The structure of binary zinc phosphate glasses

The structures of (ZnO)x(P2O5)100−x glasses, with x = 49.1–69.7, have been studied by spallation neutron diffraction and high-energy X-ray diffraction methods. For all glasses the average Zn–O coordination number NZn–O is around five (4.5–4.9) as distorted (4 + 1) environment. Although no abrupt change in NZn–O between the nearly meta- and orthophosphate compositions is observed, the mean value and distribution of Zn–O distances slightly increases with an increase in the ZnO content. The distorted five-coordinations of Zn–O may be regarded as the mixture of ZnO4 tetrahedra and ZnO5 polyhedra. The Zn cation plays a typical role as a network-modifier in the process of the depolymerization PO4 tetrahedral network on the one hand, and may act as a network-former for linking the ZnO4 and PO4 tetrahedra on the other hand. The presence of the two kinds of intermediate-range ordering corresponding to PO4–PO4 linkage and ZnO4–PO4 connections is revealed by application of the reverse Monte Carlo modeling of the neutron and X-ray diffraction data. The enhancement of density fluctuation about the size of a few nanometers in small-angle neutron scattering region (Q ∼ 0.2 Å−1) arisen at near 60 mol% ZnO suggests that there are diverse stages in the intermediate-range structure composed of PO4 and ZnO4 tetrahedra and ZnO5 polyhedra depending on the compositions. These results demonstrated that the phosphate glass anomaly and the peculiar property for zinc phosphate glasses such as a behavior of glass transition temperature Tg should be understood in terms of not only the depolymerization of network composed of PO4 tetrahedra but also the stability of the chemical ordering of PO4 tetrahedra and ZnOx polyhedra.