THE OXYGEN COORDINATION OF METAL-IONS IN PHOSPHATE AND SILICATE-GLASSES STUDIED BY A COMBINATION OF X-RAY AND NEUTRON-DIFFRACTION

A combination of results from X-ray and neutron diffraction is used to obtain structural information about the metal-oxygen coordination she ll in oxide glasses. Two ways to extract structural parameters of the Me-O coordination are presented The first variant is a direct combinat ion of both distance correlation functions which are considered simult anously in a least-squares fit procedure. On the other hand a suitable difference of the two structure factors is introduced, which do not c ontain any O-O correlation. The corresponding distance correlation fun ction directly shows the Me-O peak. The samples are metaphosphate glas ses with Me = Al, Zn, Mg, Ca, Ba and Na and two sodium silicate glasse s (76.5 and 67 mol% silicon dioxide). Four oxygens are found in contac t to the Mg ion. But two additional, more distant positions are detect ed. Thus, the sum of all oxygen atoms in the coordination sphere is 6 rather than 4. The Zn cation is located in a real ZnO4-tetrahedron. Th e number of oxygens in the environment of the Na ion is of about five both in the metaphosphate glass and in the silicate glasses. But a sur prising result is a splitting observed for the Na-O distance peak in c ase of silicate glasses.