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 (β phase) of the V1−xAlx-H (x ⩽ 0.1) system was studied by means of proton nuclear magnetic resonance (1H NMR). Hydrogen atoms which occupied the octahedral (Oz) 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 1H 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 Oz sites and resulted in reducing the activation energy for hydrogen diffusion, EH, for the Oz sites. Moreover, the diffusion of hydrogen was enhanced by a shift of site occupation from the Oz sites to the T sites, which had lower EH than the Oz sites. The addition of larger substitutional Al atoms than V enhanced the diffusion of hydrogen in the β phase of V-H.