Superconductivity of MX₂H₈ (X= B or C) at low pressure

In the last decade several binary hydride compounds have been predicted to possess a high superconducting critical temperature, T_c, at high pressures. This includes an Fm-3m LaH₁₀ phase, which was synthesized, and whose T_c was measured to be 250-260 K near 200 GPa. Currently, there is significant interest in designing superconducting ternary hydrides that can be quenched to mild pressures. One way in which such phases can be designed is by removing two hydrogen atoms from the Fm-3m MH₁₀ phases and adding a p-block element that can form strong bonds. We theoretically studied such systems via high-throughput first-principles calculations. We identified dynamically stable MX₂H₈ compounds (M=alkali, alkaline or rare earth metal, X=B or C), and analyzed their superconducting properties and the factors that are responsible for them. Several phases are predicted with T_cs of over 100 K that can be quenched to mild pressures.