Oxygen-18 tracer diffusion in nearly stoichiometric single crystalline lithium niobate

Oxygen-18 tracer transport measurements were performed in nearly stoichiometric lithium niobate. To produce such crystals the vapour transport equilibration (VTE) technique was applied using a much larger mass of a lithium-rich two-phase mixture. To avoid Li2O loss the samples were also held in close proximity to the lithium-rich two-phase mixture during the diffusion annealing. This maintains the Li2O activity in the system so that a rigorous defect chemical analysis is applicable. The oxygen-18 tracer diffusivity data were rationalised on the basis of current point defect models for lithium niobate taken from literature. Considering calculated energy values for the different point defect models it can be concluded that the oxygen transport in both Li2O-rich and Li2O-poor lithium niobate occurs via oxygen interstitials rather than via oxygen vacancies.