Abstract
Hydrogen atom absorption by hcp titanium is treated using an embedding theory (Whitten and Pakkanen, Phys. Rev. B21 (1980) 4357) to describe the electronic bonding. The very low concentration limit is modelled by treating a single H atom in a 43 atom titanium cluster to compute binding energies of different interstitial sites. An energy barrier to H penetration of the surface of 23 kcal/mol is found for the unrelaxed surface, and this result is attributed simply to short Ti-H distances in the surface plane. Hydrogen in an octahedral hole blow the close packed titanium surface layer is 12 kcal/mol more stable than H in a tetrahedral hole for an unrelaxed lattice and this appears to be due to the better accommodation of the negatively charged H by the larger octahedral cavity. An energy barrier to the migration of H between interstitial sites is found.
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