The Intermediate Phase in Ternary GexAsxSe1-2x Glasses

Melt-quenched AsxGexSe1-2x glasses over the composition range, 0 < x < 0.26, are examined in Raman scattering, T-modulated Differential Scanning Calorimetry (MDSC), and Sn Mossbauer spectroscopy measurements. The non-reversing enthalpy near Tg, ∆Hnr(x), accessed from MDSC shows a global minimum (~ 0) in the xc(1) = 0.09 < x < xc(2) = 0.16 range, and increases by an order of magnitude both at x < xc(1) and at x > xc(2). Raman mode frequency of corner-sharing Ge(Se1/2)4 tetrahedra studied as a function of x, also shows three distinct regimes (or power-laws, p) that coincide with ∆Hnr(x) trends. These regimes are identified with mechanically floppy (x < xc(1)), intermediate (xc(1) < x < xc(2)), and stressed-rigid (x > xc(2)) phases. The Raman elasticity power-law in the intermediate phase, p1 = 1.04(3), and in the stressed rigid phase, p2= 1.52(5), suggest effective dimensionalities of d = 2 and 3 respectively. STRUCTURE BASED CLASSIFICATION OF GLASSES In the early 1980s, Phillips [1] and independently Thorpe [2] suggested that a network of polymeric chains (weakly crosslinked) will spontaneously stiffen or become rigid when chain-cross-linking acquires a threshold value. In covalent systems, one usually expresses the degree of cross-linking in terms of a mean coordination number, r . The floppy to stressed rigid elastic phase transition was predicted to occur near r = 2.40. However, recent Raman scattering [3-5] and independently T-modulated DSC experiments [3-8] on chalcogenide glasses have shown that there are in fact two (rigidity transitions) and not one transition as predicted by mean-field constraint theory. Thus, the onset of stressed rigidity in disordered systems display a far richer structure [9] than previously believed. Specifically, the frequency of corner-sharing Ge(Se1/2)4 tetrahedra in GexSe1-x glasses [5] display a kink (change in slope) near x = 0.20 (or r = 2.40 ; transition 1) , and a discontinuous jump between x = 0.25 and 0.26 (r = 2.55; transition 2) followed by a distinct power-law at higher x. Parallel results are now available in binary Si-Se [3], As-Se[6] and P-Se [7] glasses. These results provide evidence for the opening of intermediate phases [9-12] between floppy and stressedrigid ones in disordered networks. The physical picture of elastic phase transitions in network glasses above suggests that one can generically classify these disordered networks into three distinct phases, floppyCC8.1.1 Mat. Res. Soc. Symp. Proc. Vol. 754 © 2003 Materials Research Society