1. N. V. Dergunova, V. P. Sakhnenko, and E. G. Fesenko, Kristallografiya 23, 94 (1978) [Sov. Phys. Crystallogr. 23, 50 (1978)].
2. R. Kolesova, V. Kolesov, M. Kupriyanov, and R. Skulski, Phase Transitions 68, 421 (1999).
3. R. V. Kolesova, V. V. Kolesov, M. F. Kupriyanov, and O. A. Lavrova, Izv. Ross. Akad. Nauk, Ser. Fiz. 64(6), 1097 (2000).
4. N. Lampis, Ph. Scian, and A. Geddo Lehman, J. Phys.: Condens. Matter 11, 3489 (1999).
5. Y. X. Wang, W. L. Zhong, W. L. Wang, and P. L. Zhang, Phys. Lett. A 288(1), 45 (2001).