title: First principles investigation of helium physisorption on an α-Al2O3(0001) surface.
abstract: The interaction of helium with an α-Al2O3(0001) surface was studied by density functional theory (DFT), with consideration of the effects of He-coverage, surface defects, He-coadsorption and van der Waals interaction, respectively. Adsorption energies of helium atoms are very small as expected for a physisorbed state, varying from -20 to -5 meV, which is attributed to the small overlap between Al 3sp, O 2sp and He 1s states. A correlation is obtained for the adsorption energies and the He to nearest-neighbor Al atom distances on a clean (0001) surface. The He atom prefers to bound atop the Al site of the fourth atomic layer (Al4 hollow site), and the favorable site around an O-vacancy is atop the site of the O vacancy with less stability. The competition between O-He attraction and Al-He repulsion makes the He stable sites. As He-coverage on the surface increases, He atoms tend to form clusters, and coadsorption configuration is not solely determined by the most stable site but also by the He-He distance. The two co-adsorbed He atoms absorb on hollow sites Al4 and Al3, with a He-He distance of 2.767 Å. The OBS dispersion corrected DFT energies are 2.2-4.4 times larger than the non-corrected DFT values and He-surface distances are smaller. Finally, implications on H/He interaction within α-Al2O3 as a tritium permeation barrier are discussed.
authors.affiliation: Institute of Materials, China Academy of Engineering Physics, Jiangyou, 621908, China. zhangguikai@caep.cn.
authors.forename: Guikai
authors.initials: G
authors.lastname: Zhang
authors.forename: Xin
authors.initials: X
authors.lastname: Xiang
authors.forename: Feilong
authors.initials: F
authors.lastname: Yang
authors.forename: Lang
authors.initials: L
authors.lastname: Liu
authors.forename: Tao
authors.initials: T
authors.lastname: Tang
authors.forename: Yan
authors.initials: Y
authors.lastname: Shi
authors.forename: Xiaolin
authors.initials: X
authors.lastname: Wang