X-ray photoelectron spectroscopy study of the germanate anomaly in the Na2O-GeO2 system

Photoelectron spectra of the O is and Ge 3d levels have been obtained from a series of Na 2 O-GeO 2 glasses of general formula xNa 2 O.(1-x)GeO 2 (x=0.125, 0.175, 0.2 and 0.3) and from powders of quartz-like GeO 2 and rutile-like GeO 2 . The O 1s binding energy is unchanged in both crystalline polymorphs but the binding energies of Ge 3d differ by 0.7±0.1 eV. Symmetric O Is peaks from samples with x≤0.2 are assigned to bridging oxygens connecting combinations of 4 and 6 coordinated Ge. The O is spectrum from the x=0.3 sample contains contributions from bridging and nonbridging oxygens. Ge 3d spectra from the glasses have been fitted to contributions from 4 and 6 coordinated Ge giving values for N 6 (the fraction of 6 coordinated Ge) consistent with the theoretical relationship N 6 =x(1-x) -1 for x≤0.2. For the x=0.3 sample N 6 is lower than predicted due to replacement of 6 coordinated Ge by 4 coordinated Ge plus nonbridging oxygens. This lower value of N 6 would require there to be 20±3% nonbridging oxygens present and this is reasonably consistent with the value of 25±2% obtained by fitting the O 1s spectrum.