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.

中文翻译：

---

Na-Al-H三元化合物在高压下的相图和超导性

受到高压下三元氢主导化合物的高温超导性的启发，我们使用ab initio方法系统地探索了三元 Na-Al-H 系统的高压相图、电子性质、晶格动力学和超导性。我们发现了三种稳定的化合物${\mathrm{氯化钠}}_4$, ${\mathrm{氯化钠}}_6$， 和 ${\mathrm{氯化钠}}_8$，以及亚稳态化合物 ${\mathrm{氯化钠}}_7$在高压下。除了${\mathrm{氯化钠}}_4$，它们都包含 ${\mathrm{H}}^{-}$ 和 ${\mathrm{H}}_2$单位。电子结构计算表明${\mathrm{氯化钠}}_7$ 和 ${\mathrm{氯化钠}}_8$ 是金属的，而 ${\mathrm{氯化钠}}_4$ 和 ${\mathrm{氯化钠}}_6$是半导体。此外，${\mathrm{氯化钠}}_8$在沿 AM 方向的多个声子分支中表现出声子异常（倾角）。费米面嵌套引起的声子异常增强了电子-声子耦合强度，因此，超导转变温度${吨}_C$ 在 300 GPa 下产生了 55 K。