Effect of Pb excess in sol–gel process on phase composition in PFN ceramics

Lead iron niobate Pb(Fe0.5Nb0.5)O3 (PFN) precursors were prepared using sol–gel synthesis by mixing acetates Pb and Fe with Nb-ethylene glycol–tartarate (Pechini) complex at 80 °C and calcination of gels at 600 °C. Single pyrochlore phase with structure close to Pb3Nb4O13 was formed in stoichiometric precursor and Pb3Nb4O13 with small amount of perovskite phase Pb(Fe0.5Nb0.5)O3 in nonstoichiometric precursor prepared with the excess of Pb in molar ratio (Pb:Fe:Nb = 1.2:0.5:0.5). Average particle sizes of PFN calcined powders were ∼120 nm. The metastable pyrochlore phase was partially decomposed to perovskite phase at sintering temperature of 1150 °C for 2, 4 and 6 h. Excess of Pb caused increasing of the density (7.4 g/cm3) and content of the perovskite phase (∼53 vol.%) in ceramics sintered for 4 h. In microstructures of PFN ceramics sintered at 1150 °C for different times, the bimodal grain size distribution was observed with small spherical grains of perovskite phase and larger octahedral grains, which represent the pyrochlore phase. Results of EDX analysis confirm that complex types of pyrochlore phases that differ in iron content were present in ceramics.