Transições de fase no B-LiNH4SO4

• Ano de defesa: 1988
• Autor(a) principal: Martins, Antonio Roberto Mendes
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: Raman, Espectroscopia de
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/61629

In this work we studied β-LiNH4SO4 (β-LAS) single crystals through Raman scattering and dielectric constant measurements as a functions of temperature in the range from 15K to 300K. The dependence of the Raman polarized spectra with the scattering geometry was studied for various polarizations in order to characterize the polar nature of the crystal phases in the temperature range mentioned above. These studied showed that β-LiNH4SO4 undergoes three phase transitions between 15K and 300K following the sequence given below: […] All these phase transitions are reversible and nondestructive. The transition at 284K is first-order and ferro-elastic while the other two are second-order. The transition at 256K has the usual characteristics of an order-disorder transition and the transition at 28K is driven by a doubling of the unit cell. The transition at 256 was detected for the first time in the present work. We give a clear evidence for this new phase that exists from 256K to 28K in β-LAS. For all phase we givem for the first time, an assignment of the normal vibration modes of β-LAS as a function of temperature together with the polar nature of each phase. For the new phase we propose a determination of the space group on the basis of our polarized Raman scattering results as a function of the scattering geometry. The reversibly of the phase transitions is clearly evidenced in the dielectric constant measurements as a function of temperature where no thermal hysteresis were observed. We also show that the new phase is characterized by the ions occupying two non-equivalent sites of C1 symmetry. This is made evident by the Raman polarized spectra where we could observe the existence of two v1 -type modes, four v2-type modes, six v3-type modes and six v4-type modes for the SO4 ion. Our Raman spectra showed the existence of two v1 -type modes with frequencies 1006cm-1 and 1014cm-1 that are related to two C1 distinct sites that ate occupied by the SO4 ions. We associate the 1O14cm-1 mode with SO4 ions that display a greater deformation as compared to the other SO4 ions forming the LAS crystalline lattice in this range of temperature. In the last transition, occuring at 28K, there is evidence of a doubling of the unit cell manifested by the appearing of a normal mode of low frequency that hardens to 28cm-1 at 15K. We believe that this mode is double degenerated, coming from the border to the center of the Brillouin zone when the cell is doubled.