EFFECT OF EXCESS OF IRON-OXIDE AND LEAD-OXIDE ON THE MICROSTRUCTURE AND DIELECTRIC-PROPERTIES OF LEAD IRON TUNGSTATE CERAMICS

Lead-iron tungstate (Pb(Fe2/3W1/3)O3; PFW) perovskite ceramics were prepared by the conventional mixed oxides method. Additional amounts of Fe2O3 and PbO were used to examine the role of excess oxides on the phase development, densification behaviour and dielectric properties. The stoichiometric mixture of the oxides results in a residual phase (Pb2WO5) besides the perovskite. Excess Fe2O3 eliminates the Pb2WO5, with a marked increase in the dielectric constant, whereas excess PbO leads to an unexpected increase of the amount of the second phase and a consequent decrease of the dielectric constant. The densification behaviour and the microstructures obtained after firing were very dependent on the starting stoichiometry. The PFW phase obtained using PbO or Fe2O3 excesses shows various degrees of ordering which seem to be correlated with the presence of either W6+ vacancies or of Pb2+ vacancies.