Estudo de cristais de Li(NH4)1-xNaxSO4 por meio de espalhamento Raman, constante dielétrica e difração de raios-x

• Ano de defesa: 1999
• Autor(a) principal: Feio, Waldeci Paraguassu
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: Raman, Espectroscopia de; Moléculas
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/63880

This work shows the study of β - Li(NH4)1-xNaxSO4 (LASS), with x = 0.0 (LAS); 0.01 (LASS1 %) e 0.2 (LASS20%) by x-ray diffraction, polarized Raman and dielectric constant measurements in the temperature range between 500 and 80 K. The main objective was to investigate the modifications in the phase transition of LAS due to the introduction of Na+ ions in the crystal structure of β - LAS. The study shows that the β — LASS crystals undergo five phase transitions between 500 and 80 K. The first phase transition occurs at T = 453 K (for LASS1%) and T = 450 K (for LASS20%). Such transitions has characteristics of a ferroelectric transition and the crystal changes its symmetry from D162h to C92v. The second phase transition occurs at T = 284 K, for LASS1 % and at T = 281 K, for LASS20%. The new phase is monoclinic belonging to the C52h space group. The unit cell in both phase are characterized by four molecules in equivalent sites with symmetry C1. The third transition was observed at T≅245 K and T = 240 K, for LASS1 % and LASS20%, respectively. The new structure belongs to the space group C1,22 with four molecules per unit cell. Two new phase transitions were observed for temperatures lower than 210 K. One at T = 185 K (LASS1%), T = 208 K (LASS20%), that it seems to be of second order and reversible. The symmetry of new phase is C2,12. Other phase transition was observed at T = 120 K (LASS1%) or T = 133 K (LASS20%). This last phase transition is reversible and by Raman spectroscopy it was observed changes in the external mode region. The distribution of all vibrational modes of LASS for all irreducible representations of the respective factor group were performed. Finally it was observed that increasing of Na+ content decreases the temperature of the phase transitions for those occurring for T > 230 K and increases the phase transition temperature for transitions occurring for T < 230 K. The effect of volume decreasing and the decreasing of hydrogen bonds due the NH4 and SO4 ions is discussed.