Interstitial H in body-centred cubic metals was studied theoretically by using first-principles density-functional calculations. The effect of self-trapping was investigated in detail, and the calculated energies, forces and displacements for H at tetrahedral sites were found to be in good agreement with experiment. The local motion of H and D was treated quantum-mechanically by mapping out potential energy surfaces and solving a Schrödinger equation for the ground state and vibrationally excited states. Diffusion between sites was considered in both the classical and quantum regimes. At low temperatures, the small-polaron theory of phonon-assisted tunnelling was applied, and excellent agreement was found with experiment for both the calculated coincidence energy and bare tunnelling-matrix elements. At higher temperatures, the results indicated that H migration could best be described in terms of over-barrier motion, rather than tunnelling from excited states.

Self-Trapping and Diffusion of Hydrogen in Nb and Ta from First Principles. P.G.Sundell, G.Wahnström: Physical Review B, 2004, 70[22], 224301 (7pp)