Rheology of the λ transition in liquid sulfur: Insights from arsenic sulfide liquids.

The frequency dependence of the storage and loss shear moduli and viscosity of AsxS100-x glass-forming liquids (x = 5, 10, 15, 40) are obtained over a frequency range covering nearly 15 orders of magnitude using parallel plate rheometry. The S-rich (x ≤ 15) liquids are characterized by a ring-to-chain structural transition near Tλ ∼ 120 °C, and their rheological behavior below Tλ strongly resembles that of long-chain and entangled polymers as well as that recently speculated for liquid sulfur above its λ transition. These AsxS100-x liquids are characterized by the coexistence of a slow and a fast relaxation process with similar activation energy. Both processes are coupled to viscosity, but differ in time scale by several orders in magnitude and are assigned, respectively, to the reptation and the Rouse dynamics of the Sn chains in these liquids. Such complex polymer-like rheological behavior disappears in the As40S60 liquid, characterized by corner-shared AsS3/2 pyramids, where a single average shear relaxation time typical of simple liquids instead emerges.