The 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 NGeO 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 GeOn 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.