First-principles density functional methods were used to investigate the atomistic

behaviour of hydrogen, helium, and oxygen in β-phase ErH2. The ground state for

hydrogen was deduced to be the tetrahedral position; as was usually assumed.

However, if the surrounding tetrahedral sites were filled, any additional hydrogen

occupied the octahedral site. Only a small number of thermally generated

tetrahedral-vacancy octahedral-occupancy pairs were predicted to exist at

equilibrium since the formation energy was 1.21eV. Other possible criteria that

resulted in octahedral hydrogen occupation included a H/Er ratio greater than 2 and the presence of oxygen in the lattice. The present calculations indicated that

oxygen impurities would reside in tetrahedral sites; even if that site were already

occupied and hydrogen had to be displaced into a neighbouring octahedral site.

Oxygen could migrate at moderate temperatures by jumping between tetrahedral

and octahedral sites. The extent of hydrogen self-diffusion depended upon the

concentration of tetrahedral vacancies and/or octahedral hydrogen, and could

therefore be modified by changing the H/Er ratio or by impurities such as oxygen

which created an octahedral hydrogen occupation. In samples where some of the

hydrogen was replaced by tritium, the helium generated by tritium decay was

expected to favour a tetrahedral site being left vacant by transmuted tritium. The

barrier to helium migration between two unoccupied neighbouring tetrahedral sites

was 0.49eV, where the path maximum corresponded to the octahedral site. If an

extended network of neighbouring vacancies existed, the relatively small barrier

allowed helium to move through the network at room temperature. Given the

energy to escape from tetrahedral sites, 1.31eV, the helium could continue to

migrate via a 0.88eV concerted-motion mechanism: temporarily displacing

hydrogen as it moved between empty octahedral sites and filled tetrahedral sites.

First Principles Site Occupation and Migration of Hydrogen, Helium, and Oxygen

in Beta-Phase Erbium Hydride. R.R.Wixom, J.F.Browning, C.S.Snow,

P.A.Schultz, D.R.Jennison: Journal of Applied Physics, 2008, 103[12], 123708