Análise vibracional e de transições de fase em dielétricos ferróicos e de baixa simetria

Abstract

Under the scope of this thesis, it was proposed the investigation of the vibrational properties, through Raman and infrared (IR) spectroscopies, and of the phase transitions in three crystal systems, namely: single crystal fibers of CaTa2O6 (CTO), single crystals of Li3ThF7 (LTF) and montebrasite, LiAl(PO4)(OH). The investigation of such dielectric systems, with very distinct physical properties, led to the maximum exploitation of the adopted experimental techniques to study them. Particularly, this unusual exploitation of the experimental procedures led to the understanding of structural phases, which are poorly addressed in the literature, as in case of CTO and LTF, and of fundamental problems of infrared Dispersion Analysis in a lowsymmetry crystal as in montebrasite. The former of them exhibits polymorphic phase transitions and, according to synthesis conditions, it can show three roomtemperature phases: a disordered cubic perovskite phase (Pm3m space group), a cubic phase (space group Pm3) and an orthorhombic phase (space group Pnma). The interest in CTO was based on two objectives: (i) to investigate the cubicorthorhombic transition, Pm3 Pnma, and (ii) to determine the vibrational spectrum by Raman and infrared spectroscopies. The experimental issue about CTO first goes by the difficulty in obtaining Raman and IR spectra of a very small sample (0.4 mm in diameter x 0.8 mm long) and after by the measuring in the far IR region (from 50 to 700 cm1). This was successfully attained with the aid of commercial FTIR spectrometer and microscope, but adapted at the UFMG to perform these special measurements. It can be specially highlighted that the CTO specimen studied here was the smallest sample ever studied by IR and under microscope analysis in the far IR region. Regarding LTF Raman and infrared spectroscopy techniques were used to investigate two hightemperature phase transitions experienced by the material, in order to identify the space group of the highest temperature tetragonal phase. To perform this it was necessary to compare the experimental results with group theory predictions for the number of normal modes of each phase. The main experimental difficulty in this system was to get the IR spectra at high temperature, since it is difficult to find a furnace window transparent to the IR radiation and that could stand for appreciable thermal variations. Concerning montebrasite, it is a triclinic crystal that was used as a model system to study the phononic properties in lowsymmetry crystals (monoclinic or triclinic) at various angles of polarization of light. Such study permitted, in the case of polar phonons, the identification of transition moments directions, which in highsymmetry crystals lie along the crystallographic axes. The nonpolar phonons, on the other hand, were studied in order to check the Raman and infrared selection rules and also to check possible defects. This work in montebrasite provided the background to perform future Raman and IR spectroscopic measurements of lowsymmetry crystals. Until now the experimental procedures reported in the literature failed in this attempt and left an opened field in IR Dispersion Analysis, that is fortunately fulfilled under the scope of this thesis.