Hydrogen diffusion in proton-exchanged congruent Lithium Niobate during post-annealing

We investigated the diffusion of hydrogen during post annealing of proton-exchanged congruent LiNbO3 single crystals in the temperature range between 200 and 400 degrees C. Proton exchange was done in benzoic acid with 3.6 mol% lithium benzoate added. This process results in a 180 nm thin surface layer with a sharp interface where Li is substituted by H for about 45%. Depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, diffusivities were extracted which follow the Arrhenius law with an activation enthalpy of diffusion of (1.39 +/- 0.17) eV. The diffusivities are about two orders of magnitude lower than hydrogen tracer diffusivities in pure and in proton-exchanged LiNbO3 but are in good agreement to effective interdiffusion coefficients, governing the proton exchange. Also the activation energy for the latter case of (1.24 +/- 0.04) eV is identical within error limits. The results indicate that the diffusional redistribution of high amounts of hydrogen in LiNbO3 is an interdiffusion process between H and Li that is governed by the migration of Li.