Aqueous Hafnium Sulfate Chemistry: Structures of Crystalline Precipitates

Crystalline precipitates resulting
from the hydrolysis and subsequent condensation of HfIV 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.