On the effect of thermal treatment and hydrogen vibrational dynamics in sodium alanates: an inelastic neutron scattering study

We have measured inelastic neutron scattering (INS) spectra from Ti-doped polycrystalline alanates (NaAlH_4 and Na_3AlH_6), at low temperature, in the energy transfer range 3-500 meV, both for thermally treated and untreated samples. From the spectral range corresponding to the fundamental vibrational bands of these aluminohydrides, accurate one-phonon spectra and hydrogen-projected densities of phonon states have been extracted and analyzed using ab initio lattice dynamics calculations. Satisfactory agreement has been found for the untreated samples. In the case of thermally treated samples, due to thermal decomposition, different ionic species are present and the sample composition could be quantitatively evaluated. No evidence for the existence of intermediate species such as AlH_3 or AlH_5^{2-} has been found.