Espectroscopia Raman no molibdato de magnésio e molibdato de lítio sob altas pressões

• Ano de defesa: 2014
• Autor(a) principal: Coelho, Marcelo Nunes
• Orientador(a): Freire, Paulo de Tarso Cavalcante
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Palavras-chave em Português: Espectroscopia de Raman; Molibdatos; Alta pressão (Tecnologia); Transição de fase
• Link de acesso: http://www.repositorio.ufc.br/handle/riufc/8969

In this work it was carried out an investigation by Raman spectroscopy, of the vibrational modes of the magnesium molybdate MgMoO4 subjected to high pressures up to 8.5 GPa, and lithium molybdate Li₂MoO₄ - in two experiments – up to the limits of 5.0 and 7.0 GPa, respectively. The analysis of the spectra obtained for MgMoO₄ revealed a significant change in the spectral pattern of the material around the pressure of 1.4 GPa. For the lithium molybdate in first experiment, such changes occured between 0.0 and 3.1 GPa and above 4.5 GPa. In second experiment, changes similar to those observed (between 0.0 and 3.1 GPa) in the first experiment, appeared between 1.1 and 3.8 GPa, and additional changes occurred above 5.0 GPa. All changes were confirmed by analysis of the ω × P curves. For MgMoO₄, these changes in the spectra were interpreted as being due to a phase transition undergone by the material at 1.4 GPa. It was also possible to see that the transition is irreversible, taking into consideration that the spectrum of the material measured 16 hours after the pressure releasing to 1 atm was similar to that obtained above 1.4 GPa (the pressure transition). It is made a discussion about of the possible nature of the high-pressure phase, which takes into account some similarities between MgMoO₄ and other molybdates and tungstates crystals. Among some possibilities it is suggested that the phase transition is of type C2/m → P2/c (C₂ₕ⁴), which discards the possibility of an isostructural phase transition. For Li₂MoO₄ in the first experiment, the first changes are interpreted as being due to a structural phase transition. This phase transition, in the second experiment, seems to happen between 1.1 and 3.8 GPa However the additional changes that happen in both experiments at the higher pressures, were explained as being derived either from a possible amorphization (first experiment) or from a new structural phase transition (second experiment). It is made a discussion of the amorphization mechanism in the first experiment, and why the phenomenon has not been observed in the second experiment. For this we took into consideration the different experimental conditions and the hydrostaticity / quasi-hydrostaticity of the compressing fluid used.