Structure of aqueous gallium(III) bromide solutions over a temperature range 80-333 K by Raman spectroscopy, X-ray absorption fine structure, and X-ray diffraction

摘要

The structure and complex formation of concentrated aqueous gallium(III) bromide (GaBr3) solutions have been investigated over a temperature range 80 - 333 K by Raman spectroscopy, X-ray absorption fine structure (XAFS), and X-ray diffraction. The Raman spectra obtained at various [Br-]/[Ga3+] molar ratios and temperatures have shown that complex formation between Ga3+ and Br- occurs as a predominant species, with [GaBr4](-) at [Ga3+] as high as 1similar to 2 M (M = mol- dm(-3)) and[Br-]/[Ga3+] ratios > similar to 2, and that cooling of the solutions favors the formation of the aqua Ga3+. The intermediate species were not seen in the Raman spectra. The XAFS data have revealed that the aqua complex has a sixfold coordination as [Ga(H2O)(6)](3+) with a Ga3+ - H2O distance of (1.96 +/- 0.02) Angstrom, whereas the [GaBr4](-) complex has a Ga3+ - Br- distance of (2.33 +/-02) Angstrom, and that vitrification of the aqueous GaBr3 solution at liquid nitrogen temperature shifts the equilibrium toward the aqua complex. The X-ray diffraction data at different subzero temperatures have shown a tendency of decreasing Ga3+ - Br- and increasing Ga3+ - H2O interactions with lowering temperature, confirming the preference of aqua Ga3+ in the supercooled liquid state as well as in the glassy state. The Ga3+ - H2O distance of similar to 1.8 Angstrom for the tetrahedral coordination was found in a 2.01 M gallium(III) bromide solution with a [Br-]/[Ga3+] ratio of 3.7 and gradually increased to a value of 1.92 Angstrom for octahedral geometry with decreasing temperature, suggesting that equilibrium shifts from [ GaBr4](-) to [Ga(H2O)(6)] (3+) through intermediate species, [GaBrn]((3-n)+) (n = 2 and 3). The Ga3+ - Br- and Br- - Br- distances within [GaBr4](-) with an almost tetrahedral symmetry are (2.35 +/- 0.02) and (3.82 +/- 0.03) Angstrom, respectively. The Ga3+ has the second hydration shell at (4.03 +/- 0.03) Angstrom and the hydration of Br- is characterized with a Br- - H2O distance of (3.35 +/- 0.02) Angstrom at all temperatures investigated.