1. L. Yiping, G. G Hadjipanayis, V. Papaefthymiou, A. Kostikas and A. Simopoulos, J. Magn. Magn. Mat., in press (1996).
2. G. Ferey, in “Mixed Valency Systems” Ed. K. Prassides, Kluwer Acad. Pub. (Netherlands) 343, 155(1991).
3. R. G. Burns, in “Mixed Valency Systems” Ed. K. Prassides, Kluwer Acad. Pub. (Netherlands) 343, 175 (1991).
4. G. J. Long and F. Grandjean, “Mössbauer Spectroscopy Applied to Inorganic Chemistry”, Vol. 3, Plenum Press, New York, (1989).
5. J. S. Swinnea and H. Steinfink, Am. Mineral., 68, 827 (1983).