1. Israel Ceramic and Silicate Institute; Haifa 32000 Israel
2. Department of Physics; Ben-Gurion University of the Negev; Be'er Sheva 84105 Israel
3. Laboratoire de Chimie de la Matière Condenée de Paris; UMR-CNRS 7574; Chimie ParisTech; Université Pierre et Marie Curie; 75231 Paris Cedex 05 France