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Pb(Yb½Nb½)O₃-PbTiO₃ : a model solid solution for the study of the different polar orders

Abstract : Piezoelectricity has attracted a considerable amount of work due to the variety of possible applications such as accelerometers, gyroscopes or transducers. Pb(Yb½Nb½)O₃-PbTiO₃ is long known to have large piezoelectric properties (d33=510pC/N) and a relatively high Curie temperature (_370°C at the MPB).This solid solution between a double (B-site ordered) perovskite (Pb2YbNbO6) and lead titanate is remarkable, in the sense that it presents all polar orders. Indeed, depending on the composition, the solid solution can exhibit at room temperature: antiferroelectric, classical ferroelectric or relaxor properties. In this work, I will present results obtained on PYN-PT ceramics of various compositions spanning the phase diagram. From experiments using X-ray diffraction and neutron scattering, dielectric spectroscopy, as well as piezoelectric and ferroelectric measurements as a function of temperature, a new phase diagram is proposed. This thorough study of the phase diagram of PYN-PT has revealed a more complex phase diagram than what was reported in the literature. In particular, the structure of the antiferroelectric PYN is peculiar compared to the other Pb-based antiferroelectric. Indeed, in PYN, the Pb atoms are displaced along the direction of the modulation vector and that the B cations are not in their prototypic positions and take part in the antipolar order observed.Moreover, the morphtropic phase boundary (x _0.50) extends over a larger compositional range than what was observed in other Pb-based solid solution. In the PT-poor side of the morphtropic phase boundary, the absence of the expected rhombohedral phase (by analogy with the other Pb-based solid solutions) was demonstrated. Furthermore, among the compositions presenting relaxor properties, differences have arisen and I suggest that several types of relaxors exist, in the PYN-PT solid solution. Finally, I will propose a model explaining continuously the change of polar order through out the phase diagram. This model is based on the correlation of Pb displacements induced by the chemical nature and the cationic arrangement on the B site.
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Charlotte Cochard. Pb(Yb½Nb½)O₃-PbTiO₃ : a model solid solution for the study of the different polar orders. Other. Ecole Centrale Paris, 2015. English. ⟨NNT : 2015ECAP0027⟩. ⟨tel-01187385⟩

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