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 CaO for MgO in metaphosphate glasses has been studied. The experimental results of density, ρ, molar volume, V, ionic volume fraction, w, and glass transition temperature, Tg, are presented. The observed departures from additivity 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 heterogeneities of electron density with dimensions greater than about 1 nm that are associated with the mixed oxide effect. The non-linearities observed for V and w are not attributable to packing effects correlated with the magnitude of the difference between the field strengths of the two modifier cations as suggested by Dietzel. These departures from additivity were found to be correlated with the molar volume of oxygen. The formation of dissimilar pairs, Me1–ONB–Me2, suggested by Dietzel is confirmed in the metaphosphate samples. However, the deviations in Tg are independent of the differences of the field strengths of the modifier cations. The intermediate range structure of the mixed metaphosphate samples were found to be determined by the differences in spatial arrangement of the twofold-linked PO4-units and the MeOn-polyhedra in the participating binary systems.