Effect of sintering conditions on the pyrochlore phase content in PMN–PFN ceramics prepared by sol–gel process

Lead magnesium niobate–lead iron niobate [(1 − x)Pb(Mg1/3Nb2/3)O3–xPb(Fe1/2Nb1/2)O3, (1 − x)PMN–xPFN] system, where x = 0.0–1.0, was prepared using sol–gel synthesis by mixing acetates Pb, Mg and Fe with Nb-ethylene glycol-tartarate complex at 80 °C. Single pyrochlore phase (Pb1.83Mg0.29Nb1.71O6.39 or Pb3Nb4O13) was formed by the calcination of gels at 600 °C. Average particle sizes of pure PMN and PFN powders were ∼80 and 150 nm. The pyrochlore phases were partially decomposed to perovskite phase at sintering temperatures of 1150 and 1250 °C. The maximum bulk density in ceramic samples increased with both sintering temperature and the content of PFN phase in samples but despite these facts—the volume fractions of porosity were too high in all prepared samples. From the point of view of perovskite phase content, the optimum sintering temperature of PMN–PFN is shifted from 1200 to 1150 °C in comparison with the sample with PMN stoichiometry. Low values of ɛr were measured in low density samples with high pyrochlore content and this characteristic is probably the main factor affecting ɛr in prepared ceramics. In microstructures of PMN–PFN ceramics sintered at 1150 °C at different times, the bimodal grain size distribution was observed with small grains of globular shape and larger grains of regular or octahedral shape. The results of EDX and XRD analysis showed that the more complex types of pyrochlore phases were present in ceramics.