Effects of substitutional Mo and Cr on site occupation and diffusion of hydrogen in the β-phase vanadium hydride by first principles calculations

The effects of substitutional Mo and Cr in β-phase VH_0.5 and V_1−xM_xH_0.5625 (M = Mo, Cr; x = 0, 0.0625, 0.125) on the site occupation and diffusion paths of hydrogen are investigated by quantum mechanical calculations based on density functional theory. Fundamental processes of the interstitial-assisted mechanisms are systematically figured out, and specific values of the site energies are obtained with zero-point energy (ZPE) corrections. Hydrogen atoms are found to occupy the octahedral (O) interstitial sites in β-phase (V + M)H_0.5 in the ground state. Upon increasing the hydrogen concentration H/(V + M) higher than 0.5, the additional H atom prefers to reside at the tetrahedral (T) interstitial sites. The minimum energy paths of hydrogen diffusion are analyzed by the Nudged Elastic Band method with ZPE corrections. The site occupation energy and activation energy for each hydrogen diffusion path are found to be strongly influenced by the substitution of Mo or Cr into vanadium hydride. The results presented in this work indicate that the additional H prefers to migrate directly from T site to the nearest neighboring T site without crossing O site. The energy barriers in the order of 0.253–0.276 eV of hydrogen migration in the V_1−xM_xH_0.5625 hydrides obtained from ab initio simulations are in good agreement with the experimental data by means of ¹H NMR measurement.