Evolution of the germanium–oxygen coordination number in lithium–lead–germanate glasses

Lithium–lead–germanate glasses in the xLi2O·(100-x)[7GeO2·3PbO] system, where 0 ≤ x ≤ 40 mol% Li2CO3 exhibit a “germanate anomaly”, which is a maximum in their density profiles with the addition of Li2CO3 up to 20 mol%. The structural characterization of the samples was performed by investigations of Raman spectroscopy and X-ray scattering analysis.An EXAFS analysis of the spectra collected at the germanium K-edge for series of lithium–lead–germanate glasses is also presented. With the EXAFS-extracted structural parameters were reconstructed to the local neighborhood of Ge atoms and were verified current opinions on the germanate anomaly in the considered glasses. The formation of higher-coordinated Ge species in the glassy network can be correlated with the density anomaly.Several structural mechanisms can be summarized by increasing of lithium carbonate in the lead–germanate glassy: i) depolymerization of germanate network; ii) conversion of 3- to 4-membered [GeO4] rings; iii) transformation of connected [GeO6] structural units into isolated [GeO6] structural units and a conversion part of higher to lower coordination number of germanium atoms.