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 $\alpha \text{−}{\mathrm{Na}}_3\mathrm{Al}{\mathrm{H}}_6$ is reproduced and $\alpha \text{−}{\mathrm{K}}_3\mathrm{Al}{\mathrm{H}}_6$ is predicted to be isostructural ($\alpha \text{−}{\mathrm{Na}}_3\mathrm{Al}{\mathrm{F}}_6$ 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 $\alpha$ modifications will not be stable at higher pressures. On application of pressure to ${\mathrm{Na}}_3\mathrm{Al}{\mathrm{H}}_6$, the $\alpha$ phase transforms into a ${\mathrm{Cs}}_3\mathrm{Nd}{\mathrm{Cl}}_6$-type structure at $19.8\mathrm{GPa}$. Similarly $\alpha \text{−}{\mathrm{K}}_3\mathrm{Al}{\mathrm{H}}_6$ transforms into two high-pressure forms: (1) $\alpha$ transforms to $\beta$ with ${\mathrm{Rb}}_3\mathrm{Tl}{\mathrm{F}}_6$-type structure at $53.4\mathrm{GPa}$ and (2) $\beta$ transforms to $\gamma$ with ${\mathrm{U}}_3\mathrm{Sc}{\mathrm{S}}_6$-type structure at $60.2\mathrm{GPa}$.

• Received 15 October 2004

DOI:https://doi.org/10.1103/PhysRevB.71.092103