1. Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52074 Aachen, Germany
2. STOE & Cie GmbH, Hilpertstraße 10, D-64295 Darmstadt, Germany
3. Institute of Physics (IA), Physics of Novel Materials, RWTH Aachen University, D-52056 Aachen, Germany