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Contribution expérimentale et théorique à l'étude des propriétés structurales, dynamiques et photoconductrices dans KTaO3 dopé au Li

Abstract : The thesis is a study of the structural, dynamic and photoconductive properties of Li doped KTaO3 system (often called KTL) which belongs to the perovskites ABO3 family, highly interesting for their applications in non-linear optics and in photorefractivity. The Li introduction role in the occurence of a short and even a long range dipolar order and in the structural (phase transition, nature, mechanism...) and dynamic (energy, damping, lattice vibration dispersion) properties is still debated in spite of the many studies on this subject. A contribution to the understanding of the questions still open in the KTL system is given by Raman spectroscopy and thermal inelastic neutron diffusion measurements. A lattice dynamic theory is developed and applied in order to interpret the very intense second-order Raman spectrum on one hand and to specify the Li role in the non-linear character of the system on the other hand. The increase of the soft mode frequency and the decrease of the oxygen polarisability non-linearity with Li concentration augmentation show that the Li ions reduce the non-linearities. This phenomenon is interpreted by the occurence, at a microscopic scale, of polar clusters induced by the Li. These clusters are placed on the Ta-O chains and restrict the polarisability along these chains. The first-order Raman lines activation, in principle not allowed by symmetry, is interpreted with a theory taking into account the relaxational polar fluctuations induced by the offcentered Li ions and their interaction with the crystalline lattice. The existence of a polar phase with microregions of spatial extension depending on Li concentration and on temperature is evidenced. For high Li concentrtions, the results are characteristic of a cubic tetragonal phase transition with long-range order. Photoconductivity measurements in KTL, with light energy lower than the bang gap, show the existence of photocurrents higher than 0.1 uA for temperatures lower than 80K. Thecureents intensities are of the same order as in conventional semi-conductors and are strongly influenced by the Li concentration, the temperature and the sample treatment. Experimental results are satisfactorily interpreted by a theorical model. The proposed electronic structure scheme assumes the existence of electronic levels due to the oxygen displacement induced by the off-centered Li ions and to uncontrolled iron impurities
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Régine Klein Saint-André. Contribution expérimentale et théorique à l'étude des propriétés structurales, dynamiques et photoconductrices dans KTaO3 dopé au Li. Matière Condensée [cond-mat]. Université Paul Verlaine - Metz, 1994. Français. ⟨NNT : 1994METZ037S⟩. ⟨tel-01776729⟩



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