Structural origin of fragility in Ge-As-S glasses investigated by calorimetry and Raman spectroscopy.

The fragility index (m) of a series of Ge–As–S glasses covering a wide range of mean coordination (⟨r⟩) and stoichiometry is measured by differential scanning calorimetry. The evolution of the fragility index appears to be better predicted by the stoichiometry than the mean coordination, and m shows a well-defined dependence on the departure from stoichiometry quantified as the at. % excess or deficiency in sulfur. The effect of stoichiometry on the fragility of Ge–As–S glasses is very similar to that observed in the Ge–As–Se system. A systematic Raman spectroscopy investigation also indicates the presence of large fractions of molecular species such as S8, As4S4, and As4S3 in the structure of some glasses. The presence of molecular species is shown to increase the fragility and decrease the glass transition temperature (Tg). These results emphasize that short-range order rather than long-range characteristics such as structural rigidity appears to control the fragility of chalcogenide glasses.