1. Department of Inorganic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden, Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway, Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany, and Centre d'Elaboration de Matériaux et d'Etudes Structurales (CEMES/CNRS), 29 rue Jeanne Marvig, BP 4347, Toulouse Cedex 4, France