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 ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$ single crystals from different suppliers up to a temperature of $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, using isotope-enriched $^{6}\mathrm{LiNbO}_{3}$ and $^{6}\mathrm{LiTaO}_{3}$ 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 ${150}^{\ensuremath{\circ}}$ to $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Furthermore, the electrical conductivity is determined between $400{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ 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 ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$ are identical within the error margins with the overall conductivities obtained from impedance spectroscopy measurements. This indicates that the migration of ${\mathrm{Li}}^{+}$ is able to explain the overall electrical conductivity below $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ down to $180{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$.