First-Principles Determination of Crystal Structures, Phase Stability, and Reaction Thermodynamics in the Li-Mg-Al-H Hydrogen Storage System*

First-principles DFT calculations have been used to investigate the crystal structures, thermodynamic stability, and decomposition pathways of Li-Mg-Al-H hydrogen storage compounds. We find that the recently discovered LiMg(AlH$_4)_3$ compound is marginally stable with respect to decomposition into LiAlH$_4$ and Mg(AlH$_4)_2$; however, we also find that LiMg(AlH$_4)_3$ is unstable with respect to H$_2$ release and decomposes exothermically into LiMgAlH$_6$, Al, and H$_2$ in excellent agreement with measurements. Using ICSD crystal structures database, we predict that the hypothetical MgAlH$_5$ compound should assume the orthorhombic BaGaF$_5$ prototype structure. We also discuss that phonon vibrations have sizeable effects on the enthalpies and entropies of hydrogen release reactions of Li-Mg-Al-H compounds.