Design of superconducting compounds at lower pressure via intercalating XH₄ molecules (X = B, C, and N) into fcc lattices

Recently, many encouraging experimental advances have been achieved in ternary hydrides superconductors under high pressure. However, the extreme pressure required is indeed a challenge for practical application, which promotes a further exploration for high temperature (T_c) superconductors at relatively low pressure. Herein, we performed a systematic theoretical investigation on a series of ternary hydrides with stoichiometry AX₂H₈, which is constructed by interacting molecular XH₄ (X = B, C, and N) into the fcc metal A lattice under low pressure of 0–150 GPa. We uncovered five compounds which are dynamically stable below 100 GPa, e.g., AcB₂H₈ (25 GPa), LaB₂H₈ (40 GPa), RbC₂H₈ (40 GPa), CsC₂H₈ (60 GPa), and SrC₂H₈ (65 GPa). Among them, AcB₂H₈, which is energetically stable above 2.5 GPa, exhibits the highest T_c of 32 K at 25 GPa. The superconductivity originates mainly from the coupling between the electron of Ac atoms and the associated low-frequency phonons, distinct from the previous typical hydrides with H-derived superconductivity. Our results shed light on the future exploration of superconductivity among ternary compounds at low pressure.