First-principles study on crystal structures and superconductivity of ternary metal hydride La-Sr-H under high pressure

The discovery of cage hydrides (Y,Ca)H6, (La,Ce)H9, and (La,Y)H10 indicates the appeal of ternary hydrides as contenders for high-temperature superconductors. Herein, we systematically studied a La-Sr-H system and predicted interesting stable and metastable hydrides at 200 GPa by first-principles calculations. The results revealed that LaSrH21 has a high superconducting transition temperature (Tc ) of 211 K at 200 GPa, mainly attributed to its unique H12 rings. LaSrH12, LaSr2H18, and LaSr3H24 contain distorted H24 cages and exhibit high Tc values of 160, 167, and 169 K, respectively. Notably, LaSrH12 maintained dynamic stability down to 30 GPa, which is extremely low for H24 cage hydrides. In addition, the Tc s of LaSrH12 increased at a rate of 0.43 K/ GPa with the increase in pressure. Further analysis revealed that the positive pressure-dependent Tc was mainly due to the softening of the low-frequency phonon modes and Lifshitz transition. Our work will guide the study of ternary rare- and alkaline-earth hydrides.