Structure of potassium germanophosphate glasses by X-ray and neutron diffraction. Part 1: Short-range order

It has been reported that the addition of K ₂O or P ₂O ₅ to binary germanate glasses increases the Ge−O coordination numbers ( N _GeO ). The present work describes X-ray and neutron diffraction studies aimed at clarifying the concomitant effects of both oxides on the structures of ternary K ₂O–GeO ₂–P ₂O ₅ 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 PO ₄ 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 N _GeO which is based on an increase of the fraction of GeO ₆ units, at the expense of GeO ₄ 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 GeO ₅ 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 GeO ₄ and GeO ₆ with bond lengths of ∼0.175 and ∼0.190 nm, respectively.