1. L. L. Boyer and J. R. Hardy, Phys. Rev. B 24(5), 2577 (1981).
2. M. B. Smirnov, Phys. Rev. B 59(6), 4036 (1999).
3. A. D. Bruce and R. A. Cowley, Structural Phase Transitions (Taylor and Francis, London, 1981).
4. V. L. Aksenov, N. M. Plakida, and S. Stamenkovič, Neutron Scattering by Ferroelectrics (Énergoatomizdat, Moscow, 1983; World Scientific, Singapore, 1990).
5. A. N. Rubtsov, J. Hlinka, and T. Janssen, Phys. Rev. E 61(1), 126 (2000).