Changing hafnium speciation in aqueous sulfate solutions: A high-energy X-ray scattering study

The relationship of solution speciation and the structures of corresponding precipitates is examined for an aqueous Hf⁴⁺ sulfate series. High-energy X-ray scattering (HEXS) and Raman spectroscopy data are used to probe atomic correlations in solutions. Hf⁴⁺ in acidic perchlorate solution shows no evidence of a mononuclear metal species but instead has a peak in the pair-distribution function (PDF), generated from the HEXS data, at 3.55 Å, indicating Hf⁴⁺-Hf⁴⁺ solution correlations. The peak intensity is consistent with clusters that are, on average, larger than the tetramic unit [M₄(OH)₈(H₂O)₁₆] ⁸⁺ usually attributed to Zr⁴⁺ and Hf⁴⁺ solution speciation under these conditions. Addition of sulfate results in a breakup of hydroxo-bridged oligomers into sulfate-capped dimers and, for higher concentrations, Hf-sulfate monomers. The bidentate coordination mode of sulfate dominates the dissolved precursors, although it is not found in the structure of the final crystallized product, which instead is comprised of bridging-bidentate sulfate ligation. Neither the PDF patterns nor the Raman spectra show any evidence of the larger oligomers, such as the octadecameric metal clusters, found in similar Zr⁴⁺ solutions. The oligomeric units found in solution provide insights into possible assembly routes for crystallization. In addition to expanding our understanding of synthesis science this study also reveals differences in the aqueous chemistries between Hf and Zr, two elements with ostensibly very similar chemical behavior.