1. J. VALASEK,Phys. Rev. 15 (1920) 537.
2. F. JONA and G. SHIRANE, “Ferroelectric Crystals” (Pergamon, London, 1962).
3. W. KANZIG, “Solid State Physics”, Vol. 4, edited by F. Seitz (Academic, New York, 1957).
4. H. D. MEGAW, “Ferroelectricity in Crystals” (Methuen, London, 1957).
5. J. C. ROCARIES and P. BOLDRINI,Appl. Phys. Lett. 20(1) (1972).