Polycrystalline BIMGVOX.13 studied by impedance spectroscopy
Abstract The impedance of the polycrystalline oxide ion conductor BIMGVOX (Bi 2 Mg x V 1− x O 5.5−1.5 x − δ ) for composition x  = 0.13 was measured in the frequency range from 10 MHz to 0.01 Hz at temperatures between 20 °C and 680 °C using an automated set-up which allowed measurements of high impedance. Impedance spectra were analysed by non-linear least squares fitting of equivalent circuit, which models grain interiors conductivity with charge carrier relaxation represented by the Cole–Cole function, grain boundaries resistance with parallel non-ideal capacitance, and impedance of platinum electrodes. Determination of electrical properties of the polycrystalline sample was possible up to about 300 °C. At higher temperature, electrode impedance dominated the impedance spectrum and only the total ionic conductivity of the sample could be estimated. Temperature dependence of the intragrain conductivity and charge carrier relaxation was studied as well as of the macroscopic grain boundary conductance and capacitance. Assuming that the onset frequency of increase of the intragrain conductivity is a good estimate of the effective hopping rate, the concentration of charge carriers was found to be independent of temperature. For the average grain size of 10 μm, the thickness of grain boundaries, estimated based on their capacitance, was about 8 nm.