1. Max Planck Institute for Solid State Research Heisenbergstr. 1 70569 Stuttgart Germany
2. New address: Institute of Inorganic Chemistry University of Regensburg 93053 Regensburg Germany
3. Laboratory for Electron Microscopy Microscopy of Nanoscale Structures and Mechanisms Karlsruhe Institute of Technology Engesserstr. 7 76131 Karlsruhe Germany
4. Department of Chemistry Ludwig-Maximilians-Universität München Butenandtstr. 5–13 81377 München Germany