1. W. Doering, Z. Naturforsch, A3, 373 (1948).
2. B. E. Argyle and A. P. Malozemoff, AIP Conf. Proc. 10, 344 (1972).APCPCS0094-243X
3. F. de Jonge and W. Druyvesteyn, Festkorproblem XII, 531 (1972).
4. A. A. Parker, thesis, The Ohio State University, 1980
5. P. E. Wigen, H. Doetsch, and J. Morkowski, Phys. Stat. Sol., to be published.