First principles site occupation and migration of hydrogen, helium, and oxygen in β-phase erbium hydride

Abstract

First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in β-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 formation energy is 1.21 eV. Other possible scenarios that result in octahedral hydrogen occupation include 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 and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahedral vacancies and/or octahedral hydrogen and therefore can be modified by changing the H/Er ratio or by impurities (such as oxygen) that create octahedral hydrogen occupation. 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 helium 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 network 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 concerted-motion mechanism—temporarily displacing hydrogen as it moves between empty octahedral sites and filled tetrahedral sites.