1. K. Aizu, Phys. Rev. B: Solid State 2, 754 (1970).
2. G. A. Smolenskii, V. A. Bokov, V. A. Isupov, N. N. Krainik, R. E. Pasynkov, A. I. Sokolov, and N. K. Yushin, Physics of Ferroelectric Phenomena (Nauka, Leningrad, 1985) [in Russian].
3. M. Lines and A. Glass, Principles and Applications of Ferroelectrics and Related Materials (Oxford University Press, Oxford, 1977; Mir, Moscow, 1981).
4. G. A. Smolenskii and I. E. Chupis, Sov. Phys.-Usp. 25(7), 475 (1982).
5. W. Zhong and D. Vanderbilt, Phys. Rev. Lett. 74, 2587 (1995).