Hydrogen self-dynamics in orthorhombic alkaline earth hydrides through incoherent inelastic neutron scattering

Inelastic neutron scattering patterns from polycrystalline CaH2, SrH2 and BaH2, 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 optical phonon bands, high-quality generalized self-inelastic structure factors are extracted and compared to new ab initio lattice dynamics simulations, accurately reproducing the hydride lattice structures. The overall agreement is found satisfactory, even though not perfect, especially in the first optical phonon zone of BaH2. In addition, the simulations provide a compelling support to a recent physical interpretation of the recorded spectral features and allowed to separate the contributions produced by the two non-equivalent hydrogen atoms.