Theoretical Prediction And Physical Properties Of Alkaline-earth Metal Borosilicide Compounds Under High Pressure

As a typical functional material,high-temperature superconductor has attracted much attention because of it has exhibited promising application in energy and medicine.It is a hot research topic in the forefront of science and technology in the world.In recent years,new breakthroughs have been made in the superconducting transition temperature of hydrogen rich cage compounds found under high pressure,such as,Ca H₆（150 GPa,T_c=235 K）,Th H₁₀（100 GPa,T_c=241 K）,YH₆（120 GPa,T_c=264 K）and La H₁₀（210 GPa,T_c=286 K）.However,the properties of these high-temperature superconducting hydrogen-rich cages are unstable at atmospheric pressure,which limit their applications.Compared with hydrides,metal silicides adopt cage structure can exist stably under atmospheric pressure.In this work,we used the crystal structure prediction method,combined with first-principles calculations to explore structures and properties of AB₃Si₃（Mg,Ca,Sr,Ba）systematically under high pressure.The main research results are as follows:1.Applying the first-principles calculations and the CALYPSO crystal structure prediction method based on the particle swarm optimization,the structure of ternary alkaline-earth metal borosilicide compounds MgB₃Si₃,CaB₃Si₃,SrB₃Si₃and BaB₃Si₃are explored in the pressure range of 0-50 GPa systematically.Nine energetically favorable crystal structures were found:For MgB₃Si₃compounds,the Amm2 structure is stable in the range of 6.5-13.3 GPa,and P2₁/m structure is energetically stable in the range of 13.3-50 GPa;For CaB₃Si₃compound,P-6m2 structure is the most stable in 0-5.3 GPa range,and the P2₁/m structure is stable in the range of 5.3-26.3 GPa,when the pressure is higher than 26.3 GPa,the C2/m structure energy is more advantageous;The thermodynamic properties of the C2/m structure of SrB₃Si₃compound in the range of 0-6.7 GPa.When the pressure reaches 6.7 GPa,it phases to P4₂/mmc structure.With the increase of pressure,the P-1 structure is stable in the pressure region of 26.5-50 GPa;No phase transitions are found in BaB₃Si₃compound in the pressure range of 0-50 GPa,and the space group is Pm-3n.2.The results show that there are no imaginary frequencies in the phonon dispersion curves of CaB₃Si₃,SrB₃Si₃and BaB₃Si₃under atmospheric pressure and high pressure,determine the dynamical stability,which provides theoretical support for experimental synthesis.3.The calculated results of the energy band structure and density of states show that the four compounds are all metals.The hardness values calculated under atmospheric pressure are greater than 10 GPa,which are hard materials.In addition,SrB₃Si₃-P4₂/mmc and BaB₃Si₃-Pm-3n with sodalite-like cage structure with superconducting properties are obtained.The superconducting transition temperatures at atmospheric pressure are 14.1 K and 7.6 K,respectively.The results of this work provide new schemes and theoretical support for searching new superconducting materials,enrich the research on ternary alkaline-earth metal borosilicide system,and also provide research ideas for the search for new functional materials.