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 chalcogen ide glasses, GexS1-x and GexSe1-x, with 0 less than or equal to x less than or equal to 0.42 in the average coordination number, [r], from 2 to 2.84. The relation between the rigidity percolation, based on the prediction of t he mean-held constraint theory, and a relative degree of fragility, represe nting 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 fra gility at about [r] = 2.4 is attributed to the predicted rigidity percolati on threshold where the glass changes from floppy to rigid, and mesoscopic s tructural changes beyond the mean-field theory. For [r] > 2.4, fragility de pends on chemical bonds between Ge-Se and Ge-S systems, since the local str ucture directly contributes to the properties of the overcoordinated networ k. (C) 2000 Elsevier Science B.V. All rights reserved.