Structural specifics of phosphate glasses probed by diffraction methods: a review

The present work illustrates the different contributions to the structure of phosphate glasses which are made by diffraction studies. The resolution in real space of the neutron diffraction experiments resolves two P-O distances. The lengths of the P-O bonds to the terminal and to the bridging oxygen atoms change as a function of the P2O5 content and of the species of the modifier cation. The model about the role of the properties of the modifier atoms, Me, in the structure of phosphate glasses predicts three different types of structural incorporation of these atoms. The experimental findings of a network change at approximate to 20 mol% modifier oxide content in range I are explained by a change of the interaction between the Me sites and the twofold-linked PO4 groups. For the intermediate cations a change of the Me-O coordination number, N-MeO, is obtained which indicates a stabilization of Me-O-P bridges in range II. With increasing modifier content a situation commonly described as a modified random network ensues (range III) where clusters of MeOn, polyhedra are formed. The corresponding consequences for the Me-Me distances from the relation between N-MeO and the available number of terminal oxygen atoms per modifier cation are simulated by the reverse Monte Carlo method. This approach which makes use of the scattering information about the medium-range order is applied to the structures of binary metaphosphate glasses with Me = Zn, Ca, Sr, Pa, Na and K. (C) 2000 Elsevier Science B,V. All rights reserved.