Melt dynamics, nature of glass transition and topological phases of equimolar GexAsxS100−2x ternary glasses

Abstract

• Flexible, Intermediate and Stressed-Rigid Phases of Ge x As x S 100−2x glasses observed. • Abrupt Rigidity- and Stress- Elastic Phase Transitions observed near x = 9.0% and 16.0%. • Melt Fragility index, m, are< 20 in the Intermediate Phase (IP) but> 20 outside the IP. • Kinetics of melt/glass homogenization slow down as x increases to the IP compositions. • Ge- and As centered Local Structures of Ge x As x S 100−2x glasses deduced from Raman scattering. The Topological Phases (TPs) in specially homogenized equimolar Ge x As x S 100−2x ternary glasses are established here by performing detailed Raman scattering, together with Modulated-DSC and Volumetric measurements over a wide range of compositions, 5%< x < 25%. Our results show the presence of an Intermediate Phase (IP) residing in the 9.0%< x < 16.0% range, with compositions x < 9.0% in the flexible phase, and compositions x > 16% in the stressed-rigid phase. The novel use of ex-situ Raman profiling on the entire batch of compositions indicates very slow dynamics that is tracked with time and composition and reveals the impact of homogenization on physico-chemical properties of glasses. Stressed-Rigid glasses exceeding the chemical threshold composition are all found to be nanoscale phase separated. In the presently synthesized specially homogenized melts/glasses, we observe (i) evidence for the elusive 537 cm −1 Raman active mode of the S = As stretch in quasi-tetrahedral S = As(S 1/2 ) 3 local structure, (ii) a square-well like variation of the non-reversing enthalpy of relaxation ∆H nr (x) at T g displaying the Reversibility window and defining the Intermediate Phase (IP), (ii) a square-well like variation of molar volumes, V m (x) coinciding with the IP composition range defining a Volumetric window, (iii) Melt fragility index, m(x), variation showing a global minimum in the IP compositions with m(x)< 20, and with the fragility index, m(x)> 20 for non-IP compositions, defining a Fragility Anomaly, and finally a variation in the specific heat jump near T g , ∆C p (x), that tracks part of the observed anomalies in m(x) and ∆H nr (x). The location of each of these anomalies/windows coinciding with those of the IP highlights the privileged nature of the window edge compositions that represent respectively rigidity- (x r = 9.0%)- and the stress- (x s = 16.0%) elastic phase transitions determined within the Topological Constraint Theory of glasses.