Pyrochlore phase and microstructure development in lead magnesium niobate materials

Microstructure characteristics of powder and sintered samples of lead magnesium niobate (PMN) have been compared and related to the presence or not of pyrochlore phase. Two kinds of samples obtained from two different batches of powders, batch A and B, containing 1 and 0 vol.% of pyrochlore phase respectively, were analysed. The two powders show the same morphology. The sintered samples, with pyrochlore content around 13 vol.% for samples from batch A and 0 vol.% for samples from batch B, exhibit totally different microstructures The grain size increases drastically from about 1 μm for samples with pyrochlore to 4–5 μm for pyrochlore free samples although starting from powders with same granulometry. The maximum dielectric constant of samples as fired, ground and annealed were measured and related to microstructures and amount of pyrochlore. On the basis of these results and data reported in literature, mostly relating dielectric properties to stoichiometry of the starting powder, it has been hypothesised that different sintering mechanisms come into play and govern the microstructure. Consequently the dielectric properties obtained were correlated to the sintering mechanism occurred rather than to the absolute value of pyrochlore phase present, as itʹs commonly accepted.