Pirogermanatos de lantanídeos: obtenção e investigação das propriedades estruturais, vibracionais, magnéticas e luminescentes

Abstract

Lanthanide pyrogermanates (Ln2Ge2O7 – Ln = lanthanides except for Ce and Pm) were prepared via synthesis in molten salt (NaCl as flow) at 900ºC for 4h. The ceramics produced were characterized from the structural, chemical, morphological, optical-vibrational, luminescent, magnetic and thermal point of view. The materials obtained presented high purity and crystallinity, belonging to two structural phases: triclinic (P1, #1) for higher ionic radius lanthanides (La-Eu), and tetragonal (P41212, #92) for lower ionic radius lanthanides (Gd-Lu). Raman spectra were obtained for both crystalline phases. In the case of samples with triclinic structure, it was possible to observe all 63 phonons predicted by group theory; for tetragonal ceramics, 53 modes of a total of 81 predicted were determined. The complete and first-time characterization of optical properties through DRS was performed, in addition to obtaining the band gap values for the entire series. This study assisted in the luminescent investigation of these compounds that presented relevant emissions in both visible and IV. Pyrogermanates were produced by the solid-state synthesis method, at temperatures between 900 and 1300 ºC for 4h, for comparison between the synthesis routes and investigation of polymorphism at high temperatures in some known chemical compositions. Only Gd2Ge2O7 presented polymorphism with increasing temperature polymorphic transition was also observed by Raman spectroscopy and DRS, presenting great coherence with the characterization by XRD. The polymorphism of Gd2Ge2O7 was also investigated using Eu3+ as a structural probe. Studies were conducted on solid solutions of Eu2-xGdxGe2O7 (x=0-2) at 1100 ºC in 4h. The purpose of this study was to understand the changes in structural, vibrational and optical properties due to the exchange of Ln3+. Polymorphic transformation was also observed in this set in an unprecedented way by Raman, DRS and luminescence spectroscopy, both in accordance with previous studies. Finally, the last study was luminescence measurements in different pyrogermanates doped with other Ln3+ ions. It was possible to observe the wide range of options of luminescent materials by both downshifting and up conversion energy process, which allows the proposed use of these materials as emitting devices and optical thermometers.