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Descripción
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| The work described in this thesis explores the synthesis and characterization of a number of new double scheelites and double perovskites. The trivalent cerium silver tungstate, AgCe(WO4)2, with scheelite-like structure has been synthesized by solid-state method despite the relatively high instability of the trivalent Ce ion. This behavior could be explained by the redox properties of Ag2O in the solid state reaction at high temperature in air, which stabilizes the trivalent valence state. X-ray diffraction analysis was carried out and the refinement was performed in the tetragonal S.G. I 41/a (No. 88). Initially differential thermal analysis (DTA) and thermal gravimetric (TG) were used to analyze the reaction process and chemical mechanism. The results of magnetic susceptibility measurement gave confirmed evidence that the trivalent cerium cation existed in the compounds Ce(1-x)Ag(1+x)(WO4)2 (x from 0 to 1/3), and no transition from trivalent Ce3+ cation to tetravalent Ce4+ cation was observed with the decreasing of cerium concentration. The fluorescence properties of Eu3+ and Tb3+ doped AgCe(WO4)2 were investigated in order to determine the crystal field energy level diagrams. The results of FTIR and Raman showed that the compounds EuxCe(1-x)Ag(WO4)2 (x from 0.00 to 0.50) have tetrahedral symmetry with C_4h^6 symmetry group, there is no appreciable distortion of the tetragonal structure as the increasing of Eu3+ concentration because of the analogous of the radius of Ce3+ ion and Eu3+ ion. It is also found that the Tb3+ is qualified as a local structure probe rather than the Eu3+ within the AgCe(WO4)2 compound because the characteristic bands of Eu3+ were quenched with the appearance of cerium ions. Polycrystalline powders of the silver rare earth tungstate-molybdate family, AgLn(WO4)(MoO4), with scheelite-like structure have been synthesized by solid-state method. X-ray diffraction analysis was carried out and the refinement was performed in the tetragonal S.G. I 41/a (No. 88). The FTIR and Raman Spectra were discussed on the basis of factor group analysis and normal coordinate calculations. Photo luminescent excitation and emission spectra of the rare earth ions was presented with characteristic bands in the silver based host material especially for Ln= Ce, Pr and Tb despite the relatively high instability of these trivalent ions. The so-called double perovskites A2B¿BX6, in which the octahedral sites are occupied by B and B¿ cations either in a random or an ordered fashion, have been widely studied concerning the structure and properties that they present. In this present work, Sr2RESbO6 double perovskites (RE = La to Lu and Y) were synthesized by ceramic method. The structure and phase purity of the prepared double perovskites were examined by X-ray diffraction pattern and vibrational spectroscopy. A systematic analysis of the compounds structure was carried out by Raman and IR. A simple inspection of the diffraction patterns shows that these compounds have lower symmetry than the cubic which can be usually found in the Ba2RESbO6 double perovskites. The four active modes (A1g, Eg, and two T2g) in the Raman spectra and the active mode (T1u) in the IR spectra which previously described in the spectroscopic data of Ba compounds have changed. According to our data, the Sr2RESbO6 double perovskites can be described by a monoclinic symmetry cell, space group P21/n with the lattice parameters , and and ? ? 90º, being the lattice constant of the cubic perovskite. However, in the cases of Sr2LaSbO6 and Sr2PrSbO6 an alternative structure should be searched by neutron diffraction technique. Furthermore, the lattice parameters decrease linearly with the decreasing of the size of the R3+ cation according with the well-known lanthanide contraction. Magnetic susceptibility measurements for this family of compounds reveal a paramagnetic behaviour down to temperatures of 2 K. The downwards or upwards deviation behaviour observed a | |
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Internacional
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Tipo de Tesis
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Doctoral |
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Calificación
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Apto |
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Fecha
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07/10/2016 |