1. G. A. Smolenskii, V. A. Bokov, and V. A. Isupov, Ferroelectrics and Antiferroelectrics (Nauka, Leningrad, 1985) [in Russian].
2. R. Blinc and B. Zeks, Soft Modes in Ferroelectrics and Antiferroelectrics (North Holland, Amsterdam, 1974; Mir, Moscow, 1975).
3. N. A. Kulagin and D. T. Sviridov, Introduction to the Physics of Activated Crystals (Vysshaya Shkola, Kharkov, 1990) [in Russian].
4. V. V. Lemanov, E. P. Smirnova, P. P. Syrnikov, and E. A. Tarakanov, Phys. Rev. B 545(2), 151 (1996).
5. V. V. Lemanov, Phys. Solid State 39(3), 1468 (1997).