High-Pressure Study on Structural Phase Transformation and Physical Properties of SrB₂C₂

By employing the first-principles calculations and the structure prediction method, which are based on density functional theory and particle swarm optimization algorithm respectively, this work conducted a structural search of SrB₂C₂ in the pressure range of 0−350 GPa, and successfully determined the structure of tetragonal phase tI20-SrB₂C₂ at ambient pressure and orthorhombic phase oF40-SrB₂C₂ at high pressures. Based on the enthalpy difference curve of SrB₂C₂, the phase transition pressure was determined to be 44.7 GPa. Moreover, the stability and the possibility of experimental synthesis of tI20-SrB₂C₂ and oF40-SrB₂C₂ at the corresponding pressure were verified by calculating the phonon spectrum, elastic constants and formation enthalpy. In addition, the tI20-SrB₂C₂ has higher degree of mechanical anisotropy than oF40-SrB₂C₂, which can be seen from the Young’s modulus and shear modulus as a function of orientations. It can be ascribed to the fact that the sp²-hybridized boron-carbon bonds form the layered structure of tI20-SrB₂C₂, while the boron-carbon bonds of oF40-SrB₂C₂ are mainly sp³-hybridized covalent bonds, forming a more stable three-dimensional tetrahedral network structure. The electronic structure calculations show that SrB₂C₂ is an indirect band gap semiconductor, and the calculated electronic localization function reveals that the boron-carbon bonds in tI20-SrB₂C₂ and oF40-SrB₂C₂ are sp² and sp³ covalent bonds, respectively.