NEUTRON AND X-RAY-DIFFRACTION STUDY ON THE STRUCTURE OF ULTRAPHOSPHATE GLASSES

The high real-space resolution of neutron diffraction experiments whic h is provided by use of the epithermal neutrons from spallation source s was exploited in order to differentiate the unlike P-O bonds existin g in the PO4 units of phosphate glass networks. The 2 P-O distance pea ks, separated by about 12 pm, which were found in the zinc and the cal cium ultraphosphate glasses studied are assigned to oxygen sites on br idging (O-B) and terminal (O-T) positions. The mean P-O distances are nearly invariable versus the growing metal oxide content which results from an elongation of the P-O-B and P-O-T bonds. The bond lengths whi ch are known from the related crystal structures and from ab initio ca lculations show almost the same behaviour. The discussion of further d etails of the crystal structures leads to the conclusion that P-O-B ra ther than P-O-T distances should show more details in case of diffract ion measurements of even higher real-space resolution. The change of t he Zn-O coordination number from 6 to 4 versus increasing ZnO content, which was obtained in previous X-ray diffraction experiments, is conf irmed by the recent combination of neutron and X-ray diffraction data. On the other hand, the Ca-O coordination number of about 6 is almost invariable.