Xin Xiang

A new perspective on the process of intrinsic point defects in α-Al2O3

First-principles plane-wave pseudopotential calculations have been performed to study the charge states and energetics of intrinsic point defects as vacancies, interstitials and antisite atoms in α-Al2O3, and thus a new perspective on the process of intrinsic point defects has been proposed. Considering the various charge states for each intrinsic point defects, V(Al)(3-), V(O)(0), Al(i)(3+), O(i)(2-), Al(O)(3+), and O(Al)(3-), not all in their fully ionized states are found to be most stable and in pure Al2O3. From the formation energies of individual point defects, the antisite atom O(Al) will be readily formed in α-Al2O3 in an O-rich environment. By combination of charge states and formation energies, the defect types of Schottky, Al Frenkel and antisite pairs formed are found to be dependent on the O condition, and the most stable Schottky defect type is not the commonly considered {3V(O)(2+):2V(Al)(3-)}. There are two types of possible O Frenkel defects under both O conditions, yet the most stable defect is {O(i)(1+):V(O)(1-)} rather than the commonly believed {O(i)(2+):V(O)(2-)}. The bizarre configuration and the charge state of Schottky and Frenkel defects predicated in this work provide a new perspective on the process of intrinsic point defects in α-Al2O3.

Fe effect on the process of intrinsic point defects in α-Al 2 O 3

Fe is a common existing impurity in α-Al2O3, and thus the effect of Fe on the process of intrinsic point defects in α-Al2O3 has been investigated based on first-principles calculations. It is found that the formation, charge state, relative stability and equilibrium configuration of isolated intrinsic point defects in α-Al2O3 will be remarkably influenced by Fe, resulting in the variation of defect process, i.e., the formation of defect complex such as Schottky defect, Frenkel defect and antisite pair in α-Al2O3. Generally speaking, depending on the O-condition, the most stable configurations, types and relative proportions of defect complexes will be varied by Fe doping in α-Al2O3. From the viewpoint of defect formation energy, Fe is favorable for Frenkel defects and antisite pairs in α-Al2O3 under both O-rich and O-deficient conditions; while for Schottky defects, Fe is favorable for the defect formation under the O-rich condition, yet unfavorable under the O-deficient condition.

H/He interaction with vacancy-type defects in α-Al2O3 single crystals studied by positron annihilation

To probe the interaction of H and He, produced by tritium decay, with vacancy-type defects of α-Al2O3 as a tritium permeation barrier (TPB) in fusion reactors, α-Al2O3 single crystals were treated in pure Ar gas, D2 gas and T2 gas with subsequent tritium aging, respectively, and then their positron annihilation lifetimes and the type of defects that may contribute to the observed positron lifetime components were studied, in combination with DFT results. More monovacancies and vacancy clusters were formed in the thermally hydrogenated samples when compared to the fresh and Ar-annealed samples, indicating the stabilizing effect of hydrogen; this was consistent with the Fermi level position of α-Al2O3 moving towards the conduction band minimum (CBM) in the presence of hydrogen impurities, resulting in VAl3− and [VAl3−–H+]2− becoming more stable, as observed by DFT calculations. The monovacancies were slightly eliminated when the samples were thermally annealed and then aged in T2 gas at room temperature, indicating that He filled the vacancies. This was consistent with it being favourable for He atoms to occupy Al vacancies, with HeAl3− forming most readily, whilst more vacancy clusters were continuously induced, suggesting that Al–O bonds weakened and thus nano-hardness decreased with an external load. This study provides the first evidence that Al vacancies can be stabilized by H and filled with He, which will provide further novel TPB design opportunities.