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.