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 shell in oxide glasses. Two ways to extract structural parameters of the Me-O coordination are presented The first variant is a direct combination of both distance correlation functions which are considered simultanously in a least-squares fit procedure. On the other hand a suitable difference of the two structure factors is introduced, which do not contain any O-O correlation. The corresponding distance correlation function directly shows the Me-O peak. The samples are metaphosphate glasses with Me = Al, Zn, Mg, Ca, Ba and Na and two sodium silicate glasses (76.5 and 67 mol% silicon dioxide). Four oxygens are found in contact to the Mg ion. But two additional, more distant positions are detected. 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. The 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 surprising result is a splitting observed for the Na-O distance peak in case of silicate glasses.