc N₈ layered structure with unique structure,excellent energetic characteristics and explosion characteristics is proposed.The structure is a sandwich structure composed of N layers and Sc elements.The P (?)-Sc N₈ structure is a metal phase,and the N-N bond and Sc-N bond in the structure are strong covalent bond and weak ionic bond,respectively.The energy density of P (?)-Sc N₈ structure is 7.53 kJ/g,which is much higher than that of TNT(4.3 kJ/g).The explosion pressure is better than TNT,and the explosion velocity value(22.63 Km/s)is more than three times that of TNT(6.9Km/s).