Raman-spectroscopy studies on rigidity percolation and fragility in Ge–(S,Se) glasses

We have investigated the Raman spectra in a range 3–100 cm−1 in chalcogenide glasses, GexS1−x and GexSe1−x, with 0⩽x⩽0.42 in the average coordination number, 〈r〉, from 2 to 2.84. The relation between the rigidity percolation, based on the prediction of the mean-field constraint theory, and a relative degree of fragility, representing the structural relaxation of the network, are demonstrated by varying the structures of the glasses. The obtained fragility of the glasses for x<0.20 (〈r〉<2.4) is larger than that for x>0.20. A decrease of the fragility at about 〈r〉=2.4 is attributed to the predicted rigidity percolation threshold where the glass changes from floppy to rigid, and mesoscopic structural changes beyond the mean-field theory. For 〈r〉>2.4, fragility depends on chemical bonds between Ge–Se and Ge–S systems, since the local structure directly contributes to the properties of the overcoordinated network.