Silicatos e Germanatos de terras raras: propriedades estruturais, ópticas, elétricas e vibracionais dos sistemas Sr2RE8(SiO4)6O2, (Sc,In)2Ge2O7 e Sc2-xLaxGe2O7

Abstract

The presence of rare earth elements in the most diverse hosts has increased scientific and economic interest, due to the great potential of the properties of these materials in the face of the growing demand for high technology, which can be applied as magnetic and luminescent devices, and as fuel cell electrolyte materials, for example. Therefore, in this work, the structural, optical, electrical and vibrational properties of Sr2RE8(SiO4)6O2, (Sc,In)2Ge2O7 and Sc2-xLaxGe2O7 systems were investigated. Rare earth silicates and germanates were obtained by conventional solid-state reaction route and investigated by several techniques to better understand the behavior of rare earth elements in these hosts. For the apatites Sr2RE8(SiO4)6O2(RE = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb, Y), Raman and far-infrared reflectivity spectroscopies were used to determine the characteristic vibrational modes and the intrinsic dielectric properties, which could be compared to extrinsic values obtained by direct measurements. Furthermore, the optical properties of apatites were studied by diffuse reflectance and photoluminescence emission and excitation spectroscopies. These techniques proved to be complementary on the identification of intraconfigurational transitions for all ceramics under study. Additionally, diffuse reflectance spectroscopy proved to be a more complete and accessible technique for studying optical properties and determining the positions of charge transfer bands and band gap energies. For indium and scandium germanates, chemical and structural characterization was carried out by X-ray diffraction and transmission electron microscopy. Both materials had their vibrational properties investigated under ambient conditions by Raman and infrared spectroscopy and were subjected to high pressures so that a group-subgroup phase transition was observed. Finally, for the Sc2-xLaxGe2O7 system (0.00 ≤ x ≤ 2.00), the experimental X-ray diffraction data were analyzed by Rietveld refinement to determine each of the phases present in the system and identify the intervals where solid solutions are formed. The evolution of the system was also investigated by Raman and mid infrared spectroscopies, which helped to understand the phase transformation due to variation on the composition. Finally, diffuse reflectance spectroscopy measurements were performed so that band gap energy values could be obtained and discussed in terms of the system composition. All the results obtained fill existing gaps in the literature or add non-existent knowledge about the properties of rare earth silicates and germanates, expanding the possibilities for future applications.