Estudos de monocristais de trifluoretos de ítrio e de lantanídeos e tetrafluoretos de sódio e lantanídeos por espectroscopias Raman e infravermelho

Abstract

Polarized Raman and Infrared spectra have been obtained for lanthanide trifluoride crystals YF3, LnF3 (Ln = Tb, Dy, Er, Yb and Lu) and also for the sodium lanthanide tetrafluoride crystals NaLnF4 (Ln = La, Ce, Pr, Sm, Eu and Gd). The trifluoride crystals exhibit an orthorhombic symmetry belonging to the Pnma (D16 2h) space group, known as ¯¡YF3. The Raman spectra of DyF3 and LuF3 and infrared-reflectivity spectra of ¯¡YF3 crystals have been obtained for the first time. The observed modes have been assigned in accordance with the irreducible representations of the point group of the structure. A study of the modes behavior in the Raman spectra of the different trifluoride was made. By analysing the spectral evolutions with the Ln mass and volume, we have shown that some modes are more influenced by the motion of the Ln+3 ions, others by the F¡1 ones. The intensity of some modes has shown to be quite dependent on the orthorhombic elastic distortion parameter. The frequency splitting of TO-LO modes have been used in the ionic effective charges calculations. The results were used to discuss the potentiality of crystals with structure ¯¡YF3 as laser matrices. There is a controversy in the literature concerning the tetrafluoride structure. Based on the analysis of the Raman and infrared measurements, we found the most likely point group, PG, for these crystals. This PG confirms one of the structures reported in the literature. The factor group analysis of Rousseau was employed to predict the number and assignment of the vibrational modes for this structure. The few divergences between this structure and the experimental results is explained.