First-principles study of the surface properties of γ-LiAlO2: Stability and tritium adsorption

Abstract

γ-LiAlO2 enriched with 6Li isotope exhibits excellent performance under neutron radiation and is used in tritium-producing burnable absorber rods (TPBARs) to produce tritium (T, 3H) in pressurized water reactor (PWR). To better understand the 3H formation and diffusion on the surfaces, the first-principles calculations are performed to study the stability of γ-LiAlO2 surface and the 3H adsorption on the surface. By considering all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces, two most stable surfaces, one stoichiometric (100) and one nonstoichiometric (101), were identified. The 3H adsorption and diffusion on the most stable (100) surface were further explored, and the obtained results show that there are four stable sites (three O and one Al atop) for 3H adsorption. The calculated Bader charges of atoms reflect that 3H adsorption on the top of O site is a reduction process, while on the top of Al site is an oxidation one. Under low adsorption coverage of 1/4 per surface unit cell, 3H atoms are adsorbed on the most stable OA atop site. With increasing 3H diffused from bulk to surface, the AlG site is the second choice for 3H adsorption, and then the OB site. The coverage of 2/4 is the most favorable 3H adsorption state due to the counteracted charge transfer. When the coverage is up to 4/4, the interaction between 3H atoms leads to partial formation of T2 molecule that desorbs from surface. Therefore, T2 molecule should be the main product from the perfect surface of γ-LiAlO2 at high 3H coverage.