Quasielastic Thermal Neutron Scattering by Hydrogen in α-Vanadium Hydride

Abstract

The diffusion process of hydrogen in vanadium has been studied by quasielastic thermal neutron scattering. Neutron linewidths associated with diffusion broadening have been determined at 485°K for VH_0.198 and VH_0.570. These measurements cover a range of momentum transfers (Q) for elastic scattering up to 4.1 Å⁻¹. This is much larger than the momentum transfers obtained in earlier cold‐neutron experiments, where no conclusive comparison was possible between data and proposed models involving diffusive jumps between octahedral and tetrahedral sites. The linewidth results for α‐VH_0.57 are compared with these models and with an extension developed in the present paper. Although no complete agreement was obtained, this comparison leads to the conclusion that jumps between tetrahedral sites are predominant in the diffusion process. Differences in the linewidth behavior for VH_0.198 and VH_0.570, and the fact that the measured widths at large Q increase rapidly with Q rather than approaching an asymptotic value as predicted by the theory, are discussed. It is suggested that these effects and the large Debye‐Waller factors found in the present and previous neutron experiments are connected with the relative magnitudes of diffusion relaxation time and the jump time between sites.