Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V-H system studied by ¹H NMR

The diffusion of hydrogen in the monohydride phase (β phase) of the V _1-xCr _x-H (x ≤ 0.1) system was studied by means of ¹H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, E _H, and the Korringa constant, T _1eT, were estimated from the temperature and frequency dependence of ¹H spin-lattice relaxation times, T ₁. The value of E _H increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T _1eT, which was accompanied by reducing in the stability of the monohydride phase of the V-H system.