Impedance spectroscopy of PZT ceramics - measuring diffusion coefficients, mixed conduction and Pb loss

Sintering of PZT at high temperatures results in loss of Pb unless an ambient Pb activity is maintained. The tell-tale sign of Pb loss is an increased conductivity, usually manifested in unacceptably high values of tan δ. The conductivity is caused by oxygen vacancies and/or electron holes which are a by-product of Pb evaporation. In the first part of this paper it is shown how impedance spectroscopy can be used to separate ionic and electronic conductivity in a properly designed sample by selection of appropriate boundary conditions. Subsequently, impedance is used to probe defect concentrations in PZT during prolonged annealing at 700°C. It is found that oxygen vacancies are generated during annealing in air but the rate of generation actually decreases on lowering the ambient pO₂. These results are explained by a model of Pb evaporation which, in this case, leads predominantly to oxygen vacancy generation. In principle this effect could be used to generate a specific vacancy concentration in similar Pb based oxides.