Stoichiometry dependence of Li ⁺ diffusivity in LiNbO ₃ crystal in off-congruent, Li -deficient regime

Diffusivity at 1130°C of Li ⁺ in LiNbO ₃ crystal in off-congruent, Li-deficient regime has been studied as a function of Li ₂O-content. A diffusion-limited Li ₂O-content reduction depth profile in an X- or Z-cut plate, ranging from 47.2 to 48.5 mol%, was produced by carrying out Li-poor vapor transport equilibration (VTE) treatment on 1-mm-thick, initially congruent plates at 1130°C for 150 h. The profile was obtained by alternatively doing the optical polish to one crystal surface and the Li ₂O-content characterization on the polished surface through the measurement on birefringence. The Boltzmann-Matano mathematical analysis was applied to the profile to obtain the Li ₂O-content dependence of Li ⁺ diffusivity. The results show that the diffusivity decreases with the decreased Li ₂O-content and the decrease is by at least one order of magnitude as the Li ₂O-content decreases by 1 mol% from 48.5 mol%. The diffusion shows slight anisotropy and is faster along the optical axis direction of crystal. In combination with previously reported result in the near-stoichiometric regime, the diffusivity over the Li ₂O-content range from 47 to 50 mol% is presented.