Relationship between density, viscosity and structure of GeO 2 melts at low and high pressures

Abstract

A correlative study of the viscosity, density and structure of GeO 2 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 × 10 3 poise at 1 bar to 1.2 × 10 3 poise at 9.5 kbar. The maximum variation in the density of quenched GeO 2 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 GeO 2 melt quenched at 1 atm (1425°C) and at 18 kbar (1700°C) implies that the scattering units in GeO 2 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 GeO 2 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 GeO symmetric stretching mode in the two polymorphs. A comparison of the Raman spectra of GeO 2 glasses with those of crystalline GeO 2 polymorphs shows that the local ordering in GeO 2 glass is similar to that of hexagonal GeO 2 in which Ge is tetrahedrally coordinated. The decrease in the viscosity of GeO 2 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 GeO bond with increasing pressure. The increase in the degree of local ordering in GeO 2 melts quenched at high pressures does explain the progressive increase in melt density.