Li self-diffusion and ion conductivity in congruent LiNbO3 and LiTaO3 single crystals

Lithium niobate and lithium tantalate crystals are technologically important metal oxides with exceptional combinations of ferroelectric, piezoelectric, acoustic, optical, and electrical properties. The self-diffusion of both, the ionic constituents and the underlying point defects, is especially important for the overall electrical conductivity. To get insight into their dynamics, we investigate in this work Li self-diffusion in congruent LiNbO3 and LiTaO3 single crystals from different suppliers up to a temperature of 800 degrees C, using isotope-enriched 6LiNbO(3) and 6LiTaO(3) tracer layers in combination with secondary ion mass spectrometry depth-profile analysis. The diffusivities of the two isostructural materials are identical within error limits and can be described by the Arrhenius law with an activation energy of 1.35 eV in the range from 150 degrees to 800 degrees C. Furthermore, the electrical conductivity is determined between 400 degrees C and 600 degrees C and can described by an activation energy of about 1.34 eV. This is in excellent agreement with the energy barrier for the diffusion of a single Li vacancy as determined by nudged elastic band calculations based on density-functional theory. The Li-ion conductivities calculated from the diffusivities in LiNbO3 and LiTaO3 are identical within the error margins with the overall conductivities obtained from impedance spectroscopy measurements. This indicates that the migration of Li+ is able to explain the overall electrical conductivity below 600 degrees C down to 180 degrees C.