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+.