Abstract
First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in �-phase ErH2. The ground-state for hydrogen was indeed determined to be the tetrahedral position as commonly assumed, but if the surrounding tetrahedral sites are filled any additional hydrogen will occupy the octahedral site. Only a small amount of thermally generated tetrahedral-vacancy octahedral-occupancy pairs are predicted at equilibrium since the barrier and formation energies are 1.90 and 1.21 eV respectively. Two other possible scenarios will result in octahedral hydrogen occupation, a H/Er ratio > 2.0 and the presence of oxygen in the
lattice. Our calculations indicate that oxygen impurities will reside in tetrahedral sites, even if that site is already occupied and hydrogen must be displaced into a neighboring octahedral site. Oxygen will migrate at moderate temperatures by jumping between tetrahedral sites and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahedral vacancies and/or octahedral hydrogen and therefore will be enhanced by overloading hydrogen or the presence of significant oxygen contamination. In samples where some of the hydrogen is replaced with tritium, helium generated by tritium decay will favor a tetrahedral site left vacant by a transmuted tritium. The barrier to migration between two unoccupied neighboring tetrahedral sites is 0.49 eV, where the path maximum corresponds to the octahedral site. If an extended network of neighboring vacancies exists, the relatively small barrier provides that helium may move throughout that at room temperature. Given enough energy to escape the
tetrahedral site(s), 1.31 eV, helium may continue to migrate by a 0.88 eV concertedmotion mechanism---temporarily displacing hydrogen as it moves between empty octahedral sites and filled tetrahedral sites.
