The change of the Ge–O coordination number in potassium germanate glasses probed by neutron diffraction with high real-space resolution

Neutron diffraction experiments of high real-space resolution are used for studying the Ge–O coordination numbers and the Ge–O bond lengths in two K 2O–GeO 2 glasses (11 and 20 mol% K 2O). On the long-distance side of the first-neighbour Ge-O peak lying at 175 pm, which is likewise found for vitreous GeO 2, appears a clear shoulder. This feature indicates the existence of GeO n units larger than GeO 4 tetrahedra. The shoulder is not resolved as a separate peak which could be attributed to Ge–O bonds of GeO 6 octahedra unambiguously. Hence, it cannot be differentiated between GeO 5 or GeO 6 polyhedra being formed in addition to the GeO 4 tetrahedra. The increase of the Ge–O coordination number complies with the concept of a lack of non-bridging oxygen atoms in alkali germanate glasses up to concentrations of about 20 mol% K 2O where the additional oxygen atoms are incorporated in Ge–O–Ge bridges.