Hydrogen vibrational dynamics in ionic metal hydrides revealed through inelastic neutron scattering

Inelastic neutron scattering spectra from polycrystalline alkali (Li, Na, Rb and Cs) and heavy alkaline-earth (Ca, Sr and Ba) hydrides, measured on TOSCA-II spectrometer at low temperature in the energy transfer range 3 meV<E<500 meV, are reported. From the medium-energy regions, coinciding with the optical phonon bands, accurate generalized self inelastic structure factors and (if possible) hydrogen-projected densities of phonon states are extracted and compared to ab-initio lattice dynamics results. The overall agreement is found satisfactory. In addition, in CaH2, simulations provide a compelling support to a recent physical interpretation of the recorded spectral features and allow to separate the contributions produced by the two non-equivalent hydrogen atoms. In conclusion, incoherent inelastic neutron spectroscopy proves to be a stringent validation tool for lattice dynamics simulations of H-containing materials.