Phase diagrams and superconductivity of ternary Ca-Al-H compounds under high pressure

The search for high-temperature superconductors in hydrides under high pressure has always been a research hotspot. Hydrogen-based superconductors offer an avenue to achieve the long-sought goal of superconductivity at room temperature. Here we systematically explored the high-pressure phase diagram, electronic properties, lattice dynamics and superconductivity of the ternary Ca-Al-H system using ab initio methods. At 80 GPa, CaAlH₅ transforms from Cmcm to P2₁/m phase. Both of Cmcm-CaAlH₅ and Pnnm-CaAl₂H₈ are semiconductors. At 200 GPa, P4/mmm-CaAlH₇ and a metastable compound Immm-Ca₂AlH₁₂ were found. Furthermore, P4/mmm-CaAlH₇ shows obvious softening of the high frequency vibration modes, which improves the strength of electron-phonon coupling. Therefore, a superconducting transition temperature T_c of 71 K is generated in P4/mmm-CaAlH₇ at 50 GPa. In addition, the thermodynamic metastable Immm-Ca₂AlH₁₂ exhibits a superconducting transition temperature of 118 K at 250 GPa. These results are very useful for the experimental searching of new high-T_c superconductors in ternary hydrides. Our work may provide an opportunity to search for high T_c superconductors at lower pressure.