First-Principles Study On Boron Carbide Of Alkali Metal,Alkali Earth Metal And Metal Thorium

The ternary metal boron carbide has a flexible boron-carbon framework,which results in a particularly rich and diverse structural chemistry.Therefore,it presents a variety of excellent physical and chemical properties,which has attracted extensive research.In this paper,using the first-principles calculation method based on density functional theory,we mainly studied two aspects of problems:first,we studied the mechanical and thermodynamic properties of rare earth metal boron carbide ThB2C;Secondly,the possible stable phase in ternary alkali metal and alkaline earth metal boron carbon system was predicted theoretically.Through the first-principles calculation based on density functional theory,we calculate the crystal structure,electronic properties,mechanical properties and thermodynamic properties of R3m-ThB2C.We calculated the elastic constant of R3m-ThB 2C,then used to derive the elastic modulus,Poisson’s ratio and elastic wave velocity.The results show that ThB2C is brittle.Thermodynamic properties are mainly determined by phonon vibration.Both volume and bulk modulus increase with the increase of temperature.The phonon calculation results reveal the thermodynamic stability of ThB2C.We obtain Gibbs free energy,volume,bulk modulus,entropy and heat capacity at constant pressure in the temperature range of 0-1000 K by quasi-harmonic approximation.Using the method of combining atom substitution and first-principles calculation,we theoretically predicted the possible stability of ternary alkali metal and alkaline earth metal boron carbon compounds under environmental conditions.The ternary phase diagram shows that we have found more than 20 new compounds with phonon stability,namely Amm2-LiBC5、Amm2-LiB2C4、P4n2-Li2B24C、Amm2-NaBC5、Imma-NaB13C2、P63/mmcNaBC、Amm2-NaB6C、Imma-NaB12C2、Amm2-KBC5、Imma-KB13C2、Amm2-RbBC5、Imma-RbB 13C2 and Amm2-CsBC5 and in alkali earth metal system.Amm2-BeB6C、Amm2MgB2C4、Imm2-MgB 13C2、I4/mcm-SrB2C2、Amm2-SrB2C4、Amm2-SrBC5、ImmaSrB12C2、Amm2-BaBC5、I4/mcm-BaB2C2 and Amm2-BaB2C4 in alkaline metal system.We have analyzed the crystal structure of these ternary metal boron carbides,which can be divided into two types:the layered structure of metal layer and non-metallic layer composed of B and C atoms,and the B-rich structure containing B12 icosahedron.The calculated electronic band structure and corresponding density of states show that Imma-NaB 13C2 and Imma-KB 13C2 have insulator properties,I4/mcm-SrB2C2、Imm2-MgB13C2、Imma-SrB12C2、P4n2-Li2B24C、Imma-NaB 12C、Amm2-NaB6C、P63/mmc-NaBC and Imma-RbB13C2 have semiconductor properties,Amm2-SrB2C4、Amm2-BeB6C、Amm2-MgB2C4、I4/mcmBaB2C2、Amm2-BaB2C4、Amm2-NaBC5、Amm2-KBC5、Amm2-RbBC5 and Amm2-CsBC5 have semi-metallic properties,Amm2-SrBC5、Amm2-BaBC5、Amm2-LiB2C4 and Amm2LiBC5 has metallic properties.The calculated phonon spectra show that these compounds are thermodynamic stable.In summary,we have theoretically studied the mechanical and thermodynamic properties of R3m-ThB2C,added data in this area.And by combining atomic substitution and first principles calculations,new ternary stable compounds that may exist in alkali metal and alkali earth metal boron carbides were predicted,and the crystal structure of new stable ternary compounds was explored,laying a foundation for further research on the physical and chemical properties of ternary alkali metal and alkali earth metal boron carbide systems.