1. (a) M. M. Godneva and D. L. Motov, Chemistry of Zirconium and Hafnium Fluoride Compounds [in Russian], Nauka, Leningrad (1971);
2. (b) A. G. Babkin, V. G. Maiorov, and A. I. Nikolaev, Extraction of Niobium, Tantalum, and Other Elements from Fluoride Solutions [in Russian], Nauka, Leningrad (1988); c) È. G. Rakov, Chemistry and Technology of Inorganic Fluorides [in Russian], Moscow Chem. Technol. Institute, Moscow (1990).
3. (a) R. D. Rogers, A. N. Rollins, and M. M. Benning, Inorg.Chem., 27, 3826–3835 (1988);
4. (b) G. G. Talanova, N. S. A. Elkarim, V. S. Talanov, etal., J. Am. Chem. Soc., 121, 11281–11290 (1999);
5. (c) R. D. Rogers, S. E. Huggins, R. F. Henry, and A. H. Bond, Supramol. Chem., 1, 59–63 (1992);