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

First-principles density-functional theory ͑DFT͒ 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 LiMg͑AlH 4 ͒ 3 compound is stable with respect to solid-state decomposition into LiAlH 4 and Mg͑AlH 4 ͒ 2 ; however, we also find that LiMg͑AlH 4 ͒ 3 is unstable with respect to hydrogen release and decomposes exothermically into LiMgAlH 6 , Al, and H 2 with a calculated T = 300 K enthalpy of −7.3 kJ/ ͑mol H 2 ͒, in excellent agreement with the weakly exothermic value of −5 kJ/ ͑mol H 2 ͒ obtained from differential scanning calorimetry measurements ͓M. Mamatha et al., J. Alloys Compd. 407, 78 ͑2006͔͒. LiMgAlH 6 is a stable intermediate, which has two competing endothermic decomposition pathways for H 2 release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate Li 3 AlH 6 phase, with room-temperature enthalpies of +18.6 and +16.6 kJ/ ͑mol H 2 ͒, respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical MgAlH 5 compound should assume the orthorhombic BaGaF 5 prototype structure, in contrast to a previous DFT study of MgAlH 5 , ͓A. Klaveness et al., Phys. Rev. B 73, 094122 ͑2006͔͒. However, the decomposition enthalpy of MgAlH 5 is only weakly endothermic, +1.1 kJ/ ͑mol H 2 ͒, 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 H 2 at ambient temperatures and significantly lower reaction entropies.