Lithium niobate and lithium tantalate crystals as well as LiNb1-xTaxO3 solid solutions are technologically important polar metal oxides with exceptional combinations of ferroelectric, piezoelectric, acoustic, optical and ion conductivity properties. The self-diffusion of the ionic constituents and underlying point defects is important for overall electrical conductivity, structural disorder, ferroelectric domain wall pinning, high-temperature stability, and optical and piezoelectric applications. Li self-diffusion in congruent LiNbO3 and LiTaO3 single crystals as well as in solid solutions is investigated up to a temperature of 800 °C using 6LiNbO3 or 6LiTaO3 tracers and secondary ion mass spectrometry. The diffusivities of LiNbO3, LiTaO3 and LiNb1-xTaxO3 (x < 0.1) single crystals are identical within error limits below 800 °C and can be described by the Arrhenius law with an activation enthalpy of 1.3 eV. The Li-ion conductivities calculated from the diffusivities in LiNbO3 are identical to the overall conductivities obtained from impedance spectroscopy measurements. This indicates that Li is one of the dominating species determining the electric conductivity below 800 °C.
The diffusion of hydrogen is also of importance because it occurs in as-grown crystals as an impurity with concentrations below 0.1 at.%, contributing also to the overall electric conductivity. Consequently, we present first experiments on the diffusion of hydrogen. Diffusivities are determined by post-annealing of crystals which were proton-exchanged in benzoic acid. First results indicate that the tracer diffusion of hydrogen is faster than that of lithium.