Hydrogen release from Li alanates originates in molecular lattice instability emerging at ∼100 K

Abstract

Lattice vibrations from 20 K to room temperature in lithium alanates LiAlH4 and Li3AlH6, which decompose to release hydrogen over 423 K, were investigated by neutron spectroscopy. For both alanates, the overall spectra already start to broaden at 100–150 K with increasing temperature. The spectral lines assigned to the librational (rotational) modes of the polyhedral Al hydrido complexes, [AlH4]− and [AlH6]3−, exhibit not only broadening but also softening at around 250–300 K. These results suggest that the decomposition stems from low-temperature bulk lattice instability/anharmonicity, in particular, due to the weakening of binding between the complexes and their surroundings.