THE ROLE OF EXCESS MAGNESIUM-OXIDE OR LEAD-OXIDE IN DETERMINING THE MICROSTRUCTURE AND PROPERTIES OF LEAD MAGNESIUM NIOBATE

Near‐phase pure perovskite lead magnesium niobate (PMN) with MgO or PbO additives was produced by reacting PbO with MgNb2O6 at 800°C and sintering at 1200°C. Dense ceramics were characterized by scanning electron microscopy, X‐ray diffraction, and dielectric measurements. The microstructural studies showed that excess MgO exists as micrometer spherical particles either in the grain boundary as a discrete particle or in the perovskite grain as an inclusion. The pyrochlore phase exists in large isolated grains in the microstructure. The 10 mol% MgO excess composition had a peak dielectric constant of 19 500 at 100 Hz, which suggests very “clean” or uninhibiting grain boundaries. The excess addition of PbO did not improve the yield of perovskite PMN phase and decreased the dielectric constant. PMN grain boundaries are the dominant path of fracture. This paper, to a certain degree, explores the chemistry and characteristics of these grain boundaries. Copyright © 1990, Wiley Blackwell. All rights reserved