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

TL;DR

This study investigates rigidity percolation, negative heat capacity, and the intermediate phase in a fast ion conducting glass system, revealing compositional thresholds for fragility, structural units, and phase transitions using calorimetry and spectroscopy.

Contribution

It introduces a novel calorimetric approach to detect nanoclusters and characterizes the intermediate phase in a specific glass system with detailed compositional insights.

Findings

Negative heat capacity observed at certain compositions.

Identification of an intermediate phase between floppy and rigid networks.

Formation of covalent units and molybdenum oxides in the glass structure.

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.