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

Using the first-principles calculations based on density functional theory and the structure prediction method based on particle swarm optimization algorithm, this paper conducted a structural search for SrB₂C₂ in the pressure range of 0-350 GPa, and successfully determined that SrB₂C₂ belongs to the tetragonal system tI20-SrB₂C₂ under normal pressure, and belongs to the orthorhombic system oF40- SrB₂C₂ under high pressure. Based on the enthalpy difference curve, the phase transition pressure of SrB₂C₂ was determined to be 44.7 GPa. The stability of tI20-SrB₂C₂ and oF40- SrB₂C₂ at the corresponding pressure and the possibility of experimental synthesis were verified by calculating the phonon spectrum, elastic constants and formation enthalpy. It can be seen from the Young's modulus and shear modulus as a function of orientations that tI20-SrB₂C₂ has higher degree of mechanical anisotropy than oF40- SrB₂C₂, which 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 calculation of the electronic structure shows that SrB₂C₂ is an indirect band gap semiconductor, and the calculation of the electronic localization function shows that the boron-carbon bonds in tI20-SrB₂C₂ and oF40-SrB₂C₂ are sp² and sp³ covalent bonds, respectively.