Dual-guest-metal control of stability and superconductivity in ambient-pressure B-C clathrates X0.5Y0.5B2C8.

Phonon-mediated superconductivity in boron-carbon clathrates has become an active area of research, but how mixed guest-metal occupancy affects stability and superconductivity is still unclear. Using high-throughput first-principles calculations, we study quaternary X0.5Y0.5B2C8 clathrates (X, Y = Li, Na, K, Rb, Cs, Mg, Ca, Sr, and Ba) at ambient pressure. We identify 15 dynamically stable phases, all involving combinations of two large-radius guest metals. Fourteen of these phases are predicted to be superconducting: compounds with an average formal oxidation state at the metal sites of +1.5 and +1 show Tc of 34 to 42 K and 78 to 90 K, respectively, whereas the remaining +2 phase is semiconducting. The contrast in Tc fits a rigid-band picture in which lower guest formal oxidation states increase the B-C density of states at the Fermi level and enhance coupling to mid-frequency B-C phonons. Ab initio molecular dynamics at 300 K reveals no structural transformation, indicating that these dual-guest clathrates are viable candidates for further study.