1. Wigner Research Centre for Physics; Institute of Solid State Physics; Budapest H-1525 Hungary
2. Department of Materials Science and Engineering; University of Arizona; Tucson Arizona 85721
3. Equipe Verres et Ceramiques; Laboratoire Sciences Chimiques de Rennes UMR-CNRS 6226; Rennes France
4. Institute of Materials Science; TU Dresden; Dresden 01062 Germany
5. Institute for Complex Materials; IFW Dresden; Dresden 01171 Germany
6. Laboratoire Léon Brillouin; CEA-Saclay 91191 Gif sur Yvette Cedex Saclay France
7. Deutsches Elektronen-Synchroton DESY; Notkestrasse 85 Hamburg 22603 Germany