Correlation between structural and physical properties in Ge-Sb-Se glasses

Three groups of Ge-Sb-Se glasses with compositions Ge_xSb ₁₀Se_90-x, Ge_xSb₁₅Se_85-x, and Ge_xSb₂₀Se_80-x have been systematically studied with the aim of understanding the role of chemical composition and mean coordination number (MCN) in determining their structural and physical properties. For each group of glasses, it was found that the optical bandgap increases and the refractive index decreases with increasing Ge concentration up to a transition point which corresponds to glasses that have chemically stoichiometric compositions. Raman spectra were measured and decomposed into different structural units. While the relative number of the heteropolar bonds changes in a reasonable manner with chemical composition, the evolution of the optical bandgap and refractive index correlated closely with the number of the homopolar bonds, suggesting that the band-tails formed by homopolar bonds could reduce the optical bandgap. On the other hand, the transitions at the chemically stoichiometric compositions could be attributed to "demixing" of networks above the chemical thresholds. These transition thresholds in the three groups of glasses demonstrated that the chemical composition has significant effects on the physical properties in the Ge-Sb-Se system.