Structure of potassium germanate glasses - a combined X-ray and neutron scattering study

X-ray scattering experiments on potassium germanate glasses of 11 and 20 mol% K₂O confirm the change of the Ge-O coordination state which was obtained by earlier neutron scattering studies. The O-O and Ge-Ge coordination numbers are extracted by help of the different contrast of the X-ray and neutron data. Their behavior agrees with the model assumption of the increase of N_GeO and the absence of non-bridging oxygen atoms up to about 20 mol% K₂O concentration. Reverse Monte Carlo (RMC) simulations are used to analyze the changes in the medium-range order (MRO). The modified glasses are assumed to be quasi-binary systems formed of K atoms and GeO_n units where the latter are represented by their central Ge positions. According to the Bhatia/Thornton formalism the first diffraction peak is identified as a chemical order prepeak which indicates the preference of K-Ge neighbors in the packing of K and Ge sites. The Ge-Ge pair distributions show additional features which are explained by the needs of the network linkages and by steric effects of the K⁺ cations in the environments of the GeO₄ tetrahedra.