This article reports on Li self-diffusion in lithium containing metal oxide compounds. Case studies on LiNbO 3 , Li 3 NbO 4 , LiTaO 3 , LiAlO 2 , and LiGaO 2 are presented. The focus is on slow diffusion processes on the nanometer scale investigated by macroscopic tracer methods (secondary ion mass spectrometry, neutron reflectometry) and microscopic methods (nuclear magnetic resonance spectroscopy, conductivity spectroscopy) in comparison. Special focus is on the influence of structural disorder on diffusion.

Lithium diffusion mechanisms in β-LiMO2 (M = Al, Ga) were studied in a combined experimental and theoretical approach based on Li tracer diffusion experiments and climbing-image nudged-elastic-band (cNEB) calculations at density functional theory (DFT) level, respectively. Secondary ion mass spectrometry (SIMS) investigations were carried out for β-LiAlO2 and β-LiGaO2 polycrystalline films in the temperature range between 473 and 773 K. A thin layer of ion-beam sputtered isotope-enriched 6LiAlO2 or 6LiGaO2 was used as a tracer source. The diffusivities of β-LiGaO2 polycrystalline films are in good agreement with those measured on single crystals of the same type. The diffusivities of β-LiAlO2 are higher than in β-LiGaO2 by almost 2 orders of magnitude. This can be traced back to a lower activation energy for diffusion in β-LiAlO2. Our computational study shows that the formation energy of a Li vacancy (VLi) is much higher than that of the Li Frenkel pair (VLi + Lii) showing that Li vacancies are not abundant in both systems. Irrespective of the defect types, the defect formation energy values are smaller in β-LiAlO2 than in β-LiGaO2, indicating that Li ion migration could be facile for the former case. In both systems, the most likely Li migration pathways involve Li diffusion from a regular LiO4 tetrahedral location to the first and/or second nearest tetrahedral sites by octahedral interstitial sites. On the basis of calculated activation energies it is concluded that Li diffusion is faster in LiAlO2 than in LiGaO2. Our calculated data are in good accord with the experiments.

Monoclinic gallium oxide (β-Ga2O3) is an ultra-wide bandgap semiconductor with importance in various technological areas. We investigated oxygen tracer self-diffusion in (100) oriented β-Ga2O3 single crystals at high temperatures between 1200 °C and 1600 °C. Isotope enriched 18O2 gas was used as a tracer source. The isotope exchanged samples were analyzed by secondary ion mass spectrometry in depth profile mode. The diffusivities can be described by the Arrhenius law with an activation enthalpy of (3.2 ± 0.4) eV. Possible diffusion mechanisms are discussed using defect equilibria and density functional theory calculations as found in the literature. As a result, oxygen interstitials are more likely than vacancies as defects governing diffusion.