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 methods. At 80 GPa, CaAlH transforms from to 2/ phase. Both of -CaAlH and -CaAlH are semiconductors. At 200 GPa, 4/-CaAlH and a metastable compound -CaAlH were found. Furthermore, 4/-CaAlH shows obvious softening of the high frequency vibration modes, which improves the strength of electron-phonon coupling. Therefore, a superconducting transition temperature of 71 K is generated in 4/-CaAlH at 50 GPa. In addition, the thermodynamic metastable -CaAlH exhibits a superconducting transition temperature of 118 K at 250 GPa. These results are very useful for the experimental searching of new high- superconductors in ternary hydrides. Our work may provide an opportunity to search for high superconductors at lower pressure.