Hydrothermal Chemistry, Structures, and Luminescence Studies of Alkali Hafnium Fluorides

This paper describes the hydrothermal chemistry of alkali
hafnium fluorides, including the synthesis and structural characterization
of five new alkali hafnium fluorides. Two ternary alkali hafnium fluorides
are described: Li2HfF6 in space group P3̅1m with a = 4.9748(7)
Å and c = 4.6449(9) Å and Na5Hf2F13 in space group C2/m with a = 11.627(2) Å, b = 5.5159(11) Å, and c = 8.4317(17) Å.
Three new alkali hafnium oxyfluorides are also described: two fluoroelpasolites,
K3HfOF5 and (NH4)3HfOF5, in space group Fm3̅m with a = 8.9766(10) and 9.4144(11) Å, respectively,
and K2Hf3OF12 in space group R3̅m with a = 7.6486(11)
Å and c = 28.802(6) Å. Infrared (IR) spectra
were obtained for the title solids to confirm the structure solutions.
Comparison of these materials was made based on their structures and
synthesis conditions. The formation of these species in hydrothermal
fluids appears to be dependent upon both the concentration of the
alkali fluoride mineralizer solution and the reaction temperature.
Both X-ray and visible fluorescence studies were conducted on compounds
synthesized in this study and showed that fluorescence was affected
by a variety of factors, such as alkali metal size, the presence/absence
of oxygen in the compound, and the coordination environment of Hf4+.