1. Eduard-Zintl-Institute of Inorganic and
Physical Chemistry, Technische Universität Darmstadt, Alarich-Weiss-Str.
12, 64287 Darmstadt, Germany
2. Department of Chemistry, University of Houston, Houston, Texas 77204, United States
3. Institute of Inorganic Chemistry, Rheinische Friedrich-Wilhelms-Universität Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany