Structural studies on ferroelectric and ferrodistortive materials

The structure of the piezoelectric material 0.68PbMg_1/3Nb _2/3O₃-0.32PbTiO₃ have been studied by single crystal, powder x-ray diffraction techniques over the temperature range from 25°C to 200°C. The existence of twinned structures or coexistence of rhombohedral and tetragonal phases has been shown by the peak distortion of Bragg reflections. Superlattice structure was observed for all experimental PMN-PT crystals. Refinement results showed that the 2 x 2 x 2 superlattice resulted from anti-parallel displacement of oxygen in the adjacent conventional perovskite unit cells. No cation displacement in the paraelectric phase and little in the ferroelectric phase were shown by the refinement results. This unique feature associated with the ferroelectric mechanism of the material was explained by comparison with PbMg_1/3Nb_2/3O₃.; The crystals were extensively characterized by using powder x-ray diffraction, Laue back-reflection and electron backscatter diffraction (EBSD) techniques. The detailed orientation information such as misorientation of grains, location of grain boundaries and the orientation distribution was obtained from the automatic orientation mapping with the EBSD technique. The uniform orientation was confirmed for crystals with a “cellular-like” structure. A crystal growth model, the two-dimensional layer mechanism, was proposed by orientation analysis. Based on the model, some important comments were made on orientation problems under general growth conditions.; The ferrodistortive phase transitions of tertramethylphosphonium tetrabromozincate [P(CH₃)₄]₂ZnBr₄ and tertramethylphosphonium tetraiodonzincate [P(CH₃)₄]₂ZnI₄ were thoroughly studied by a single crystal x-ray diffraction technique. An order parameter analysis by application of Landau theory showed that the two compounds undergo first-order phase transitions near a tricritical Lifshitz point. Transitions for both compounds appear to be first order, but with the iodo salt the transition is nearly second order. The driving force for the transitions was found to be the uniaxial anion displacement with respect to the mirror plane in Pmcn phase, coupled with the rotation of the cation and anions. The abnormal thermal behaviors, such as thermal contraction of a-axis, non-linear behavior of thermal parameters versus temperature in both compounds and superheating of [P(CH₃)₄]₂ZnBr₄ , were explained by observing the Lifshitz point. The results from theoretical analysis of the free energy are consistent with all features of the phase transitions.