Observation of a sharp reversibility window in rejuvenated homogenized equimolar Ge_xAs_xS_100-2x bulk glasses

We have synthesized 1.5-gram sized batches of the title glasses over a wide composition range 2% ≤ x ≤ 25%, taking special care to alloy the starting materials for an extended period (several weeks) in evacuated quartz tubes, and recording FT Raman spectra along the length of the melt column till the spectra became identical. Using Modulated DSC, we then examined calorimetric properties, T_g (x), ΔC_p , and ΔH_nr (x), and find T_g(x) to increase monotonically with x, and the enthalpy of relaxation, ΔH_nr (x) to display a sharp square-well like reversibility window with an onset near x =9.0(5)% (rigidity transition) and an endpoint x = 16.0(5)% (stress transition) in rejuvenated glasses. In FT- Raman scattering we observe modes of Ge-centered CS and ES tetrahedra and the As-centered Pyramidal and Quasi-Tetrahedral units besides modes of S₈ rings and polymeric S_n chains. The mode near 537 cm^-1 due to S=As(S_1/2) QT units is observed in FT-Raman but not in dispersive Raman using 514 nm excitation. Complex C_p measurements using MDSC are being undertaken to establish variation in melt fragility index.

1. Tao Qu and P. Boolchand Phil. Mag B 85, 875(2005).