Synthesis, structure and properties of high piezo - and Ferroelectric complex perovskite systems

The complex morphotropic phase boundary (MPB) behaviour of the relaxor ferroelectric-based (1-x)Pb(Scl12Nbl12)03 - xPbTi03 (PSN-PT) solid solution system has been investigated systematically by experiments with both ceramics and single crystals. The study of PSN-PT ceramics with compositions within the MPB region by means of dielectric spectroscopy has revealed two phase t~ansitions. A new phase with monoclinic symmetry has been found by X-ray phase analysis. A new phase diagram of PSN-PT solid solution with the MPB region and a curved upper boundary has been established. Single crystals of Pb(SclI2Nbll2)O3 (PSN) and PSN-PT with compositions near or within MPB region have successfully been grown by an improved high temperature solution method using the mixtures of PbO and B203 as flux. The effects of the chemical compositions, such as the ratios of PSN-PT vs. flux and PbO vs. B2O3, on the morphology and quality of grown crystals have been studied. The optimum chemical compositions were proposed, which have led to the growth of high quality crystals. The PSN-PT single crystals of the MPB composition show two phase transitions and complex structure symmetry, typical of the MPB behaviour. The spontaneous phase transitions from the paraelectric to a relaxor, then to a normal ferroelectric state upon cooling have been disclosed in the disordered P ~ ( S C ~ ~ ~ N ~ ~ ~ ~ ) ~ ~ single crystals, with the existence of macro domain state with a possible rhombohedra1 symmetry at room temperature. In comparison, the structural phase transitions and the dielectric properties of Pb(Zn113Nb2/3)03 (PZN) single crystals were also investigated. The electric field induced phase transition from relaxor to ferroelectric state has been revealed and discussed in terms of the kinetics of phase transition in relaxors. In our search for new materials with high piezoelectric performance, we have investigated the solid solution of (1-x)PbSn03 - xPbTi03 (PbSnTi) system. Single crystals of PbSnTi with composition x = 0.60, close to the MPB have been successfully grown for the first time from high temperature solution. The grown crystals exhibit good dielectric, piezo- and ferroelectric properties, which make them a new family of piezocrystals with relatively high Curie temperature (Tc == 206 "C), potentially promising for electromechanical transducer applications.