ROLE OF EXCESS PBO ON THE MICROSTRUCTURE AND DIELECTRIC-PROPERTIES OF LEAD MAGNESIUM NIOBATE

Stoichiometric and 2 wt. % excess lead oxide containing lead magnesium niobate (PMN) ceramics have been prepared by partial oxalate route. Dielectric measurements with frequency showed a typical relaxer behavior for stoichiometric PMN, while PMN with excess PbO shows a scatter in the dielectric curves at all temperatures above T-c. Under the same processing conditions, the dielectric constant (K-max) decreases drastically from 16,300 to 9500 at 1 KHz for PMN without and with excess PbO, respectively. Microstructure studies revealed a second phase (unreacted PbO) segregated in the grain boundaries for excess PbO samples. A careful analysis of the data on dielectric properties and phases present coupled with microstructural detail indicate that the second phase in the grain boundary has a pronounced effect on the dielectric properties and not the pyrochlore phase.