Study of hydrogen diffusion in tantalum hydrides by inelastic neutron scattering

Neutron inelastic scattering spectra have been measured as a function of temperature and scattering angle for α-TaH0.15 and for Ta2H in its α and β1 phases. The widths of the quasielastic peaks vs momentum transfer (Q) have been derived and compared with several models for hydrogen diffusion. The results suggest the dominance of tetrahedral jumps in the diffusion process, although no completely satisfactory theoretical fit was obtained. Residence times τ between 1.6 (340°C) and 4.0 (148°C) psec for TaH0.15 and between 2.4 (300°C) and 7 (153°C) psec for Ta2H were derived from the fit of the observed widths to the tetrahedral model. These results indicate a significant concentration dependence of the diffusion rates in α-TaHx, with activation energies for diffusion of 10.4 ± 1.2 kJ/mol in TaH0.15 and 15 ± 1.2 kJ/mol for Ta2H. ``Effective'' mean square amplitudes of proton vibration of 0.02–0.04 Å2 were obtained from the Q dependence of the quasielastic peak intensities, in reasonable agreement with the values predicted from relatively crude vibration frequency distributions obtained from our inelastic scattering spectra in the α phase. Our results are compared in detail with previous neutron results on VHx and NbHx. On the basis of this comparison it is tentatively concluded that the very high ``Debye-Waller factors'' obtained for VHx and some other hydrides are related to high hydrogen diffusion rates.