The current detour effect observed on materials with random microstucture:: experimental evidence from Li3,La2/3-xTiO3 studied by impedance spectroscopy

Impedance spectroscopy (IS) has been used to study the influence on the low frequency part of the impedance diagrams of the microstructure of a fast ionic conductor, Li3xLa2/3-xTiO3 with x=0.10 (named hereafter LLTO). This oxide has been synthesised by sol-gel method. After synthesis, the powder of LLTO displays a large distribution of grain size and agglomerates. The grain size distribution and the porosity of the ceramic have been changed by heat-treatment from 600 degrees C to 1200 degrees C in air. The impedance spectra of these ceramics, recorded at different temperatures from room temperature (RT) to 400 degrees C, show a low-frequency depressed arc, which is characteristic of the grain boundary response of the ceramic. Its shape depends strongly on the heat-treatment of the ceramic, and therefore, on its microstructure. It is a simple arc when the pellet is well sintered but becomes very complex for non-sintered ceramics with high resistive grain boundary and pores. The observed "fish" shape indicates the presence of current "detours effect" in the material. This effect means that current detours around blocking grain boundary and/or pores occur to lower the impedance. Consequently, the brick layer model (BLM), which assumes an ideal microstructure, and then no current "detours effect", can not be used to analyse these impedance data. (c) 2005 Elsevier Ltd. All rights reserved.