Diffusion of Hydrogen in the Body Centered Tetragonal Hydride Phase of the V–H System Controlled by Multiple Substitutional Metal Elements

The diffusion and site occupation of hydrogen in the monohydride phase (β phase) of the V1−x−yCrxMoy–H (x + y ≤ 0.15) system have been studied by means of proton nuclear magnetic resonance (1H NMR). The β phase has a body centered tetragonal (BCT) structure and hydrogen atoms occupy the octahedral (Oz) sites along the c -axis. Cr has a smaller atomic radius than V but Mo has a larger one, resulting in no obvious change in the unit cell volume of the BCT lattice by substitution of both Cr and Mo for a part of V, although the multiple metal elements of V, Cr and Mo induce lattice strain and distortion. The temperature dependence of 1H spin-lattice relaxation time has demonstrated that the substitution of both Cr and Mo offsets the change in the activation energy for hydrogen diffusion EH for the Oz sites, while our previous study has indicated that Cr increases EH and Mo decreases that. The diffusion behavior of hydrogen is explained by the dimension of the interstitial sites attributing to the weighted mean atomic radius of the substitutional metal elements, which is independent of their affinities for hydrogen and the lattice strain.