Theoretical exploration of novel compounds in Sr(B,C)x (x = 7,8) at high pressure

Abstract

Boron-carbon-based compounds have attracted considerable attention due to their exceptional properties and increasing applications in industry. In this study, we conducted a systematic structural prediction of the SrBxC7-x and SrBxC8-x systems under ambient and high pressures up to 20 GPa using an unbiased global structure search. Through the calculations of formation enthalpy and phonon dispersion curves, we identified three new compounds, namely SrB5C2, SrB6C and SrB5C3, as thermodynamically and dynamically stable under ambient pressure and high pressures up to 20 GPa. Calculations of mechanical properties reveal that these three compounds have significant mechanical strength, with estimated theoretical Vickers hardness values ranging from 23.2 to 39.8 GPa. Electronic structure calculations suggest that both SrB5C2 and SrB5C3 are hole conductors, with the bands crossing the Fermi level, and SrB6C is a semiconductor with an indirect bandgap of 2.4 eV. The electron-phonon coupling calculations using the Migdal-Eliashberg theory indicate that SrB5C2 exhibits superconductivity at 10.5 K under ambient pressure, while SrB5C3 shows superconductivity at a much lower temperature of 0.4 K. This finding adds to our knowledge of boron-carbide materials and provides valuable insights for the development of similar materials with good mechanical properties.