Enhancement of hydrogen diffusion in the body-centered tetragonal monohydride phase of the V-H system by substitutional Al studied by proton nuclear magnetic resonance

The diffusion and site occupation of hydrogen in the monohydride phase (beta phase) of the V1-xAlx-H (x <= 0.1) system was studied by means of proton nuclear magnetic resonance (H-1 NMR). Hydrogen atoms which occupied the octahedral (O-z) sites along the c-axis in a body-centered tetragonal structure shifted to the tetrahedral (T) sites with the Al content. V0.90Al0.10H0.68 had a body-centered cubic structure and the component observed in the temperature dependence of H-1 spin-lattice relaxation time was due to the contribution of hydrogen on the T sites. The addition of Al reduced the site radius of the O-z sites and resulted in reducing the activation energy for hydrogen diffusion, EH, for the O-z sites. Moreover, the diffusion of hydrogen was enhanced by a shift of site occupation from the O-z sites to the T sites, which had lower EH than the O-z sites. The addition of larger substitutional Al atoms than V enhanced the diffusion of hydrogen in the beta phase of V-H. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.