1. Ishibashi, Y., Takagi, Y.: Improper Ferroelectric Phase Transitions, Japan. J. Appl. Phys. 15 (1976) 1621.
2. Samuelsen, E., Andersen, E., Feder, J. (Editor): Structural Phase Transitions and Soft Modes, Proceedings of the NATO Advanced Study Institute, held at Geilo (1971), Oslo: Universitetsforlaget 1971.
3. Shirane, G.: Neutron Scattering Studies of Structural Phase Transitions at Brookhaven, Rev. Mod. Phys. 46 (1974) 437.
4. McMillan, W. L.: Phys. Rev. B 12 (1975) 1187.
5. McMillan, W. L.: Phys. Rev. B 14 (1976) 1496.