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.