Oxygen Diffusion in Li(Nb,Ta)O3 Single Crystals

Oxygen tracer self-diffusion in LiNbO3, LiTaO3, and LiNb0.15Ta0.85O3 single crystals between 880 and 1050 degrees C is investigated. O-18(2) isotope-exchanged samples are analyzed by secondary ion mass spectrometry. The diffusivities of each of the three different materials can be described by the Arrhenius law with an activation enthalpy of diffusion of about 3.2-3.5 eV. The diffusivities are highest for LiNbO3 and are lower by about one order of magnitude for LiNb0.15Ta0.85O3 and LiTaO3. The change of the pre-exponential factor is identified as the reason for the difference in diffusivities. The experimental results are compared to defect formation energy calculations as given in literature and to energy barrier calculations for the diffusion of a single O vacancy as determined by nudged elastic band calculations based on density-functional theory. An oxygen vacancy mechanism is suggested to govern diffusion. The difference in diffusivities is tentatively attributed to a different number of freely migrating vacancies, probably due to defect complex formation.