Structure of Molten Oxides. II. A Density Study of Binary Germanates Containing Li2O, Na2O, K2O, and Rb2O

Abstract

An improved counterbalanced sphere viscometer—densitometer was used to obtain density information between 1050° to 1500°C for a series of binary germanate melts containing as much as 60 mole% Li2O, Na2O, K2O, and Rb2O. Densities and the expansion coefficient of molten GeO2 between 1100° to 1400°C were estimated from the binary data. The molar volume, expansion coefficient, partial molar volume and partial molar expansion deviations suggest a mode of network alteration that differs from that observed for silicate melts. This confirms the previously reported viscosity results for binary alkali germanate melts. It is suggested that a gradual change from tetrahedral to octahedral configuration occurs for GeO2 with the addition of alkali oxide. The maximum stability of octahedrally coordinated germanium at 1300°C appears to occur for melts containing from 15 to 30 mole% M2O. A rapid return to tetrahedral configuration appears to take place for most germanate melts as the alkali oxide content is increased above 30 mole%. These alkali-rich germanate melts possess properties that are similar to those found for their silicate analogs. It is suggested that discrete anions also exist in these alkali rich germanate melts.