1. Institute of Inorganic Chemistry, Christian-Albrechts University, Max-Eyth-Str. 2, D-24118 Kiel, Germany
2. Institute for Materials Science, Synthesis and Real Structure, Christian-Albrechts University, Kaiserstrasse 2, D-24143 Kiel, Germany
3. Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany
4. RWTH Aachen University, I. Institute of Physics, 52056 Aachen, Germany