Hydrogen-induced interactions in vanadium from first-principles calculations

We present a first-principles study on hydrogen-induced interactions in vanadium, such as V-H, H-H, and vacancy-H interactions, which are relevant to the H-induced embrittlement in vanadium alloys employed as H-2 purification membranes. Insertion of H at tetrahedral interstitial sites of V proceeds exothermically and lowers the energy levels of V 3d, 4s, and 4p states that form a bonding state with H 1s. However, H insertion accompanies large local atomic relaxation, thereby developing stress inside the material, which makes a good contrast with Pd where H can be added without significant structural distortion. The strength of the H-H interaction in V, which is indeed an interaction between two V-H bonding states, is negligibly small compared with that of the V-H interaction itself when the H-H distance is larger than similar to 2 angstrom. We show that six H atoms can be trapped at the six octahedral sites next to a vacancy in V. Formation of H-2 molecules is energetically unfavorable, which is different from the cases of Al and W, where H-2 molecules can be formed when enough H atoms are accumulated in a vacancy. Reasons behind this difference, together with the energetics of H-induced superabundant vacancy formation, are discussed.