Bonding Properties Of Aluminum Nitride At High Pressure

High pressure condensed matter physics is the subject of exploring structures,properties and applications of materials under extreme conditions.The material is compressed under high pressure,which the physical and chemical properties will be changed.High pressure may also introduce one phase to another unknown structure and can also decrease the chemical barrier,promoting the formation of reaction.As geting new materials under normal pressure are difficult to synthesize and most solid nature material are lived in a high-pressure condition.Therefore,high pressure science will be human to understand of nature and open the door to the key for the universe.In addition,when reached a certain degree of high pressure or high pressure and high temperature conditions,the atom arrangement,crystal structure,electronic structure,magnetic structure could be changed.At the same time,the mutation of physical and chemical properties of material,such as insulator transform to metal which always happened.High pressure can affect the chemical reaction between the elements by affecting the electronic orbit and their occupying.Some special components that are unsuitable in the ambent environment can be synthesized under high pressure,such as nitrogen compounds.Under pressure,the chemical measurement and structure have not changed,and the electronic properties of the material have basically been modified.The pressure greatly reduces the distance between atoms and strengthens the interaction between atoms,and the result makes the band widen.Further overlaps of energy caused by pressure can achieve the transition of insulators to metals or semiconductors to metals.In order to adapt to the development of aerospace,energy and military industry under high and new technology conditions,people put forward higher requirements for energetic materials.Seeking high-energy density materials has become the focus of attention for scientists and engineers all over the world.The high energy density material?high energy density material,HEDM?generally requires that the energy performance is superior to HMX?density>20 g/cm³,explosion speed>9 km/s,burst pressure>40 GPa?.Due to the bond energy of molecular nitrogen?N=N?is954 kJ/mol which is 6 times than the N-N bond energy?160 kJ/mol?,when the N-N bond breaking to polymerize into N=N and will release tremendous energy,the energy difference is about 6 times the N-N bond.At present,the synthetic pressure and temperature of the polymerized nitrogen,represented by cg-N,are too harsh?the pressure is up to 110 GPa and the temperature is 2000 K?.Because the product is clean,it has no pollution to the environment.the materials for clean energy as an environmentally friendly can be widely used in explosive and propellant,gas generating agent.Polymericnitrogen has become a hot point for researching in the field of energetic materials.However,as for energy density materials,polymericnitrogen is facing two urgent problems.One is that the synthetic pressure and temperature are too high.So,we need to reduce the synthetic pressure and temperature that could be applied to real life and production.The other one is that whether it can guarantee the stability under normal temperature and ambent pressure.Therefore,the focus of the near and mid-term research is to explore nitrogen-rich metal compounds as a means to search for high energy density materials,so as to solve the current research problems in the direction of polymerization nitrogen.A large number of nitrogen-rich metal compounds,as high-energy density materials,have made great theoretical and experimental breakthroughs in alkali metals and alkaline earth metals.Various structural characteristics of polymeric nitrogen have been revealed.Eremets et al concluded in experiment that sodium azide?NaN₃?will start polymerization at 50 GPa,which is far less than the synthetic pressure of cg-N?60 GPa?.Medvedev et al in hydrostatic pressure by X-ray diffraction and Raman spectroscopy study on the phase transformation of LiN₃,founding that under ambent pressure the NaN₃ had the same configuration structure,when the pressure is increased to 60 GPa and remains stable.Zhang et al though the structure prediction in the range of 200 GPa found a similar NaN₃ high structure with metallic properties in P6/m structure.In addition,the theoretical prediction for a number of different structures,such as the discovery of nitrogen with five-membered ring,six-membered ring in LiN₅.And in other alkali metals and alkaline earth metals at high pressure had been induced N₂^4-,N₂^2-,N₄²⁺,N₄^4-,N^5-,N₆⁺,N₆^-,N₆²⁺,N₈^4-,N_?^n-anion structures,which is a typical high energy density materials.However,there is no fundamental explanation for the formation of polymeric nitrogen under high pressure.Hence,it is also important to reveal the necessary for the formation of polymeric nitrogen and explore the electronic structure and bonding mechanism.In this work,the search technology?CALYPSO?and the first principle calculation method has been used to investigate the structures.The existence form of polymeric nitrogen,metallization and bond characteristics of energy density materials at high pressure has been detailed analysised.Details as follow:1)AlN₃ is expected to release an enormously large amount of energy?2.75 kJ/g?and driven by the sp² hybridization of nitrogen atoms.2)Because of the electrons transfer from aluminums to polymeric nitrogen chains,there is a pseudo-gap in the electronic structures of AlN₃.3)The N_p electrons form?-type chemical bonds with the neighboring atoms,resulting in the delocalization of?electrons and charge transfer in polymeric nitrogen chains.The disparities of charge density distribution between nitrogen atoms in polymeric nitrogen chains are the principal reason for the metallicity.