The structure of SiO2–GeO2 glasses: A spectroscopic study

Four glasses of the SiO2–GeO2 binary system have been synthesized via a sol–gel route followed by a heat treatment and a quench. Glass structure has been determined by Ge K-edge X-ray absorption spectroscopy (XAS) at low temperature and Raman spectroscopy. These mixed glasses present a continuous random network of interconnected GeO4 and SiO4 tetrahedra, with GeO4 tetrahedra similar to the GeO4 units in GeO2 glass and continuous compositional variations from GeO2-rich regions to SiO2-rich regions. Such a random mixture is consistent with physical properties of these binary glasses as well as with the chemical dependence of their polyamorphism at high pressure. This EXAFS-derived mean Ge–O–Si angles are close to the Ge–O–Ge mean angle in GeO2 glass, 134° and 130°, respectively. This misfit with the Si–O–Si angles might explain the ease of formation of isolated and pair defects centers, which are suspected to be at the origin of photo-induced modifications of optical properties in Ge-bearing SiO2 glasses.