Invigorated by the high-temperature superconductivity in ternary hydrogen-dominated compounds under high pressures, we systematically explored high-pressure phase diagrams, electronic properties, lattice dynamics, and superconductivity of ternary Na-Al-H systems using ab initio methods. We found three stable compounds of ${\mathrm{NaAlH}}_4$, ${\mathrm{NaAlH}}_6$, and ${\mathrm{NaAlH}}_8$, as well as a metastable compound of ${\mathrm{NaAlH}}_7$ under high pressures. Except for ${\mathrm{NaAlH}}_4$, they all containing ${\mathrm{H}}^{-}$ and ${\mathrm{H}}_2$ units. Electronic structure calculations reveal that ${\mathrm{NaAlH}}_7$ and ${\mathrm{NaAlH}}_8$ are metallic, while ${\mathrm{NaAlH}}_4$ and ${\mathrm{NaAlH}}_6$ are semiconductors. In addition, ${\mathrm{NaAlH}}_8$ exhibits a phonon anomaly (dip) in multiple phonon branches along the A-M direction. The phonon anomaly induced by Fermi-surface nesting boosts the electron-phonon coupling strength, and as a result, a superconducting transition temperature $T_c$ of 55 K was produced at 300 GPa.

• Received 30 March 2021
• Revised 5 September 2021
• Accepted 8 September 2021

DOI:https://doi.org/10.1103/PhysRevB.104.104509