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 (V_Al³⁻, V⁰_O, O_i²⁻ 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 He_i, He_Al³⁻, He_i–He_Al³⁻, [V⁰_O–He_i]⁰ and [O_i²⁻–He]²⁻ complexes, among of which He_Al³⁻ forms most readily. V_Al³⁻ can attract He to form small stable He–He_Al³⁻ clusters, whereas only a He atom is trapped by an OIS, V⁰_O and O_i²⁻. He_i is more likely to diffuse into V_Al³⁻ and V⁰_O than diffuse along the c-axis from one OIS to another. H_i⁺ trapping into He_Al³⁻ and [V⁰_O–He_i]⁰ is thermodynamically and kinetically feasible, whereas dissociation of [He_i–H⁺]⁺ is more feasible. Forms of H–He complex defects in α-Al₂O₃ are He_i, H_i⁺, [He_i–H⁺]⁺, [He_Al³⁻–H⁺]²⁻ and [H_O⁺–He_i]⁺. He_Al³⁻ and [V⁰_O–He_i]⁰ present will increase the activation energy of H migration in α-Al₂O₃, which is favored for low H transport of TPBs.