Effect of dissolved oxygen on hydrogenation of vanadium and hydrogen diffusion in the monohydride phase

We elucidate the effect of dissolved oxygen on the phase transformation and the diffusion of hydrogen in the V-H system. V forms monohydride (beta) and dihydride (gamma) phases by hydrogenation but a small amount of dissolved oxygen (O/V approximate to 0.01) leads to formation of another beta phase expanded along the c-axis in a body centered tetragonal structure (BCT) rather than the gamma phase. This change in the phase transformation is due to trapping hydrogen atoms by oxygen atoms, resulting in the shift of site occupation of hydrogen from octahedral (O) sites to tetrahedral (T) sites. Proton nuclear magnetic resonance (H-1 NMR) demonstrates that the activation energy for hydrogen diffusion is increased by the trapping effect. Oxygen dissolved in V suppresses not only the formation of the gamma phase but also the diffusion of hydrogen. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.