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

Ternary hydrides are considered as the ideal candidates with high critical temperature $({T}_{c})$ stabilized at submegabar pressure, evidenced by the recent discoveries in ${\mathrm{LaBeH}}_{8}$ (110 K at 80 GPa) and ${\mathrm{LaB}}_{2}{\mathrm{H}}_{8}$ (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 ${\mathrm{BH}}_{x}$ motifs, e.g., methanelike $({\mathrm{BH}}_{4})$, polythenelike, ${({\mathrm{BH}}_{2})}_{n}$, and ${\mathrm{BH}}_{6}$ octahedron. Among them, seven Ac-B-H compounds were found to become superconductive. In particular, ${\mathrm{AcBH}}_{7}$ was estimated to have a ${T}_{c}$ of 122 K at 70 GPa. Our in-depth analysis reveals that the B-H interactions in the ${\mathrm{BH}}_{6}$ 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-${T}_{c}$ at moderate pressure.