Structure of Potassium Germanophosphate Glasses by X-ray and Neutron Diffraction: 1. Short-range Order

It has been reported that the addition of K2O or P2O5 to binary germanate glasses increases the Ge−O coordination numbers (NGeO). The present work describes X-ray and neutron diffraction studies aimed at clarifying the concomitant effects of both oxides on the structures of ternary K2O-GeO2-P2O5 glasses. The Ge−O coordination numbers obtained range from 4.2 to 5.1, less than what is predicted according to a model which assumes all oxygen atoms form network bridges similar to those found in the related crystal structures. This implies that the glass structures must include terminal oxygen sites, likely associated with the PO4 tetrahedra, that are neutralized by coordinating K+ ions. The shapes of the high resolution first-neighbor diffraction peaks do not indicate distinctly different species of P−O and Ge−O bonds. The model for the increase of NGeO which is based on an increase of the fraction of GeO6 units, at the expense of GeO4 units, is supported by the analysis of the two main components of the Ge−O peak used in the fits. However, the existence of a GeO5 fraction cannot be excluded from the present data sets. A linear relation between the total Ge−O coordination numbers and mean Ge−O distances exists, assuming end members of units GeO4 and GeO6 with bond lengths of ~0.175 and ~0.190 nm, respectively