Diffusive oxygen ion transport in Li(Nb,Ta)O3 single crystals with congruent and near-stoichiometric composition

We investigated oxygen tracer self-diffusion in LiNb 1-x Ta x O 3 (x = 0, 0.45, 0.85, 1) single crystals with congruent and near-stoichiometric composition in the temperature range between 750 and 1400 °C. The diffusion studies were carried out using 18 O 2 / 16 O 2 gas-exchange and secondary ion mass spectrometry depth profiling. The diffusivities of each compound investigated follow the Arrhenius law with activation enthalpies in the order of 3–4 eV. In general, a higher activation enthalpy of ∼0.5 eV is found for the near-stoichiometric compounds compared to the congruent counterparts. Nevertheless, congruent and near-stoichiometric crystals show very similar diffusivities, because the higher activation enthalpy of the stoichiometric compounds is compensated by a higher pre-exponential factor. The results can be explained consistently by a model based on pseudo-Schottky defects and oxygen transport proceeding via vacancy-mediated diffusion. The diffusivities in LiNb 1-x Ta x O 3 decrease non-linearly with increasing x, resulting in an about one order of magnitude higher diffusivities in LiNbO₃ than that in LiTaO 3 across the investigated temperature range. Possible reasons, such as defect clustering are discussed.