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

Abstract

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 1∼2 M (M = mol−dm −3) and [Br−]/[Ga3+] ratios > ∼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) A, whereas the [GaBr4]− complex has a Ga3+–Br− distance of (2.33± 0.02) A, 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 ∼1.8 A 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 A for octahedral geometry with decreasing temperature, suggesting that equilibrium shifts from [GaBr4]− to [Ga(H2O)6]3+ through intermediate species, [GaBr n ](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) A, respectively. The Ga3+ has the second hydration shell at (4.03± 0.03) A and the hydration of Br− is characterized with a Br−–H2O distance of (3.35± 0.02) A at all temperatures investigated.