1. Hurley, D.C., Wolfe, J.P.: Phys. Rev. B32, 2568 (1985)
2. See also Born, M., Huang, K.: Dynamical theory of crystal lattices. New York (Oxford University Press 1954); Brüesh. P. Phonons: theory and experiments I, p. 101 Berlin, Heidelberg, New York: Springer 1982
3. Orlova, N.S.: Phys. Statics Solid.:94, 503 (1979)
4. Carles, R., Saint-Cricq, N., Renucci, J.B., Renucci, M.A., Zwick, A.: Phys. Rev. B22, 4804 (1980)
5. Borcherds, P.H.: Private communication and [4]