1. W.H. Zachariasen, J. Am. Chem. Soc. 54, 3841(1932);
2. M.F. Thorpe in Vibration Spectroscopy of Molecular Solids, edited by S. Bratos and R.M. Pick, Plenum, New York (1979), p. 341;
3. R J. Nemanich, G.A.N. Connell, T.M. Hayes and R.A. Street, Phys, Rev. B 18, 6900(1980).
4. J.E. Griffiths, G.P. Espinosa, J.P. Remeika and J.C. Phillips, Solid State Commun. 40, 1077(1981);
5. J.E. Griffiths, G.P. Espinosa, J.P. Remeika and J.C. Phillips, Phys. Rev. B 25, 1272 (1982);