1. Laboratoire CRISMAT, ENSICAEN/CNRS, 6 Bd. du Maréchal Juin, 14050 Caen Cedex 4, France, Laboratory of Inorganic Chemistry, Department of Chemistry, Helsinki University of Technology, P.O. Box 6100, FI-02015 TKK, Finland, and Institut Laue-Langevin, BP 156, 38042 Grenoble Cedex, France