Recent Advances in Processing, Structural and Dielectric Properties of PMN-PT Ferroelectric Ceramics at Compositions Around the MPB

Abstract

Ferroelectric (1-x)[Pb(Mg1/3Nb2/3)O3]-xPbTiO3 (PMN-PT) solid solutions are known for their exceptional electromechanical properties, sometimes one order of magnitude larger than classical PbZr1-xTixO3 (PZT) ceramics. Prepared with a suitable x composition, PMN-PT is technologically important for fabricating some of the most important solid state devices such as a piezoelectric transducer, actuator, FERAM, etc. PMN-PT ceramics, thin films or single crystal forms can be prepared with high piezoelectric coefficients, a high dielectric constant and a low dielectric loss. Some compositions of PMN-PT single crystals exhibit a very high piezoelectric coefficient (d33) and electromechanical coupling coefficients (k33) (d33 ~ 1240 pC/N and k33 ~ 0.923), a high dielectric constant (e ~ 3100) with low dielectric loss (tanδ ~ 0.014) compared to those of polycrystalline ceramics (d33 ~ 690 pC/N and k33 ~ 0.73) (Park & Shrout, 1997; Viehland et al., 2001). Recently, researchers have also reported that PMN-PT single crystals have high remanent polarization (Pr ~ 35 μC/cm2) at a low coercive field (Ec ~ 3.4 kV/cm), a high dielectric constant (e ~ 2500), low loss tangent (tanδ ~ 0.031), the highest piezoelectric coefficient (d33 ~ 1500 pC/N) and a high electrochemical coupling coefficient (k33 ~ 0.82) for grain-oriented PMN–PT ceramics (Sun et al., 2004). The piezoelectric coefficient dij determines the stress levels induced by a given electric field and thus is the parameter most frequently used to describe the performance of an actuator. PMN-PT solid solutions present a perovskite ABO3 structure, where the A site is occupied by the Pb2+ ion, while the B site is randomly occupied by Mg2+, Nb5+ and Ti4+ ions. Different compositions of the PMN-PT present distinct physical properties. The complex perovskite Pb(Mg1/3Nb2/3)O3 (x = 0) is a typical relaxor ferroelectric, characterized by a diffuse maximum of the dielectric constant associated with considerable frequency dispersion, that exhibits a non-polar paraelectric phase at high temperatures, similar in many aspects to normal ferroelectrics (Bokov & Ye, 2006). After cooling, a transformation occurs from the paraelectric phase to the ergodic relaxor state, characterized by the presence of polar nanoregions randomly distributed by the specimen, at the Burns temperature (TB). This transformation is not accompanied by changes in the crystal structure on the macroscopic or mesoscopic scale and therefore cannot be considered a structural phase transition. In general, the state of a relaxor crystal at T < TB is frequently considered a new phase different from the paraelectric phase, since the polar nanoregions substantially affect the behavior of