Ab initio study of metallic aluminum hydrides at high pressures

Abstract

Metallic phases of ${\mathrm{Al}}_{2}\mathrm{H}$ and AlH at megabar pressures are predicted to be possible by using ab initio density functional calculations. The ${\mathrm{Al}}_{2}\mathrm{H}$ phase is stabilized above 155 GPa, where several candidate structures are quite competitive; the structural properties suggest that ${\mathrm{Al}}_{2}\mathrm{H}$ has the phase where Al atoms form an hcp structure and H atoms occupy the octahedral sites in a random manner. The AlH phase is stable above 175 GPa, where the structure takes $R\overline{3}m$ symmetry. Superconducting transition temperature (${T}_{c}$) of the ${\mathrm{Al}}_{2}\mathrm{H}$ phase is estimated to be of the order of 1 K. In contrast, ${T}_{c}$ of the $R\overline{3}m$ AlH reaches 58 K at 180 GPa. The electronic structures around the Fermi energy in the $R\overline{3}m$ AlH are insensitive to pressure compared with those in the well-known $Pm\overline{3}n$ phase of ${\mathrm{AlH}}_{3}$. Accordingly, while theoretical ${T}_{c}$ of the $Pm\overline{3}n\phantom{\rule{4pt}{0ex}}{\mathrm{AlH}}_{3}$ rapidly decreases with compression and becomes almost zero above $\ensuremath{\sim}200$ GPa, that of the $R\overline{3}m$ AlH remains to be 21 K even at 335 GPa. This means that although superconductivity was not observed experimentally in ${\mathrm{AlH}}_{3}$, it might be achieved in AlH.