First-principles investigations of aluminum hydrides:M3AlH6(M=Na,K)

Abstract

The structural stability of ${\mathrm{Na}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ and ${\mathrm{K}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ has been systematically investigated using accurate density-functional total-energy calculations. The experimentally known crystal structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Na}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ is reproduced and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{K}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ is predicted to be isostructural ($\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Na}}_{3}\mathrm{Al}{\mathrm{F}}_{6}$ type; space group $P{2}_{1}∕n$). This structure contains a pseudo-face-centered-cubic Al sublattice and each Al atom is surrounded by distorted octahedra of hydrogen atoms with the long octahedral axis tilted from the [001] direction toward ⟨111⟩. We predict that the $\ensuremath{\alpha}$ modifications will not be stable at higher pressures. On application of pressure to ${\mathrm{Na}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$, the $\ensuremath{\alpha}$ phase transforms into a ${\mathrm{Cs}}_{3}\mathrm{Nd}{\mathrm{Cl}}_{6}$-type structure at $19.8\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Similarly $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{K}}_{3}\mathrm{Al}{\mathrm{H}}_{6}$ transforms into two high-pressure forms: (1) $\ensuremath{\alpha}$ transforms to $\ensuremath{\beta}$ with ${\mathrm{Rb}}_{3}\mathrm{Tl}{\mathrm{F}}_{6}$-type structure at $53.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ and (2) $\ensuremath{\beta}$ transforms to $\ensuremath{\gamma}$ with ${\mathrm{U}}_{3}\mathrm{Sc}{\mathrm{S}}_{6}$-type structure at $60.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$.