Prediction of high- Tc superconductivity under submegabar pressure in ternary actinium borohydrides

Ternary hydrides are considered as the ideal candidates with high critical temperature (T₂) stabilized at submegabar pressure, evidenced by the recent discoveries in LaBeH₈ (110 K at 80 GPa) and LaB₂H₈ (106 K at 90 GPa). Here, we investigate the crystal structures and superconductivity of an Ac-B-H system under pressures of 100 and 200 GPa by using an advanced structure method combined with first-principles calculations. As a result, nine stable compounds were identified, where B atoms are bonded with H atoms in the formation with diverse BHₗ motifs, e. g. , methanelike (BH₄), polythenelike, ({BH₂) }₍, and BH₆ octahedron. Among them, seven Ac-B-H compounds were found to become superconductive. In particular, AcBH₇ was estimated to have a T₂ of 122 K at 70 GPa. Our in-depth analysis reveals that the B-H interactions in the BH₆ units play a key role in its high superconductivity and stability at submegabar pressure. Our current results provide a guidance for future experiments to synthesize ternary hydride superconductors with high-T₂ at moderate pressure.