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Phys Chem Chem Phys. 2016 Jan 21;18(3):1649-56. doi: 10.1039/c5cp06496a. Epub 2015 Dec 17.

Helium stability and its interaction with H in α-Al2O3: a first-principles study.

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Institute of Materials, China Academy of Engineering Physics, 621908, Jiangyou, China.


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.


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