Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C-B Clathrates

Inspired by the synthesis of XB₃C₃ (X = Sr, La) compounds in the bipartite sodalite clathrate structure, density functional theory (DFT) calculations are performed on members of this family containing up to two different metal atoms. A DFT-chemical pressure analysis on systems with X = Mg, Ca, Sr, Ba reveals that the size of the metal cation, which can be tuned to stabilize the B-C framework, is key for their ambient-pressure dynamic stability. High-throughput density functional theory calculations on 105 Pm3̅ symmetry XYB₆C₆ binary-guest compounds (where X, Y are electropositive metal atoms) find 22 that are dynamically stable at 1 atm, expanding the number of potentially synthesizable phases by 19 (18 metals and 1 insulator). The density of states at the Fermi level and superconducting critical temperature, T_c, can be tuned by changing the average oxidation state of the metal atoms, with T_c being highest for an average valence of +1.5. KPbB₆C₆, with an ambient-pressure Eliashberg T_c of 88 K, is predicted to possess the highest T_c among the studied Pm3̅n XB₃C₃ or Pm3̅ XYB₆C₆ phases, and calculations suggest it may be synthesized using high-pressure high-temperature techniques and then quenched to ambient conditions.