1. Kahn, O. (1991), contribution in this volume.
2. Herren, M., Jacobsen, S.M. and Güdel, H.U. (1989), Inorg.Chem. 28, 504–509.
3. Jacobsen, S.M., Güdel, H.U. and Smith, W.E. (1987), Inorg.Chem. 26, 2001–2005.
4. Aebersold, M., Blank, H., Briat, B., Furrer, A. and Glüdel, H.U., submitted to Inorg.Chem.
5. Falk, U., Furrer, A., Furer, N., Güdel, H.U. and Kjems, J.K. (1987), Phys.Rev.B. 35, 4893–4900.