Neutron spectroscopy of $\gamma $-AlH$_{3}$

The density of vibrational states, G(E), for $\gamma $-AlH$_{3}$ is measured by inelastic neutron scattering. The obtained spectrum noticeably differs from that of $\alpha $-AlH$_{3}$, because $\alpha $-AlH$_{3}$ has a structure built of corner-sharing AlH$_{6}$ octahedra, while the $\gamma $-AlH$_{3}$ is composed of both corner- and edge-sharing AlH$_{6}$ octahedra. The first acoustic phonon peak in the G(E) of $\gamma $-AlH$_{3}$ appears at a lower energy and the band of translational modes extends to higher energies (55 vs. 42 meV) than in the spectrum of $\alpha $-AlH$_{3}$. The bands of Al-H bending modes are observed at about the same energies of 60--140 meV in $\gamma $-AlH$_{3}$ and $\alpha $-AlH$_{3}$, only the structures of the bands are different. The Al-H stretching modes in the $\gamma $-phase show eight peaks in the range 145 to 265 meV, while in the $\alpha $-phase they exhibits only two peaks at 200 and 235 meV. The G(E) spectrum of $\gamma $-AlH$_{3}$ is in good agreement with recent simulations [Y. Wang \textit{et al., Phys. Rev. B} \textbf{77} (2008) 014101], which took into account the existence of a unique double-bridge bond between certain Al and H atoms.