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 N_zn-O is around five (4.5-4.9) as distorted (4+1) environment. Although no abrupt change in N_Zn-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 60mol% 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 T_g 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 ZnO _x polyhedra.