Effect of Powder Purity and Second Phases on the Dielectric Properties of Lead Magnesium Niobate Ceramics

The dielectric properties of lead magnesium niobate ceramics have been studied as a function of powder purity and microstructure. Starting powder purity was shown to have d pronounced effect on the resultant dielectric properties. Values of K_max as high as 20 000 were obtained using starting powders of exceptionally high purity (99.999%), despite the formation of a pyrochlore phase. The pyrochlore phase formed as large isolated grains in the microstructure. It was found to be a cubic-type pyrochlore with a composition of Pb₂Nb_1.75Mg_0.25O_6.62. Much lower dielectric constants were obtained with samples of lower reagent-grade purity. This was partially explained by the presence of an intergranular second phase. Other factors such as lattice impurities and grain-boundary chemistry were also argued to be important.