Tesis
Técnicas de difração de elétrons e de caracterizações elétricas e térmicas combinadas para análise de transformações de simetria em estruturas perovskitas distorcidas : caso das soluções sólidas (Pb1-xCax)TiO3
Fecha
2017-03-03Registro en:
Autor
Estrada, Flávia Regina
Institución
Resumen
The main objective of this work was the unequivocally determination of the crystal symmetry
of distorted perovskite structure by direct and indirect methods, with the solid solution system
Pb1-xCaxTiO3 (PCT) as the case study. Therefore, ferroelectric ceramic of PCT were
investigated using electron diffraction techniques, electric characterization and thermal
expansion measurements at compounds with controversial results in the literature about its
crystallographic symmetries and polar states, mainly in the Morphotropic Phase Boundary
(MPB). Qualitative analysis of synchrotron X-ray diffraction pattern from PCT powders
confirms that the octahedral tilting distortions exist from the calcium concentration x = 0.40.
From this result, based on subgroup relations analysis and considering only the point group
2mm as a maximal subgroup (as a suit of the system), the symmetries Pna21 e Pcm21 were
predetermined as common subgroups of the known space groups of the ending phases (P4mm,
Pbnm, I4/mcm e Pm-3m). In turn, from the thermal expansion measurements and electric
characterization, using thermal dependence of the dielectric permittivity and the polarization
vs. electric field curves, some possibilities of crystallographic phase transition were determined,
as well as the evaluation of the polar characteristics. To determine the tilt system and the crystal
symmetry at room temperature, the electron diffraction techniques were used. The composition
choice was 47.5 mol % of Ca, which represent a challenge in PCT MPB. The tilt system
determined is the same of CaTiO3, the mix of antiphase and in phase tilt a-a-c+. Finally,
combining results of the electron diffraction techniques set with the indirect techniques set, the
orthorhombic symmetry with the point group 2mm and the space group Pna21 was found for
the solid solution Pb0,525Ca0,475TiO3. This symmetry might cover other compositions in MPB at
room temperatures. The results from the set of techniques protocol allowed concluding this
symmetry and polar states from MPB phases as a function of temperature, adding new
contribution to elucidate the PCT phase diagram.