Raman Spectra of Haloaluminate Melts Containing Oxides

Raman spectroscopy measurements on haloaluminate melts (bromides and chlorides), with or without addition of the corresponding aluminum oxohalide, have unequivocally shown that even the most carefully cleaned melts exhibit a polarized Raman band, due to the presence of aluminum oxohalide complexes. The frequency of this characteristic oxide Raman band is ∼280–270 cm⁻¹ for the chloride and ∼185 cm⁻¹ for the bromide system, respectively. Hence, being halide dependent, the observed band cannot be assigned entirely to Al-O vibrations within the skeleton of the aluminum oxohalide species but must also involve some movement of the halide atoms. At temperatures just above the melting point, studies of the oxide band intensity vs. time showed that molten AlCl₃ and AlBr₃ attack Pyrex® glass, fused quartz, and sapphire containers, causing dissolution of oxides; within few hours, the melts become almost saturated. Finally, it has been found that acidic chloroaluminate melts, which invariably tend to fluoresce when illuminated with visible light contained in a glass or silicate container, owe their fluorescing properties to a reaction between the container and the AlCl₃. This troublesome fluorescence can be quenched by addition to the melt of a reductant such as metallic aluminum.