O K-edge XANES studies of alkali germanophosphate glasses

Alkali germanophosphate glasses exhibit a ‘germanate anomaly’, which is a maximum or minimum in their density profiles with the addition of alkali oxide. Several structural mechanisms have previously been recognized to affect the density behavior of these glasses: (1) depolymerization of the PO₄ tetrahedral network; (2) conversion of 4- to 3-membered GeO₄ rings and (3) depolymerization of the GeO₄ network through the formation of non-bridging oxygens. A coordination change from 4- to 5-fold Ge has been detected in alkali germanate glasses, which also exhibit the germanate anomaly. In this study, oxygen K-edge XANES of alkali germanophosphate glasses is used to investigate the coordination change of Ge with increasing alkali content and at various Ge to P ratios. The dominant peak at ～538 eV in the spectra undergoes a shift to lower energy of ～0.4 eV, indicating the formation of 5-fold Ge similar to previous studies on alkali germanate glasses. However, the formation of 5-fold Ge does not correlate with the density anomaly compositions of these glasses suggesting that the appearance of higher-coordinated Ge species is not controlling the anomaly behavior.