Theoretical Studies On High-pressure Acquisition And Properties Of Novel High-energy Alkali-earth Metal Nitrides

Pressure is a basic physical parameter.Microscopically,it reduces the distance between atoms,greatly changes the electronic structure and bonding mode,and macroscopically changes the properties of materials.If the pressure can be used skillfully,we can adjust the physical and chemical properties of the material effectively and obtain a series of multifunctional materials.In recent years,static and dynamic high pressure experimental techniques have been developed to synthesize many functional materials with excellent properties,such as H3 S superconductors,superhard materials and high energy density materials.With the development of computer technology and the improvement of theoretical methods,it is possible to simulate the physical properties of material by theoretical calculation.In recent years,theoretical simulation has gradually become an important means of material designing,development and performance testing.Using theory to simulate the high-pressure environment,remarkable research results have been obtained in new functional materials.Recently,for the first time,our team has theoretically predicted a new type high temperature superconductor H3 S,whose superconducting transition temperature can reach 200 K.The theoretical research results have been confirmed by experimental researchers.This research results fully demonstrate that theoretical simulation can effectively guide the experiment to obtain new materials.Based on this research idea,I have done a systematic study on high energy density materials by first principle method.A variety of high energy materials with excellent performance are predicted theoretically.High energy density materials(HEDM)can be used as propellants and explosives.It is an important material for energy development and defense construction.With the rapid development of science and technology,the traditional energetic materials obviously can not meet the needs of continuous development in various fields.Therefore,it is particularly important to explore new high energy density materials.Nitrogen is abundant in air.Nitrogen atoms mainly exist in nitrogen molecular units,and two nitrogen atoms form triple bond N?N.In recent years,the research of nitrogen in high energy density materials has made more achievements.The bonding energies of N?N?N=N and N-N are different,and there is a huge energy difference between them.When a single bond N-N is converted to a double bond N=N or directly to a triple bond N?N,or a double bond N=N is converted to a triple bond N?N,a lot of energy is released in the process.If people get polymers or compounds that contain N-N or N=N,we can use the huge amount of energy released by polymerization.Thus a new type of high energy density material is obtained.The main content of this thesis is a systematic and comprehensive theoretical study of novel high energy alkaline earth metal nitrides.1.Firstly,we choose the alkaline earth metal beryllium nitrides as the research object.The beryllium nitride has a known crystalline structure of ground state Be3N2 at room temperature.However,there are few studies of beryllium nitrides under the high pressure.So,we studied the structural evolution behaviors of beryllium nitrides at high pressure by using structure searching technique.A stable N-rich compound Be N4 was predicted.It has a monoclinic P21/c space group,and its synthesis pressure is less than 40 GPa.Total energy calculations show that P21/c-Be N4 can synthesized by pressing on the Be3N2 and nitrogen.Besides,P21/c-Be N4 can be quenched to atmospheric pressure.There is a new type of nitrogen polymerization form N10 ring in P21/c-Be N4 structure.The N10 ring consists of a three-dimensional puckered network,which plays a vital role in the stability of the structure.As far as we know,this stable ring network structure is first found in alkali earth metal nitrides.The energy density released from the decomposition of P21/c-Be N4 is as high as 6.35 k J·g-1,which is close to that of pure polymeric nitrogen(cg-N).Therefore,P21/c-Be N4 is a kind of potential high energy density material.2.Based on the same research ideas,in magnesium compounds system,we use the first principle of high pressure structure searching method,and five novel magnesium nitrides(Mg N,Mg2N3,Mg N2,Mg N3,Mg N4)were predicted theoretically.With increasing of nitrogen concentration,more polymerization forms of nitrogen appear in magnesium nitrogen compounds,such as one-dimensional bending N3 unit,plane triangle N4 unite,puckered N6 ring and one-dimensional infinite chain.The N3 unit and N4 unite are first to appear in alkaline earth metal nitrides system.Released energy of decomposition about P-1-Mg N3 and P-1-Mg N4 at atmospheric pressure can reach 2.83 k J·g-1 and 2.01 k J·g-1,respectively,which are close to those of conventional high energy density materials.Our results extend the scope of research in the field of polymerized nitrogen and provide theoretical guidance for the synthesis of high-energy density compounds in the experiment.In addition,we have also done some researches on the transition metal light element superhard multifunctional materials.Superhard materials have excellent physical properties,high melting point,high hardness,corrosion resistance,and even some compounds have good electrical and magnetic properties.Cubic boron nitride(c-BN)and diamond are two kinds of traditional superhard materials.Because of their excellent physical and chemical properties,they have been widely used in various fields.However,as traditional superhard material,they all have their own shortcomings.In the process of processing steel materials,the high temperature environment will make the diamond carbonized,cause wear and shorten the service life.Cubic boron nitride materials can only be synthesized by high-temperature and high-pressure methods,and bulk materials are generally small,which are limited in industrial conditions.It has been found that a new superhard multifunctional material can be obtained by combining transition metals which have more valence electrons with light elements which are easy to form covalent bonds.1.Using the structure searching software,we find two B-rich compounds C2/m-Ta B3 and Amm2-Ta B4.When the pressure rises to 60 GPa,an orthogonal Amm2-Ta B4 falls on the convex hull diagram,and when it is optimized from high pressure to P = 0 GPa,it remains dynamic stability.This indicates that Amm2-Ta B4 is still stable at atmospheric pressure.The high-pressure Amm2-Ta B4 shows the best mechanical properties,and the predicted Vickers hardness reaches 30.2 GPa at atmospheric pressure.Our study has updated the high-pressure phase diagram of Ta-B compounds.It is proved that Amm2-Ta B4 can be obtained by means of high-pressure technology.2.For transition metal vanadium boron compounds,two novel VB3 structures were predicted by using the structure searching software.Their space groups are C2/m and Amm2,respectively,and the phase transition appears at P = 21.5 GPa.It is proved that both of these structures maintain dynamical stability and mechanical stability.The hardness values of C2/m-VB3 and Amm2-VB3 are 43.3 GPa and 37.6 GPa,respectively,especially when P = 21.5 GPa,the hardness of Amm2-VB3 reaches 42.5 GPa.So C2/m-VB3 and Amm2-VB3 can be regard as a kind of potential superhard materials.