1. Eduard-Zintl-Institut für Anorganische und Physikalische Chemie, Technische Universität Darmstadt, Petersenstrasse 20, D-64287 Darmstadt, Germany, Wilhelm-Ostwald Institute of Physical and Theoretical Chemistry, University of Leipzig, Linnéstrasse 2, D-04103 Leipzig, Germany, and Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560 064, India