We reconsider the mobility of hydrogen in bcc metals, in particular H and D in Nb, and its influence on the coherent and incoherent quasielastic neutron and x-ray scattering. An approach to describe time-dependent lattice distortions around highly mobile protons is presented, using the frequency-dependent Green’s function of the host lattice so that effects of the so-called quasielastic effects and resonances with host lattice phonons are included. We discuss within this formalism how noncubic components of the long-ranged displacement field can fade away in the presence of a high defect mobility and we demonstrate by a detailed calculation that the experimentally observed disappearance of the coherent quasielastic diffuse (x-ray and neutron) scattering in ${\mathrm{NbH}}_{\mathit{x}}$ (${\mathrm{NbD}}_{\mathit{x}}$) from these Fourier components of the displacement field points to an unusual, hitherto neglected rapid local diffusion process between adjacent tetrahedral sites with a short-time constant of ${\mathrm{\tau }}_{\mathrm{loc}}$≃${10}^{\mathrm{-}13}$–${10}^{\mathrm{-}14}$ s. The consequences of such a diffusion mechanism on the incoherent quasielastic neutron scattering are discussed in detail. We show that the elastic incoherent structure factor of the interstitial proton exhibits, in addition to the usual Debye-Waller factor behavior, distinct oscillations as a function of the neutron momentum transfer.

A neutron-scattering study of ${\mathrm{NbH}}_{0.02}$ at various temperatures between room temperature and 573 K is presented. The experimental results of the elastic incoherent structure factor displays, after a proper correction for the proton Debye-Waller factor, a pronounced oscillatory behavior in the [100] and [111] direction as predicted for a rapid local diffusion on neighboring tetrahedral sites. It is shown that this property of the proton form factor is temperature dependent. We present measurements of the quasielastic broadening of the coherent diffuse neutron scattering from the time-dependent long-ranged displacement field (‘‘Huang linewidth’’) and discuss further implications of our findings on the interpretation of structural and dynamical properties of H in Nb, as the local modes and the coherent diffuse neutron scattering from the lattice distortions around the local defect neighborhood.

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