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

Inelastic neutron scattering spectra from polycrystalline NaH, KH, RbH andCsH, measured at low temperature in the energy transfer range 3 meV < E <500 meV, are reported. From the medium-energy regions, coinciding with theoptical phonon bands, accurate hydrogen-projected densities of phonon statesare extracted and compared to ab initio lattice dynamics results. The overallagreement is very good. Further lattice dynamics calculations, based on apairwise Born-Mayer semi-empirical potential scheme, were also performed,providing only limited and qualitative agreementwith the experimental data. Inconclusion, incoherent inelastic neutron spectroscopy proves to be a stringentvalidation tool for lattice dynamics simulations of H-containing materials.