1. B. K. Vaĭnshteĭ, Crystallography and Crystal Chemistry (Nauka, Moscow, 1986) [in Russian].
2. V. A. Koptsik, Dokl. Akad. Nauk SSSR 250, 353 (1980) [Sov. Phys. Dokl. 25, 11 (1980)].
3. P. M. Zorkiĭ, Zh. Fiz. Khim. 68, 966 (1994).
4. B. A. Strukov and A. P. Levanyuk, Physical Principles of Ferroelectric Phenomena in Crystals (Nauka, Moscow, 1995) [in Russian].
5. M. E. Lines and A. M. Glass, Principles and Application of Ferroelectrics and Related Materials (Clarendon Press, Oxford, 1977; Mir, Moscow, 1982).