1. Eduard-Zintl-Institute of Inorganic and
Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str.
12, 64287 Darmstadt, Germany
2. Materials Department and Materials Research
Laboratory, University of California Santa Barbara, Santa Barbara California 93106, United States
3. Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Str. 4, 35032 Marburg, Germany