Theoretical exploration of novel boron–carbon clathrate in Sr(B,C)10 at high pressure

Abstract

The discovery of clathrate SrB₃C₃ under high pressure has inspired the exploration of new B–C clathrates. Here, we studied the structural evolution of a B–C framework over B and C contents in Sr(B,C)₁₀ at a pressure of 0–20 GPa using an unbiased structure search method in combination with first principles calculations. Analysis of the formation enthalpies and lattice dynamics suggest that a clathrate SrB₈C₂ and two ‘tubular-like’ SrB₇C₃ and SrB₉C may be synthesized at 20 GPa and quenched to ambient pressure. All compounds have excellent mechanical properties with Vickers hardness reaching 23.6–32.1 GPa. The ‘tubular-like’ SrB₇C₃ and SrB₉C are metals with their Fermi level crossing the bonding and antibonding states of B-p orbitals, respectively. However, unexpectedly, the clathrate SrB₈C₂ is a semiconductor with a direct bandgap of 0.3 eV and is fundamentally different from metallic SrB₃C₃. The calculated thermoelectric figure of merit ZT of SrB₈C₂ is 0.47, indicating that it is a potential thermoelectric material. The findings highlight the diversity of structures and physical properties of this new family of ternary B–C compounds.