Structure of zinc phosphate glasses probed by neutron and X-ray diffraction of high resolving power and by reverse Monte Carlo simulations

Neutron and X-ray diffraction experiments of high resolving power are performed on Zn polyphosphate glasses. Gaussian fitting of the P-O and Zn-O peaks yields coordination numbers of four. In combination with earlier results of ultra- and metaphosphate glasses explanations for a minimum packing density found at the metaphosphate composition are given. Atomic configurations obtained by reverse Monte Carlo simulations reproduce characteristics of oxygen atoms in the interaction of PO4 and ZnOn groups. In glasses of molar ratios n(ZnO)/n(P2O5) from 0.5 to 1.0 a change from ZnO6 to ZnO4 groups causes a decrease in the packing densities. Here, oxygen atoms are mostly found in Zn-O-P bridging positions. Finally, silica-like networks are formed at metaphosphate compositions. In the range of higher ZnO content an increasing fraction of oxygens is coordinated by two or more Zn sites which results in a strong densification of the glass structure.