Viscoelastic behavior and fragility of Se-deficient chalcogenide liquids in As-P-Se system

The viscoelastic behavior of supercooled Se-deficient liquids in the series As_xSe_100-x (40 ≤ x ≤ 60) and P_xAs_4–xSe₃ (0 ≤ x ≤ 2.8) are studied using parallel plate rheometry. The compositional variations in the viscosity and fragility of these liquids are shown to be consistent with the corresponding structural evolution. While the shear relaxation of As_xSe_100-x liquids with 40 ≤ x ≤ 50 is associated with the dynamics of AsSe bond scission/renewal, the As-rich liquids with x ≥ 55 are found to display an additional low-frequency process, which is related to a cooperative interconversion between molecular and network structural moieties. A similar behavior is also exhibited by the As-rich liquids in the P_xAs_4–xSe₃ series. In contrast, the P-rich liquids characterized by high molecule content display a power-law relaxation behavior resulting from a rather broad distribution of relaxation timescales associated with various dynamical modes of single molecules and molecular clusters.