Phase separation in GeO–GeO2 glasses

A series of transparent homogeneous amorphous solids between GeO2 and GeO has been prepared by rapid quenching of liquids and by vapour deposition. Density and refractive index increase smoothly with increasing Ge/O ratio and provide a useful index of composition. Glasses in the range GeO–GeO1.85 undergo a rapid exothermic transformation when heated above 400°C accompanied by a pronounced blackening, an increase in refractive index, and slight decrease in density. Electron microscopy and diffraction results indicate that the transformation involves the precipitation on a 50–100 A scale of an amorphous tetrahedrally coordinated Ge-rich phase. The interconnected microstructure so produced is suggestive of spinodal decomposition. A transitional region near GeO1.9 is characterized by sluggish phase separation kinetics and a dispersed particle morphology suggestive of a nucleation and growth precipitation mechanism. Glasses in the range GeO1.9–GeO2 show no evidence for phase separation.