Understanding the Impact of Microstructure on Charge Transport in Polycrystalline Materials Through Impedance Modelling

Charge transport in polycrystalline electronic or ionic conductors is usually analyzed by serial macroscopic equivalent circuits, e.g., the brick layer model, which assume a homogeneous electric potential distribution across the sample. In such analyses, the microstructure is highly idealized and usually not representative of the actual microstructure. Here, we use a network model approach to investigate the impact of the sample's microstructure on the impedance. We find that this influence can be severe and should not be ignored. The interplay between microscopic transport paths affects the impedance response which is reflected in both the frequency and the time domain. Especially in the distribution of relaxation times additional signals are identified and studied systematically. These additional contributions cannot be assigned to a microscopic transport process as usually done in a conventional analysis based on an equivalent circuit model fitted to the impedance data. The neglect of the peculiarities of the real microstructure in impedance analyses based on the brick layer model may yield deviations in the order of 100 % in terms of the derived microscopic transport parameters. The microstructures used as input for the modelling are digitalized electron microscope images of polycrystalline samples.