Ge−O Coordination in Cesium Germanate Glasses

Abstract

Neutron diffraction data with high real-space resolution are reported for an extensive series of cesium germanate glasses with 2, 5, 10, 15, 18, 21, 25, and 30 mol % Cs(2)O, and also for pure GeO(2) in its vitreous, quartz, and rutile forms. The results for pure GeO(2) show that neutron diffraction can clearly identify an increase in the Ge-O coordination number above the tetrahedral value of four. The results for cesium germanate glasses give strong evidence that the Ge-O coordination number rises to a maximum of 4.36 +/- 0.03 for 18 mol % Cs(2)O and then declines. This behavior may be associated with the germanate anomaly in the thermophysical properties. A model is developed for the composition-dependence of the Ge-O coordination number, and according to this model the rise in the coordination number involves the formation of mostly GeO(5) units, rather than GeO(6) units. Our results also show that the low alkali anomaly is a longer range phenomenon, and is not associated with a preference for the formation of nonbridging oxygens for very low alkali oxide content.

Sodium germanate glasses and crystals: NMR constraints on variation in structure with composition

“…Many spectroscopic and scattering studies of alkali germanate glasses have been reported, as reviewed recently [3,[5][6][7]. The overall pattern outlined above is generally supported, for example, by Raman scattering [8,9] and by X-ray and neutron scattering [10], including a recent, detailed study of cesium germanates [7].…”

A model for the Ge–O coordination in germanate glasses

“…1. The density of caesium germanate glasses as a function of composition [3,28]. The line is a guide to the eye.…”

“…We have recently used pulsed neutron diffraction to measure the coordination number, n GeO , for a large number of caesium germanate glasses, covering a wide range in composition [28], and these results are shown as closed circles with error bars in Fig. 5.…”

O K-edge XANES studies of alkali germanophosphate glasses

“…Recent O K-edge XANES studies of alkali germanate glasses indicate that the coordination of Ge does indeed increase with the addition of alkalis [11][12][13][14][15][16][17][18]. Oxygen Kedge XANES has proven to be a very sensitive probe to determine the coordination of the metal to which the oxygen is bound [11,19].…”