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 K2O-GeO2 glasses (11 and 20 mol% K2O). On the long-distance side of the first-neighbour Ge-O peak lying at 175 pm, which is likewise found for vitreous GeO2, appears a clear shoulder. This feature indicates the existence of GeOn units larger than GeO4 tetrahedra. The shoulder is not resolved as a separate peak which could be attributed to Ge-O bonds of GeO6 octahedra unambiguously. Hence, it cannot be differentiated between GeO5 or GeO6 polyhedra being formed in addition to the GeO4 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% K2O where the additional oxygen atoms are incorporated in Ge-O-Ge bridges. (C) 1999 Elsevier Science B.V. All rights reserved.