Effects of grain size on the dielectric properties of Pb(Mg1/3Nb2/3)O-3-30 mol % PbTiO3 ceramics

The effects of the average grain size on the phase transformation and consequent changes in the dielectric properties of Pb(Mg1/3Nb2/3)O-3-30 mol % PbTiO3 were investigated. It was observed that the size refinement down to similar to 90 nm impeded the phase transformation of the cubic phase formed in the course of sintering either into rhombohedral or into tetragonal one, and a series of phase transformation from cubic to rhombohedral and then to tetragonal phase took place as the average grain size increased from similar to 90 nm to similar to 5.7 mu m. This dependence of the crystal structure on the grain size was qualitatively explained in terms of the capillarity effect and the free volume at the grain boundaries exerting a negative hydrostatic pressure on the grains. With the change in crystal structure the typical ferroelectric behavior in dielectric properties evolved into that of a relaxor and finally into a paraelectric one. The overall changes in the dielectric behavior were interpreted on the basis of the size-driven phase transformation. An anomalous dielectric behavior, i.e., a relatively high dielectric constant over a wide range of temperature (30-330 degrees C), was observed at the average grain size of similar to 330 nm possibly due to a state of a single domain in a single grain, while below similar to 90 nm all the phases in the system practically transformed into the paraelectric cubic phase