Hydrogen Vibration in Hydrogen Storage Materials Investigated by Inelastic Neutron Scattering

Hydrides are promising hydrogen storage materials owing to their higher gravimetric and volumetric hydrogen densities compared to compressed hydrogen gas and liquid hydrogen. Hydrogen is absorbed by different compounds (formation reaction of hydride) and thus exhibits different states—elemental hydrogen (H⁰), hydride ion (H⁻), and covalently bonded hydrogen (H^cov.)—in hydrides. The absorbed hydrogen is released as hydrogen gas (decomposition reaction of hydride). Therefore, it is important to understand hydride formation and decomposition based on the nature of chemical bonding of hydrogen (hydrogen states) in hydrides. Although it is difficult to directly observe hydrogen states, inelastic neutron scattering (INS), a powerful and useful technique, can be employed for this purpose as hydrogen vibrations in hydrides depend on its state. Herein, we provide an overview of INS studies of hydrogen vibrations in representative hydrides which are potential hydrogen storage materials. In particular, mode assignments focused on hydrogen states, local atomic arrangements around hydrogen atoms, hydrogen release reactions, and hydride formation processes based on observed hydrogen vibrations through INS are reviewed in this paper.