1. Institut für Anorganische Chemie II, Universität Karlsruhe, Engesserstrasse Geb. 30.45, D-76128 Karlsruhe, Germany
2. School of Physical Sciences, University of Kent at Canterbury, Canterbury CT2 7NR, U.K.
3. School of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K.
4. Laboratoire Louis Néel, CNRS, 25 Avenue des Martyrs, BP 156, 38042 Grenoble Cedex 9, France