Impedance spectroscopy and defect chemistry of fayalite

Abstract

Impedance spectra of polycrystalline synthetic fayalite were measured at 900°C and 1 atm pressure in a CO/CO₂ gas mixing furnace at oxygen fugacities f _{O 2}=10^-13 to 10^-17 atm. The frequency range applied to the samples was 20 Hz–1 MHz. The spectra show two semicircular arcs, which are attributed to polarization processes within the bulk material and at the sample-electrode interface. Two different types of behaviour of the bulk resistance as a function of oxygen fugacity were observed. The first is proportional to the -1/4.5 to -1/6 power of f_{I 2}, which is consistent with the existing defect model. The second shows a lesser dependence on f_{O 2}, often lowered by a factor of about two, relative to the expected value, resulting in a resistance proportional to the -1/10 to -1/12 power of f_{O 2}. It is assumed, that the lesser f _{O 2}-dependence is caused by aliovalent impurities, producing an f_{O 2}-independent amount of charge carriers. The electrode arcs show a resistance proportional to the-1/2 to -1/4 power of oxygen fugacity, implying that a reaction with the gas phase is involved in the polarization process at the sample-electrode interface.