Etude des fluctuations polaires dans KTa(1-x)NbxO3 (KTN) : approche théorique des résultats de spectroscopies neutronique et Raman

Abstract : This work is devoted to the interpretation of experimental results by theoretical models in the doped system KTa(1-x)NbxO3 (KTN). Potassium tantalate is a well known compound which remains cubic down to 0K. At the opposite of phase diagram, pure KNbO3 shows a sequence of three ferroelectric phase transitions. Solid solutions of KTN are then a prototype to study the occurence of phase transitions versus temperature and concentration. Previous results available in literature are reminded in the first chapter. In the second one, we recall the fundamental aspects for the theoretical study of phase transitions. Chapter3 and 4 are devoted to the interpretation of neutron and Raman scattering results. Dispersion curves are obtained for KTN with concentrations 0.8% and 1.2%. To describe these curves, we have applied dynamical calculations based on the anisotropic shell model. In this model, the anomalous temperature dependance of the TA phonon becomes from an exchange of eigenvectors between the two lowest transverse phonons. A possible explanation of the characteristic dispersion of the soft mode close to zone centre is also given (the "drop like" behaviour). A theoretical model is developed to explain the temperature dependance of both Raman spectra and integrated intensities at temperatures above and below phase transition (precursor order and first order Raman scattering). We especially point the approximations of this model. Finally, the correlation length of the precursor order is calculated in paraelectric phase. In the last chapter we summarise all results and we discuss the general mechanism of phase transitions in KTN
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Laurent Foussadier. Etude des fluctuations polaires dans KTa(1-x)NbxO3 (KTN) : approche théorique des résultats de spectroscopies neutronique et Raman. Autre [cond-mat.other]. Université Paul Verlaine - Metz, 1994. Français. ⟨NNT : 1994METZ043S⟩. ⟨tel-01776745⟩

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