Crystal structure and thermodynamic stability of the lithium alanates LiAlH4 and Li3AlH6

The crystal structure of LiAlH₄ and Li₃AlH₆ was determined by density-functional theory (DFT) projector augmented wave ground-state (0 K) minimizations with the generalized gradient approximation (GGA). These results were in excellent agreement with the crystal structure of the deuteride analogs, LiAlD₄ and Li₃AlD₆, determined by neutron powder diffraction at 9 K. The DFT calculations were performed by starting with a number of input structures from different space groups. The cell size and shape were allowed to relax, thus making it possible to break or gain symmetry. This was an effective way of searching through a large number of possible symmetries, avoiding less favorable metastable structures. In some cases nearly degenerate structures resulted from quite different starting points, hence providing a good measure of the accuracy of the method. The cell angles differed by up to 0.17°, while the lattice constants and the atomic parameters differed by less than 3 pm, comparable in magnitude to the inherent uncertainty of the GGA. Finite-temperature thermodynamic properties of the alanates predicted with the aid of lattice phonon vibrational simulations were also found to be in good agreement with experimental data. The enthalpies of formation at 298 K for LiAlH₄ and Li₃AlH₆ were predicted to be -113.42 and -310.89 kJ mol^-1. Similarly, the two reactions, the decomposition of LiAlH₄ to form Li₃AlH₆ and the decomposition of Li₃AlD₆ to form LiH, were predicted to have endothermic reaction enthalpies of 9.79 and 15.72 kJ mol^-1 at 298 K, respectively. This has never been measured directly, and our results may contradict the commonly held belief that pure LiAlH₄ is thermodynamically unstable.