Density functional theory study on high-pressure structures and electronic properties of Mg-Al alloys

Magnesium and aluminum are abundant metals in the Earth’s crust and widely utilized in industrial engineering. Under high pressure, these elements can form elemental compounds into single substances, resulting in a variety of crystal structures and electronic properties. In this study, the possible structures of magnesium-aluminum alloys are systematically investigated in a pressure range of 0–500 GPa by using the first-principles structure search method, with energy and electronic structure calculations conducted using the VASP package. Bader charge analysis elucidates atomic and interstitial quasi-atom (ISQ) valence states, while lattice dynamics are analyzed using the PHONOPY package via the small-displacement supercell approach. Eight stable phases(MgAl<sub>3</sub>-<i>Pm</i><inline-formula><tex-math id="Z-20251017145444">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, MgAl<sub>3</sub>-<i>P</i>6<sub>3</sub>/<i>mmc</i>, MgAl-<i>P</i>4/<i>mmm</i>, MgAl-<i>Pmmb</i>, MgAl-<i>Fd</i><inline-formula><tex-math id="Z-20251017145503">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, Mg<sub>2</sub>Al-<i>P</i><inline-formula><tex-math id="Z-20251017145602">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>1, Mg<sub>3</sub>Al-<i>P</i>6<sub>3</sub>/<i>mmc</i>, Mg<sub>3</sub>Al-<i>Fm</i><inline-formula><tex-math id="Z-20251017145519">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>) and two metastable phases (Mg<sub>4</sub>Al-<i>I</i>4/<i>m</i>, Mg<sub>5</sub>Al-<i>P</i><inline-formula><tex-math id="Z-20251017145623">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>1) are identified. The critical pressures and stable intervals for phase transitions are precisely determined. Notably, MgAl-<i>Fd</i><inline-formula><tex-math id="Z-20251017145642">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, Mg<sub>2</sub>Al-<i>P</i><inline-formula><tex-math id="Z-20251017145651">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>1, Mg<sub>4</sub>Al-<i>I</i>4/<i>m</i> and Mg<sub>5</sub>Al-<i>P</i><inline-formula><tex-math id="Z-20251017145709">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>1 represent newly predicted structures. Analysis of electronic localization characteristics reveals that six stable structures (MgAl<sub>3</sub>-<i>Pm</i><inline-formula><tex-math id="Z-20251017145737">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, MgAl<sub>3</sub>-<i>P</i>6<sub>3</sub>/<i>mmc</i>, MgAl-<i>Pmmb</i>, MgAl-<i>Fd</i><inline-formula><tex-math id="Z-20251017145755">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, Mg<sub>2</sub>Al-<i>P</i><inline-formula><tex-math id="Z-20251017145828">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>1 and Mg<sub>3</sub>Al-<i>P</i>6<sub>3</sub>/<i>mmc</i>) exhibit electronic properties of electrides. The ISQs primarily originate from charge transfer of Mg atoms. In the metastable phase Mg<sub>4</sub>Al-<i>I</i>4/<i>m</i>, Al atoms are predicted to achieve an Al<sup>5–</sup>valence state, filling the p shell. This finding demonstrates that by adjusting the Mg/Al ratio and pressure conditions, a transition from traditional electrides to high negative valence states can be realized, offering new insights into the development of novel high-pressure functional materials. Furthermore, all Mg-Al compounds display metallic behaviors, with their stability attributed to Al-p<i>-</i>d orbital hybridization, which significantly contributes to the Al-3p/3d orbitals near the Fermi level. Additionally, LA-TA splitting is observed in MgAl<sub>3</sub>-<i>Pm</i><inline-formula><tex-math id="Z-20251017145844">\begin{document}${\bar {3}} $\end{document}</tex-math></inline-formula><i>m</i>, with a splitting value of 45.49 cm<sup>–1</sup>, confirming the unique regulatory effect of ISQs on lattice vibrational properties. These results elucidate the rich structural and electronic properties of magnesium-aluminum alloys as electrodes, offering deeper insights into their behavior under high pressure and inspiring further exploration of structural and property changes in high-pressure alloys composed of light metal elements and p-electron metals.