Abstract
Glasses of the system xGd2O3 · (100 − x)[7GeO2 · 3PbO] with 0 ≤ x ≤ 40 mol% were prepared using the melt quenching method. Lead germanate glasses are particularly interesting in the context of the germanate anomaly. In this paper, we investigate changes in the coordination number of germanium in gadolinium–lead germanate glasses using molar volume analysis, density measurements, FTIR and UV–VIS spectroscopy, and density functional theory (DFT). Despite some inconsistencies, the coordination change model remains the currently accepted model for the anomalous behaviour of lead germanate glasses. Based on these experimental results, we propose the following mechanism for the germanate anomaly. (i) The low thermodynamic stability of the [GeO6] structural unit and the occupation of interstices of larger dimensions (the six-coordinated interstices of the [PbO6] structural units) in the lead germanate network yield [GeO5] structural units with higher thermodynamic stability and larger ionic radii. (ii) Not linked to the terminal oxygens of the [GeO5] structural units and with the formation of smaller network cavities of the lead germanate glass, links are required with [GeO4] tetrahedra for stabilization, generating the formation of three-membered rings of [GeO4] tetrahedral structural units.
Published Version
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