Intermediate range structure of mixed phosphate glasses by X-ray diffraction

The effect of substitution of K2O for Na2O, BaO for SrO, BaO for Na2O and C aO for MgO in metaphosphate glasses has been studied. The experimental resu lts of density, rho, molar volume, V, ionic volume fraction, w, and glass t ransition temperature. T-g, are presented. The observed departures from add itivity are discussed with respect to Dietzel's model. X-ray scattering has been used to characterize the structural features on an intermediate scale of distances. In the mixed metaphosphate samples there are no heterogeneit ies of electron density with dimensions greater than about I nm that are as sociated with the mixed oxide effect. The nan-linearities observed for V an d w are not attributable to packing effects correlated with the magnitude o f the difference between the field strengths of the two modifier cations as suggested by Dietzel. These departures from additivity were found to be co rrelated with the molar volume of oxygen. The formation of dissimilar pairs , Me-1-O-NB-Me-2, suggested by Dietzel is confirmed in the metaphosphate sa mples. However, the deviations in T-g are independent of the differences of the field strengths of the modifier cations. The intermediate range struct ure of the mixed metaphosphate samples were found to be determined by the d ifferences in spatial arrangement of the twofold-linked PO4-units and the M eOn-polyhedra in the participating binary systems. (C) 2000 Elsevier Scienc e B.V. All rights reserved.