First-principles insight of hydrogen dissolution and diffusion properties in γ-Al2O3

First-principles theory is applied to identify the structure of γ-Al ₂O ₃ first. Then, hydrogen dissolution and diffusion behavior was further studied to reveal the reason that permeation reduction factor (PRF) of γ-Al ₂O ₃ is lower than that of α-Al ₂O ₃. Our calculations show that the formation energy for Al vacancy at the octahedral interstitial site ( V Al − OIS − 3 ) is lower than that at the tetrahedral interstitial site ( V Al − TIS − 3 ), and are far lower than those of other defects in both defective spinel and non-spinel γ-Al ₂O ₃. Thus, the stability of V Al − OIS − 3 is higher than that of V Al − TIS − 3 in both Al-rich and O-rich environments. However, when hydrogen atoms are introduced into γ-Al ₂O ₃, the stability of [V _Al-TIS-H] ⁻² is higher than that of [V _Al-OIS-H] ⁻² in the defective spinel γ-Al ₂O ₃ structure, while the opposite is true for the non-spinel γ-Al ₂O ₃ structure. The vibrational frequencies for OH ⁻ in [V _Al-TIS-H] ⁻² and [V _Al-OIS-H] ⁻² for defective γ-Al ₂O ₃ are calculated to be 3608 cm ⁻¹ and 3374 cm ⁻¹, respectively, which is in excellent agreement with the infrared (IR) absorption peaks observed at ∼3500 and ∼3300 cm ⁻¹. The defective spinel model is more suitable for describing the γ-Al ₂O ₃ structure. In defective spinel γ-Al ₂O ₃, there are many native Al vacancies, which are arranged in a straight line along the [ 2 1 ¯ 1 ¯ 0 ] direction. The migration barrier for H diffusion along these native Al vacancies is so low that these Al vacancies can provide a rapid diffusion channel for H. Thus, the permeability of H in γ-Al ₂O ₃ is much higher than that in α-Al ₂O ₃ leading to a lower permeation reduction factor (PRF). Our results can provide not only a sound theoretical explanation for the low PRF of γ-Al ₂O ₃ but also a direction to improve the efficiency in preventing H permeation through FeAl/Al ₂O ₃ tritium permeation barriers (TPBs).