Demonstration of Er³⁺ diffusivity and solubility increases in off-congruent, Li-deficient LiNbO₃ crystal

We demonstrate Er³⁺ diffusivity and solubility increases in off-congruent, Li-deficient LiNbO3 crystal. Li-poor vapor transport equilibration was used to reduce Li₂O content in initial congruent crystals. Local Er³⁺ in-diffusion was then performed in a wet O ₂ atmosphere. Before and after the Er³⁺ diffusion procedure, surface Li₂O content was evaluated from measured birefringence. The results show that the Er³⁺ diffusion procedure resulted in 0.3-0.5 mol% Li₂O content loss at crystal surface. Secondary ion mass spectrometry was used to measure the Er³⁺ depth profiles, from which the diffusivity and solubility are determined. It is shown that the Er³⁺ diffusivity is nearly doubled and the solubility increases at least 0.6 mol% as the Li₂O content decreases by 1.0 mol%. From the known Li₂O content reduction, the solubility increase is also predicted and the results show that the predicted data are considerably smaller than the experimental results, suggesting that the Er³⁺ ions occupy also the Nb⁵⁺ site, besides the Li⁺ site.