Superconductivity in CH₄ and BH₄ ⁻ containing compounds derived from the high-pressure superhydrides

Inspired by the synthesis of the high-pressure Fm3̄m LaH₁₀ superconducting superhydride, systematic density functional theory (DFT) calculations are performed to study ternaries that could be derived from it by replacing two of the hydrogen atoms with boron or carbon and varying the identity of the electropositive element. Though many of the resulting alkali-metal and alkaline-earth MC₂H₈ phases are predicted to be dynamically stable at mild pressures, their superconducting critical temperatures (T_cs) are low because their metallicity results from the filling of an electride-like band. Substitution with a trivalent element leads to phases with substantial metal d-character at the Fermi level whose T_cs are typically above 40 K. Among the MB₂H₈ phases examined, KB₂H₈, RbB₂H₈ and CsB₂H₈ are predicted to be dynamically stable at very mild pressures, and their stability is rationalized by a DFT-Chemical Pressure analysis that elucidates the role of the M atom size. Quantum anharmonic effects strongly affect the properties of KB₂H₈, the highest predicted T_c compound, near 10 GPa, but molecular dynamics simulations reveal it would decompose below its T_c at this pressure. Nonetheless, at ca. 50 GPa KB₂H₈ is predicted to be thermally stable with a superconducting figure of merit surpassing that of the recently synthesized LaBeH₈.