Hydrogen diffusion in proton-exchanged lithium niobate single crystals

Hydrogen diffusion in proton-exchanged congruent LiNbO₃ single crystals is investigated in the temperature range between 140 and 200 °C. The proton-exchange process carried out in a mixture of benzoic acid and lithium benzoate (1 mol. %) results in an up to 2 μm thick surface layer where Li is substituted by H for about 60% (about 12 at. % H within LiNbO₃) as determined by nuclear reaction analysis. For the diffusion experiments, deuterated benzoic acid is used as a tracer source and the hydrogen/deuterium isotope-exchange occurs at temperatures of 200 °C and below. Isotope sensitive depth profile analysis is done by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange and tracer diffusivities of deuterium within the exchanged layer are extracted. Both types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.2 eV, while the effective diffusivities are lower by three orders of magnitude. This result shows that the diffusion of hydrogen is not the rate-determining step for the proton-exchange process. Possible diffusion mechanisms of hydrogen tracer diffusion are discussed.