Prediction of novel boron-carbon based clathrates

Clathrates are inclusion compounds featured with host framework cages and trapped guest atoms or small molecules. Recently, the first boron-carbon (B-C) clathrate SrB₃C₃ was successfully synthesized at high pressures near 50 GPa. Upon the substitution of guest atoms, clathrates exhibit tunable applications. For example, LaB₃C₃ possesses an indirect band gap of near 1.3 eV, whereas the ScB₃C₃ clathrate is ferroelectric with an above-room-temperature Curie temperature of ∼370 K. To the best of our knowledge, however, there is no report on the investigation of B-C framework clathrates with non-equivalent B : C ratios. By using first-principles swarm-intelligence structure searching computations, we identified two metastable I4/mmm SrB₂C₄ and LaB₄C₂ clathrates at 50 GPa, and the framework cage contains six quadrilaterals and eight hexagons with a trapped guest metal located at the center. Their dynamic and enthalpy stabilities may be retained at ambient pressure. Moreover, the possible clathrates are extended by the substitution of the guest atoms with other metals in groups 2, 3, and 4, showing a tunable superconducting critical temperature (T_c) and considerable Vickers hardness (H). Intriguingly, a metal-to-semiconductor transition occurs in MB₂C₄ as the atomic number order of alkaline earth metals increases (M: Mg → Ca → Sr → Ba). The estimated T_c value for I4/mmm SrB₄C₂ is 19.2 K, while SrB₂C₄ and BaB₂C₄ are evaluated as superhard materials with H values of 43.6 and 41.2 GPa under ambient conditions, respectively.