On the germanium-oxygen coordination number in lead germanate glasses

Knowledge of the Ge-O coordination number, nGeO, in germanate glasses is of paramount importance for understanding the germanate anomaly and relating the glass structure to its thermophysical properties. By careful measurement using high resolution neutron diffraction on two samples of 40PbO.60GeO2 glass, it is clearly demonstrated that the average Ge-O coordination number, nGeO, is much less than the value, 4.77(5), previously obtained by use of neutron diffraction [Umesaki et al., Physica B 213 (1995) 490]. However, it is also clear that nGeO is still significantly greater than four (~ 4.1), and this is supported by the observed asymmetry of the Ge-O peak in the real-space total correlation functions. The average Pb-O coordination number is a little in excess of three, typical of Pb2 + with a sterically active lone-pair of electrons.