Hydrogen dynamics in heavy alkali metal hydrides obtained through inelastic neutron scattering

Inelastic neutron scattering spectra from polycrystalline NaH, KH, RbH and CsH, measured 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 hydrogen-projected densities of phonon states are extracted and compared to ab initio lattice dynamics results. The overall agreement is very good. Further lattice dynamics calculations, based on a pairwise Born-Mayer semi-empirical potential scheme, were also performed, providing only limited and qualitative agreement with the experimental data. In conclusion, incoherent inelastic neutron spectroscopy proves to be a stringent validation tool for lattice dynamics simulations of H-containing materials.