Phase analysis and dielectric properties of ceramics in PbO-MgO-ZnO-Nb2O5 system: A comparative study of materials obtained by ceramic and molten salt synthesis routes

Compositions of the type 3PbO-MgO/ZnO-Nb2O5 were synthesized by the ceramic route at 1000 degrees C and sintered at 1200 degrees C. Powder X-ray diffr action studies of the 1000 degrees C heated products show the presence of t he cubic pyrochlore and the columbite (Mg/ZnNb2O6) type phase in the ratio of 3 : 1 for all possible combinations of MgO and ZnO. Further heating at 1 200 degrees C led to a decrease in the cubic pyrochlore phase and an increa se in the columbite phase by around 10%. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthe sized using the KCl-NaCl molten salt method at 900 degrees C for 6 h gave a significant amount of the cubic perovskite related phase of the type Pb(Mg /Zn)(1/3)Nb2/3O3 for all compositions containing MgO. The amount of the per ovskite phase was nearly 55% for the Mg rich compositions and decreased wit h increase in Zn content, the pure Zn composition yielding mainly the cubic pyrochlore phase. On sintering these phases at 1000 degrees C the perovski te phase content decreased. The dielectric constant of the composite materi als formed by the ceramic route was in the region of 14 to 20 and varied li ttle with frequency. The composites obtained by the molten salt method, how ever, showed much larger dielectric constants in the region 40-150 at 500 k Hz for various compositions. The dielectric loss tangent of these composite s were lower by an order (0.005-0.03 at 500 kHz) compared to the ceramic ro ute.