1. Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany, Eduard-Zintl-Institute, Darmstadt University of Technology, Petersenstrasse 20, D-64287 Darmstadt, Germany, and Max Planck Institute for Solid State Research, Heisenbergstrasse 1, D-70569 Stuttgart, Germany