Structure of Sodium Aluminogermanate Melts with Na2O/Al2O3 Ratio Greater Than Unity

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Abstract
The viscosities and densities of several series of ternary sodium aluminogermanate melts with Na2O/Al2O3 ratios greater than unity have been determined for temperatures between 1050° and 1450°C. The viscosity data suggest that aluminum (III) takes part in network formation in the form of AlO4 tetrahedra for all compositions studied. The molar‐volume data for the 40‐mole % Na2O melts and the 53‐ and 62‐mole % GeO2 melts can best be interpreted in terms of a model that involves GeO4 tetrahedra, AlO4 tetrahedra, and v̄ Na2O increases that are related to the Al2O3 contents. Molar volumes for 75‐mole % GeO2 melts are interpreted in terms of a model that involves GeO6 octahedra. Similar data for the 25‐mole % Na2O melts suggest the sudden elimination of GeO2 octahedra and a v̄ Na2O minimum with increase of Al2O3 content. This is supported by the expansion coefficient and viscosity results. It is concluded that GeO6 octahedra are stable at 1300°C only within a 10‐mole % Na2O semicircular ternary sweep of the 23‐mole % Na2O binary point.