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2. A. D. Fedoseev, T. A. Makarova, N. I. Nester?uk, and D. P. Sipovskij, Kristall und Technik,3, 95 (1968).
3. N. E. Prikhid'ko, Dissertation [in Russian], Leningrad (1955).
4. H. T. S. Britton, J. Chem. Soc., 425 (1927).
5. F. Ephraim, Inorganic Chemistry [Russian translation], Leningrad (1933), Chapter 2.