Fast grain‐boundary diffusion paths in ionic solids: Space‐charge layers versus interfacial core

The accelerated diffusion of ions along grain boundaries in ionic solids may occur along two possible paths: (a) the interface core (IC), as known for metals; and (b) space‐charge layers (SCLs), located adjacent to the core, in which the point defects responsible for diffusion are accumulated. In this study, we examined whether it is possible, on the basis of diffusion profiles, to discriminate between IC and SCL diffusion paths. Taking cation diffusion along grain boundaries in MO2 oxides as our example system, we used finite‐element‐method (FEM) calculations to obtain cation diffusion profiles for various combinations of fast grain‐boundary diffusion along the two possible diffusion paths. Three characteristics of the grain‐boundary feature in diffusion profiles were investigated: the quantitative description of this grain‐boundary “tail”, its dependence on diffusion time, and its dependence on temperature (activation enthalpy). For all calculated diffusion profiles, with varying ratios of IC to SCL diffusion, no criterion was found that allowed the dominant diffusion path to be identified. Consideration of the activation enthalpies, however, may provide an indication of the dominant path: If the activation energy for grain‐boundary diffusion approaches or equals the activation enthalpy for bulk diffusion, one may conclude that SCL diffusion dominates.