Impedance spectroscopy and defect chemistry of fayalite

Impedance spectra of polycrystalline synthetic fayalite were measured at 900 degrees C and 1 atm pressure in a CO/CO2 gas mixing furnace at oxygen fugacities f(O2) = 10(-13) to 10(-17) atm. The frequency range applied to the samples was 20 Hz-l 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(O2), which is consistent with the existing defect model. The second shows a lesser dependence on f(O2), 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(O2). It is assumed, that the lesser f(O2)-dependence is caused by aliovalent impurities, producing an f(O2)-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.