Glass formation and anomalous annealing effects in the mixed oxyanion system AgIAg₂SO₄Ag₂WO₄

The glass-forming region in the ternary mixed anion system AgIAg₂SO₄Ag₂SO₄Ag₂WO₄ has been delineated and the enthalpic manifestation of the glass transition has been examined. By selecting compositions along the tie-line 60AgI · (40 - x)Ag₂SO₄ · xAg₂WO₄ and varying x it was possible to study the effect of replacing the SO₄^2- ion by the larger WO₄^2- ion, at a fixed AgI content. The glass-to-liquid transition is found to evolve over an unusually wide range of temperature in the ternary glasses relative to the binary ones indicating that the mixing of anions of different sizes leads to a dramatic broadening of the spectrum of structural relaxation times. Consistent with this observation, the DSC scans of annealed ternary samples reveal prominent pre-T_g endothermic peaks which have previously been seen only in polymer melts (such as pure PVC) known for highly non-exponential relaxation. Possible structural origins, and applications of the T_g broadening phenomenon are discussed.