1. C. Kittel: Introduction to Solid State Physics, John Wiley & Sons, NY (1966)
2. W. Kinase, Y. Uemura and M. Kikuchi: J. Phys. Chem. Solids, 30, 441 (1969)
3. J. F. Nye: Physical Properties of Crystals, Oxford University Press, London, p.123, p. 140 (1972)
4. A. F. Devonshire: Adv. Phys. 3, 85 (1954)
5. H. F. Kay: Rep. Prog. Phys. 43, 230 (1955)