Quasi-elastic neutron scattering studies of collective and correlated tracer diffusion in the systems α′-NbDx and α′-NbHx☆

Abstract The technique of neutron polarization analysis has been applied to the problem of separating the coherent and incoherent quasi-elastic scattering from deuterium in α-NbD 0.7 . The coherent scattering has provided a first experimental test of a theory due to Sinha and Ross (S. K. Sinha and D. K. Ross, Physica B, 149 (1988) 51) with some simplifying assumptions. The coherent quasi-elastic spectra exhibit a behaviour analogous to a De Gennes narrowing, corrected for tracer and mobility correlation factors. The tracer correlations have been studied in more detail by incoherent quasi-elastic neutron scattering from α′-NbH 0.79 . The results indicate a significant Q and ω dependence of the tracer correlation factor for which the Q dependence in the long time limit indicates an expected periodicity in the Brillouin zone of the interstitial lattice.