Structural specifics of phosphate glasses probed by diffraction methods: areview

The present work illustrates the different contributions to the structure o f phosphate glasses which are made by diffraction studies. The resolution i n real space of the neutron diffraction experiments resolves two P-O distan ces. The lengths of the P-O bonds to the terminal and to the bridging oxyge n 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 ra nge 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 th e Me-O coordination number, N-MeO, is obtained which indicates a stabilizat ion of Me-O-P bridges in range II. With increasing modifier content a situa tion commonly described as a modified random network ensues (range III) whe re clusters of MeOn, polyhedra are formed. The corresponding consequences f or the Me-Me distances from the relation between N-MeO and the available nu mber of terminal oxygen atoms per modifier cation are simulated by the reve rse Monte Carlo method. This approach which makes use of the scattering inf ormation about the medium-range order is applied to the structures of binar y metaphosphate glasses with Me = Zn, Ca, Sr, Pa, Na and K. (C) 2000 Elsevi er Science B,V. All rights reserved.