Hydrides optical phonons

Fig. 30. Optical-phonon spectra from inelastic neutron scattering measurements on neptunium hydrides and deuterides. (a) Scattering law data for NpHj, 3 and NPD2.13. (b) Scattering lawdata for NpHj 55 at 90 K and room temperature. After Goldstone et al. (1989).

Here is the momentum transferred from the neutron and x is one of the reciprocal lattice vectors of the palladium lattice. Thus, the incoherent scattering sees all the vibration modes but the coherent scattering selects one particular phonon for a particular experimental value of Q. It is now clear that both the incoherent and coherent one-phonon scattering will depend on the shape of the optical dispersion curves and hence will be influenced by hydrogen-hydrogen interactions. Indeed, one of the first observations of inelastic scattering from a hydride [10] interpreted the shape of the optical peak in terms of a frequency distribution broadened by H-H interactions. 

---