1. Hayes, W., Rushworth, A.J., Ryan, J.F., Elliott, R.J., Kleppmann, W.G.: J. Phys. C10, L111 (1977) Elliott, R.J., Hayes, W., Kleppmann, W.G., Rushworth, A.J., Ryan, J.F.: preprint
2. Burns, G.: to be published in Proc. Int. Conf. on Lattice Dynamics, Paris (1977) Alben, R., Burns, G.: to be published
3. Hanson, R.C., Fjeldly, T.A., Hochheimer, H.D.: Phys. Stat. Sol. (b)70, 567 (1975)
4. Winterling, G., Senn, W., Grimsditch, M., Katiyar, R.: to be published in Proc. Int. Conf. on Lattice Dynamics, Paris (1977)
5. Geisel, T.: Solid State Commun.24, 155 (1977)