1. Department of Chemistry, Laboratory of Coordination Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, bus 2404, B-3001 Leuven, Belgium, Anorganische Chemie I - Festkörperchemie and Materialien, Ruhr-Universität Bochum, D-44780 Bochum, Germany, and Department of Physics and Astronomy, Laboratory for Acoustics and Thermal Physics, Katholieke Universiteit Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium.