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 ZnO_n 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.