1. V. A. Sanina, L. M. Salozhnikova, E. I. Golovenchits, and N. V. Morozov, Fiz. Tverd. Tela (Leningrad) 30, 3015 (1988) [Sov. Phys. Solid State 30, 1736 (1988)].
2. E. I. Golovenchits, N. V. Morozov, V. A. Sanina, and L. M. Salozhnikova, Fiz. Tverd. Tela (Leningrad) 34, 108 (1992) [Sov. Phys. Solid State 34, 56 (1992)].
3. T. Doi and K. Kohn, Phase Transit. 38, 273 (1992).
4. H. Tsujino and K. Kohn, Solid State Commun. 83, 639 (1992).
5. Y. Tanaka, K. Saito, H. Tsujino, and K. Kohn, Ferroelectrics 161, 125 (1994).