First-principles determination of crystal structures, phase stability, and reaction thermodynamics in the Li-Mg-Al-H hydrogen storage system

First-principles density-functional theory (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(AlH₄)₃ compound is stable with respect to solid-state decomposition into LiAlH₄ and Mg(AlH₄)₂; however, we also find that LiMg(AlH₄)₃ is unstable with respect to hydrogen release and decomposes exothermically into LiMgAlH₆, Al, and H₂ with a calculated T=300 K enthalpy of -7.3 kJ/(mol H₂), in excellent agreement with the weakly exothermic value of -5 kJ/(mol H₂) obtained from differential scanning calorimetry measurements [M. Mamatha et al., J. Alloys Compd. 407, 78 (2006)]. LiMgAlH₆ is a stable intermediate, which has two competing endothermic decomposition pathways for H₂ release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate Li₃AlH₆ phase, with room-temperature enthalpies of $+18.6$ and +16.6 kJ/(mol H₂), respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical MgAlH₅ compound should assume the orthorhombic BaGaF₅ prototype structure, in contrast to a previous DFT study of MgAlH₅, [A. Klaveness et al., Phys. Rev. B 73, 094122 (2006)]. However, the decomposition enthalpy of MgAlH₅ is only weakly endothermic, +1.1 kJ/(mol H₂), and therefore this compound is not expected to occur in the high-temperature decomposition sequence of Mg alanate. We also present a comprehensive investigation of the phonon spectra and vibrational thermodynamics of Li-Mg-Al-H compounds, finding that vibrations typically decrease reaction enthalpies by up to 10 kJ/mol H₂ at ambient temperatures and significantly lower reaction entropies.