title: Helium stability and its interaction with H in α-Al2O3: a first-principles study.
abstract: Little is known about hydrogen interaction with helium, an extrinsic defect, present in α-Al2O3 TPBs due to tritium decay and (n, a) reaction. Using density functional theory (DFT), the stability, structure and diffusion of He-related complexes at the different positions (VAl(3-), V, Oi(2-) and octahedral interstitial sites (OISs)) in α-Al2O3, as well as the interactions with H, are determined under H2-rich conditions. A He atom favors occupation of Al vacancies, the centers of OISs or forms a dumbbell around Al vacancies, forming Hei, HeAl(3-), Hei-HeAl(3-), [V-Hei](0) and [Oi(2-)-He](2-) complexes, among of which HeAl(3-) forms most readily. VAl(3-) can attract He to form small stable He-HeAl(3-) clusters, whereas only a He atom is trapped by an OIS, V and Oi(2-). Hei is more likely to diffuse into VAl(3-) and V than diffuse along the c-axis from one OIS to another. Hi(+) trapping into HeAl(3-) and [V-Hei](0) is thermodynamically and kinetically feasible, whereas dissociation of [Hei-H(+)](+) is more feasible. Forms of H-He complex defects in α-Al2O3 are Hei, Hi(+), [Hei-H(+)](+), [HeAl(3-)-H(+)](2-) and [HO(+)-Hei](+). HeAl(3-) and [V-Hei](0) present will increase the activation energy of H migration in α-Al2O3, which is favored for low H transport of TPBs.
authors.affiliation: Institute of Materials, China Academy of Engineering Physics, 621908, Jiangyou, China. xlwang@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: Xuexing
authors.initials: X
authors.lastname: Peng
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