1. E. K. Bröch, Z. Phys. Chem.,1, 409 (1929).
2. N. J. Dunning and H. D. Megaw, Trans. Faraday Soc.,42, 705 (1946).
3. V. A. Atsarkin, É. A. Gerasimov, I. G. Matveeva, and A. V. Frantsesson, Zh. Éksp. Teor. Fiz.,43, 1272 (1962).
4. R. O. Kelling, Acta Crystallogr.,10, 209 (1957).
5. A. V. Chichagov, V. V. Ilyukhin, and N. V. Belov, Dokl. Akad. Nauk SSSR,166, 87 (1966).