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We discuss oxygen and cation diffusion processes in oxygen ion conductors. While the high oxygen diffusivity determines the proper oxygen ion conductivity, slow cation diffusion processes are important for sintering and degradation processes. In the first part of the paper we discuss an analytical model for the ionic conductivity of a strongly acceptor doped, fluorite-type oxygen ion conductor, i.e. a concentrated solution of AO2 and BB2O3. The model can be applied, e.g., to yttria doped zirconia (YSZ) and gives a qualitative explanation of the observed maximum of the conductivity as a function of the dopant fraction. The model considers nearest neighbor interactions between oxygen vacancies and dopant cations, which may be negligible, attractive or repulsive, and jump barriers that depend on the nature of the cation-cation edge that has to be crossed during a jump between adjacent oxygen sites. In the second part we discuss cation diffusion processes in doped lanthanum gallates (LSGM). The experimental results of nearly identical cation diffusion coefficients in the A- and B-sublattices of the perovskite LSGM can be explained by a bound defect cluster mechanism containing cation vacancies of both the Aand the B- sublattice and anion vacancies.

Li diffusion in spinel-type structured cubic and hexagonal LixTiS2 with x = 0.6 and 0.7, respectively, was investigated by solid state NMR spectroscopy. In layered hexagonal LixTiS2, Li diffusion is known to be confined to the van-der-Waals gap between the TiS2 planes, i. e., it is two-dimensional (2D). In cubic LixTiS2, however, Li diffusion takes place in the three-dimensional (3D) interstitial space formed by the TiS6-polyhedra. Both polymorphs form a unique pair for studying the effects of dimensionality on Li diffusion within a host structure of the same chemical composition. 7Li spin-alignment echo and spin-lattice relaxation NMR methods were applied to measure Li jump rates. Additionally, solid echo NMR spectroscopy was used to estimate the Li diffusivity in cubic Li0.6TiS2. The activation energies for Li jumps in the two materials are very similar. Nevertheless, Li diffusion in the 3D host is slower than in the layered modification by a factor of about ten.

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