RELATIONSHIP BETWEEN DENSITY, VISCOSITY AND STRUCTURE OF GEO2 MELTS AT LOW AND HIGH-PRESSURES

A correlative study of the viscosity, density and structure of GeO2 melts has been carried out at low and high pressures. It is observed that under isothermal (1425°C) conditions the viscosity decreases from 6.0 × 103 poise at 1 bar to 1.2 × 103 poise at 9.5 kbar. The maximum variation in the density of quenched GeO2 glasses is from 3.62 ± 0.02 g cm-3 for glass formed from a liquid quenched at 1 atm and 1425°C to 3.95 ± 0.04 g cm-3 for glass formed from a liquid quenched at 18 kbar and 1700°C. The similarity of the Raman spectra of GeO2 melt quenched at 1 atm (1425°C) and at 18 kbar (1700°C) implies that the scattering units in GeO2 glasses quenched at low and high pressures are the same. The intensity of the Rayleigh tail, however, decreases in glasses quenched at successively higher pressures, the structure apparently becoming more ordered with increasing pressure. The Raman spectra of the GeO2 polymorphs with quartz and rutile structures, crystallized at 1150°C and at 4 and 6 kbar, respectively, are distinct because of the difference in Ge coordination, resulting in a large difference in the frequency and intensity of the GeO symmetric stretching mode in the two polymorphs. A comparison of the Raman spectra of GeO2 glasses with those of crystalline GeO2 polymorphs shows that the local ordering in GeO2 glass is similar to that of hexagonal GeO2 in which Ge is tetrahedrally coordinated. The decrease in the viscosity of GeO2 melt with increasing pressure cannot be attributed to a pressure-induced coordination change [1]. More likely, there is a systematic change in the nature of the GeO bond with increasing pressure. The increase in the degree of local ordering in GeO2 melts quenched at high pressures does explain the progressive increase in melt density. © 1979.