First-principles study of the surface properties of LiAl5O8: Stability and tritiated water formation

Abstract

• Two most stable surfaces in an O-rich condition were identified for LiAl 5 O 8 . • The 3 H saturation is favorable for the nonstoichiometric (111) surface. • The stoichiometric (001) surface does not prefer 3 H saturation. • The 3 H 2 O desorption is preferable from the fully 3 H-saturated (111) surface. LiAl 5 O 8 is an important secondary phase of γ-LiAlO 2 ceramics in tritium ( 3 H, T)-producing burnable absorber rods (TPBARs), present in small quantities in the as-fabricated condition and in larger quantities after irradiation due to Li burnup and mobility in the radiation-damaged lattice. In this work, we performed first-principles calculations to study the stabilities and structures of LiAl 5 O 8 surfaces the possibility of T 2 O formation on the (111) surface. By calculating the surface energies of all possible symmetrical stoichiometric and nonstoichiometric low-index surfaces, we identified the two most stable surfaces, one stoichiometric (001) surface and one nonstoichiometric (111) surface in an O-rich condition. The surface oxygen atoms on the nonstoichiometric (111) surface have remarkable displacements after relaxation, due to the large dangling O bonds. Considering a Li-poor and 3 H-rich condition relevant to TPBARs, we further studied these two stable surfaces with 37.5%, 75%, and 100% surface O atoms covered by 3 H atoms. Our results show that the 3 H saturation is favorable for the nonstoichiometric (111) surface but not for the stoichiometric (001) surface. In addition, the structure of nonstoichiometric (111) surface is well stabilized when the surface O atoms are 100% saturated by 3 H atoms. Based on the fully 3 H-saturated (111) surface, the 3 H 2 O desorption with an energy barrier of 0.69 eV is preferable in the 3 H-rich condition.