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Contribution à l'étude structurale et spectroscopique de céramiques pérovskites PLZT [...] et PZTN [...] et de leurs transitions de phases

Abstract : The composition study for the PZT system shows that the substitution of the Zr ions by the Nb (or Ti) ions decreases the ratio Zr/Ti below critical value, as consequence a phase transition occurs at room temperature. The Raman and Ir spectra analysis shows the low frequency side is of a great interest. Indeed, there appears the phase transitions precursor phenomena as the modes softening at low temperatures and the occurrence of a relaxationnal component at higher temperatures. Many authors reported the order-disorder character of the phase transition to the paraelectric phase. In chapter 3 we have evidenced and characterised a phase transition of order-disorder type below room temperature. For the partially disordered phase, in the dynamic behaviour contribute a displacive and order-disorder mechanisms. The fluctuations state in the critical regime of the interactions at the vicinity of transition points is strongly influenced by the material microhomogeneity. We have observed a deviation from the classical behaviour in the mean field theory to the tridimentional Ising model as function of the La content values. The confrontation of the analysis and interpretations of the Raman spectra lead to informations about : i) identification and assignment of the vibrational modes, ii) correlation between the phase transitions and the distribution of the different ions, as function of the nature and concentration of the dopands. A new information concerning the cell doubling in the phase transition FeBT-FeHT, the authenticity of which has been controversed. The study on the PLZT system allows the identify the called "antiferroelectric mode" and "ferroelectric mode" which are predicted by the Cochran theory and the Samara analysis. In chapter 5 we report the correlation between the dielectric constant behaviour and the polarisation fluctuations. Above the Curie point, the Raman data are corroborated with the depolarised ferroelectric domains and their screening by space and electrons charges processes. The relaxor behaviour is also related to resultant concentration polarisation fluctuations
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Imad El Harrad. Contribution à l'étude structurale et spectroscopique de céramiques pérovskites PLZT [...] et PZTN [...] et de leurs transitions de phases. Autre [cond-mat.other]. Université Paul Verlaine - Metz, 1994. Français. ⟨NNT : 1994METZ019S⟩. ⟨tel-01776045⟩



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