Structural Design And Experimental Synthesis Of Ga-N And Sc-Ta-B Compounds Under High Pressure

As the third thermodynamic parameter independent of temperature and chemical composition,pressure can shorten the distance between atoms,reduce the chemical reaction barrier,change the bonding mode at the microscopic level,and prepare many new functional materials that are difficult to develop under conventional conditions.High pressure technology has played a vital role in the fields of superconductivity,super-hard and extremely energetic materials,among which the latter two have outstanding properties of high hardness and high energy density,which are of great value in basic scientific research and practical applications.Nitrogen is the main component of the Earth’s atmosphere,accounting for about78%by volume.At normal temperature and pressure,nitrogen is a stable diatomic molecule with N≡N combination.Under high pressure,nitrogen can be dissociated into a solid polynitrogen structure containing N=N or N-N.Because there is a huge energy difference between N≡N and N-N,N=N,the transformation process is accompanied by huge energy release,so the polymeric nitrogen material is the high energy density material that attracts much attention.However,the experimental preparation of polymeric nitrogen can only be done in the environment with atmospheric pressure of over million（100 GPa）,so the synthesis conditions are too harsh.In recent years,scientists have found that the introduction of metal elements into nitrogen can effectively reduce the synthesis pressure of polynitrogen and form a rich variety of polynitrogen configurations.Therefore,how to design new nitrogen-rich compounds with high nitrogen content and low synthetic pressure is one of the research hotspots in physics,chemistry,and materials.Transition metal elements have high electron density and provide high resistance to compressibility in materials.Boron can form a strong covalent network structure,which can provide strong shear resistance,so the combination of boron and Boron is always an excellent candidate system for superhard materials.However,the hardness of the synthesized transition metal borides is much different from that of the traditional superhard materials diamond and boron nitride.Therefore,increasing the hardness of transition metal borides and exploring the strengthening mechanism have important research and application value.Based on the above background,in this paper,a new extreme energetic material Ga N_xand a new superhard material Sc_0.5Ta_0.5B₂were designed using the CALYPSO structure prediction method independently developed by our research group.At the same time,the corresponding high-pressure experimental technology was used to prepare and characterize these two material systems,to achieve the theoretical prediction results.The specific innovation achievements are as follows:1.Design and preparation of a new extremely energetic material Ga N_xIn this work,the thermodynamic stability of Ga-N system under high pressure was systematically studied by using CALYPSO crystal structure prediction method.Three new nitrogen-rich compounds Ga N_x（x=5,10,15）with stoichiometric ratios were predicted theoretically.These Ga-N compounds exhibit abundant chemical bonding behavior and novel topological structure of polymeric nitrogen.In Ga N₁₅,N atoms constitute the basic structural unit of five-membered nitrogen ring and are discretized distributed in the lattice.Ga N₁₀is made up of infinite chain of nitrogen and nitrogen molecules,Ga atoms and the infinite chain constitute two mutually perpendicular plane at（011）and 01 1 two plane along the axis formed a square channel to accommodate the nitrogen molecules;Ga N₅has a layer-like structure of polymeric nitrogen,in which nitrogen atoms form a nonplanar 12 binary nitrogen ring,which is distributed indefinitely in the ab plane.Subsequently,a new nitrogen-rich compound in Ga-N system was prepared under the pressure and temperature of 85 GPa and 2000 K by laser assisted heating of diamond against anvils.The diffraction peaks of Ga N₁₀and Ga N₅were found in the X-ray diffraction spectra of high pressure synchrotron radiation.The structure of the diffraction peaks was consistent with the theoretical prediction results,and the synthesis pressure was much lower than the preparation conditions of polymeric nitrogen（>100 GPa）,indicating that the introduction of Ga effectively reduced the preparation conditions of polynitrogen.In addition,the theoretical calculation shows that their energy density is 4.1-5.3 k J/g,which is comparable to traditional explosives（such as TNT,HMX,etc.）,indicating that the nitrogen-rich Ga-N system is a typical extremely energetic material.The present study proposes a new idea of designing materials with high energy density through the chemical reaction of p-region elements with N₂under high pressure,which expands the family members of nitrogen-rich compounds.2 Structure design and experimental preparation of a new superhard material Sc0.5Ta0.5B2In this work,the thermodynamic stability of ternary transition metal diborides was systematically studied by using CALYPSO structure prediction software,and the new ternary compound Sc_0.5Ta_0.5B₂was predicted to be a good candidate system for superhard materials.The theoretical calculation shows that the hardness of Sc_0.5Ta_0.5B₂is increased by about 10%,and the ideal shear strength in the weakest direction and the dimensional shear strength of Sc_0.5Ta_0.5B₂are increased by about 30%compared with the parent phase Sc B₂and Ta B₂.By studying the structural snapshot of the fracture point of the stress-strain curves of parent phases Sc B₂and Ta B₂,it is found that the bonding fracture between transition metal and boron is the main cause of structural damage of transition metal diborides.Therefore,optimizing the bonding form can effectively enhance the mechanical strength of the system.In the theoretical prediction of Sc_0.5Ta_0.5B₂,it was found that the charge distribution in the Sc-Ta-B system could be changed by adjusting the proportion of metal atoms,which enhanced the bonding between the transition metal and boron,and thus increased the hardness of the material.Subsequently,single-phase Sc_0.5Ta_0.5B₂samples were successfully prepared by high energy ball milling combined with high pressure and high temperature conditions.The hardness of Sc_0.5Ta_0.5B₂is indeed higher than that of Sc B₂and Ta B₂,which is consistent with the theoretical prediction results.Therefore,Sc-Ta-B system is an excellent candidate system for metal superhard materials.Current research shows that multi-element cooperative regulation is an effective scheme to optimize superhard materials,which provides a feasible exploration idea and theoretical experimental basis for further searching for transition metal boride superhard materials.