Density functional theory study of the point defect energetics in γ-LiAlO2, Li2ZrO3 and Li2TiO3 materials

Abstract

The defect energetics in γ-LiAlO2, Li2TiO3, and Li2ZrO3 materials used in tritium-producing burnable absorber rods and fusion solid breeder applications was investigated using density functional theory calculations. A comprehensive analysis of the charged defects was performed for cation and anion vacancies, interstitials, antisite defects, and 3H interstitial and substitutional defects to understand the defect structures at different charge states and their stability as a function of the electron chemical potential across the electronic band gap of crystals at operating temperature and oxygen partial pressure conditions. Through investigation of the local bonding configurations and projected electronic density of states of the 3H related charged defects, including 3H interstitial (3Hint), 3Hint bound with an oxygen interstitial, and substitutional 3H defects on the Li, O, and M (M = Al, Ti, and Zr) sites (3HLi, 3HO, and 3HM), the 3H states of these defects were distinguished, i.e. triton (3H+), neutral 3H (3H0), and tritide (3H−). The two stable 3H related defects under the Li2O-LiOH rich condition at T = 1000 K, between P(O2) = 10−5 and 10−25 atm were found to be 3H interstitial (3Hint) with charge of +1 and neutral 3HLi (substitutional 3H on the Li site) in γ-LiAlO2, Li2ZrO3, and Li2TiO3 materials. For Li2TiO3, the stability of the 3HO with charge of +1 and 3HM with charge of −3 becomes comparable to that of 3Hint atm at P(O2) = 10−25 atm. Overall, our results indicate the energy cost to form the stable 3H defect species (3Hint with charge of +1 and neutral 3HLi) in γ-LiAlO2 material are higher than those of Li2TiO3 and Li2ZrO3 materials at T = 1000 K, between P(O2) = 10−5 and 10−25 atm. Interactions between the point defects and the 3H species as a function of the electron chemical potential across the band gap of the materials were also examined. All the three materials exhibit strong interactions between the 3Hint and the O vacancy, Li vacancy, and O interstitial defects with the strongest interactions found in the case of γ-LiAlO2 phase, indicating a higher tendency to trap 3H at such point defects in γ-LiAlO2.