Evidence of Negative Heat Capacity, Rigidity Percolation and Intermediate Phase in Fast Ion Conducting Conditional Glass Forming System

Abstract: In this work, we observe the rigidity percolation phenomena in a fast ion conducting, conditional glass forming system (AgI)75-x-(Ag2O)25-(MoO3)x. To find out where, why and how the rigidity percolation phenomenon occurs within the range of 20 < x < 37.5, calorimetry and photoelectron spectroscopy experiments are performed. The temperature dependence of heat capacity (normalized) at glass transition temperature (Tg), exhibits fluctuations for samples with higher AgI concentration. This specific quality attributes to fragile glass. The wide range of composition accommodates both the fragile and strong glasses, and therefore a fragility threshold. The heat capacity (absolute) values, at Tg when plotted over the whole range of compositions, exhibits an abrupt sign shift, from negative to positive, revealing the fragility threshold. The appearance of negative heat capacity has been corroborated with the thermodynamic behavior of nanoclusters. This technique has been identified as a novel method to recognize the existence of nanoclusters in this type of glasses. The photoelectron spectroscopy study shows the formation of essential covalent structural units, [- Mo - O - Ag - O -] and complex molybdenum oxides in the positive heat capacity region. Finally, the non-reversing enthalpy profile has been studied over the whole composition range. The global, square well minima sandwiched between floppy and stress rigid region has been identified to be the intermediate phase, within the range 32.25 < x < 35.