Aqueous hafnium sulfate chemistry: structures of crystalline precipitates.

Crystalline precipitates resulting from the hydrolysis and subsequent condensation of Hf(IV) aqueous acidic solutions at 60-95 °C are examined and compared. By varying the concentrations of the acid and sulfate source, a variety of complex hafnium-oxo-hydroxo-sulfate clusters are isolated and structures accessed. Four novel compounds were discovered, while the structures of two known compounds, an 18-mer and a planar hexamer, were updated. In total, the compounds described herein each contain one of four cluster architectures: 18-mer, 11-mer, nonamer, and planar hexamer. In addition, one compound contains small amounts of 19-mers together with 18-mers. As well as examining the individual structure of each complex cluster, we relate them to one another, as well as to the dense phases of HfO2, to gain an understanding of their formation and stability. Finally, the solution conditions under which each cluster forms are identified by plotting the crystallization regions of each cluster against acidity and sulfate concentration. Most clusters form under slightly acidic conditions, in decreasing size as the sulfate concentration is raised. The flat hexamer is the single exception; it appears to require more acidic solutions. The degree of hydroxo- versus oxo-bridges with changing solution conditions is assessed within the broader context of the condensates. Of specific interest is the identification of these products as they relate to the use of hydrolysis reactions in designing new materials.

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 Hf(4+) sulfate series. High-energy X-ray scattering (HEXS) and Raman spectroscopy data are used to probe atomic correlations in solutions. Hf(4+) 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 Hf(4+)-Hf(4+) 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 Zr(4+) and Hf(4+) 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 Zr(4+) 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.

A non-oxide sol-gel route to synthesise silicon imidonitride monolithic gels and high surface area aerogels.

Monolithic gels have been produced by reaction of Si(NHMe)(4) with ammonia in THF solution and converted to high surface area aerogels by critical point drying with ammonia saturated diethylamine solvent.