Lithium tracer diffusion in proton-exchanged lithium niobate

Abstract

We studied lithium tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 150 and 230 °C. Proton exchange was done in benzoic acid with 1 mol% lithium benzoate added, resulting in a surface layer where Li is substituted by H for about 60% (about 12 at.% H within LiNbO3). For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a tracer source. Isotope sensitive depth profile analysis was carried out by secondary ion mass spectrometry in depth profile mode. From the experimental results effective diffusivities governing the lithium/hydrogen exchange and lithium tracer diffusivities are extracted. Both types of diffusivities are identical within error limits and follow the Arrhenius law with an activation enthalpy of about 1.2 eV. In contrast, the lithium tracer diffusivities are lower by three orders of magnitude than hydrogen tracer diffusivities which were recently determined by our group. The results indicate that the diffusion of Li is the rate determining step for the proton-exchange process.