Formation Mechanism of the Ga₁₃ Keggin Ion:  A Combined EXAFS and NMR Study

The base hydrolysis of gallium chloride and gallium nitrate was followed by combining 71 Ga NMR and Ga K-edge X-ray absorption spectroscopy measurements. Using such an approach, it can be shown unambiguously that the solution contains various trimeric and tetrameric species in the early hydrolytic stages before the formation of the Ga 13 polycation. This Keggin-type structure would then form by condensation of trimers and tetramers in disagreement with all of the previously published models that assume the existence of a monomer either tetrahedral or octahedral. In view of the parallel behavior of aluminum and gallium, this work can certainly be used to reach a better understanding of the hydrolytic chemistry of aluminum. This is of prime importance regarding the crucial role of this metal in natural systems and its widespread use as a coagulant in water treatment operations.