Neutron-quasielastic-scattering study of hydrogen diffusion in a single crystal of tantalum

The diffusion of hydrogen in a single crystal of bcc tantalum (Ta${\mathrm{H}}_{0.02}$) at 584 K has been investigated by neutron-quasielastic scattering at a variety of crystal orientations, and over a range of wave-vector transfer $\left|\overrightarrow{\mathrm{Q}}\right|$, from 0.8 to 2.5 ${\mathrm{\AA }}^{-1\mathrm{}}$. A detailed analysis of the observed quasielastic line shapes and widths shows that the results cannot be fitted by any simple jump-diffusion model involving instantaneous jumps between octahedral and tetrahedral interstitial sites. The quasielastic width curves (full width at half-maximum versus $\overrightarrow{\mathrm{Q}}$) are much more isotropic than those predicted by any of the hydrogen-jump models, although the general shape of the widths at large $Q$ is closer to that predicted by a tetrahedral-site model. These results are in distinct contrast to a recent neutron study of hydrogen diffusion in single-crystal (fcc) palladium, where the details of the quasielastic scattering were fitted well by a model assuming instantaneous jumps between octahedral sites. The Ta${\mathrm{H}}_{0.02}$ quasielastic peaks suggest a diffusion "relaxation time" between 1 and 2 ps at 584 K. Analysis of the data also provides an average "mean-square hydrogen vibration amplitude" of 0.040 ${\mathrm{\AA }}^2$. The present single-crystal results are in reasonable agreement with the results of a previous neutron study of polycrystalline ($\alpha$-phase) $\mathrm{Ta}{\mathrm{H}}_x$. In addition, a value for the macroscopic diffusion constant at 584 K of 2.8 × ${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^2$ ${\mathrm{s}}^{-1\mathrm{}}$ is derived from the low-$Q$ results, which is in excellent agreement with the value predicted from Gorsky-effect measurements.