Linking molecular origin of melt fragility index with topological phases of network glasses

Network glasses are composed of three topological phases: flexible, intermediate, and stressed-rigid. Intermediate Phase (IP) glasses form compact, rigid, and stress-free networks with no Open Degrees of Freedom (ODF). IP melts are observed to be super-strong with the lowest fragility index, m = 15 due to the absence of ODF in the parent glass. This feature is general and observed in multiple selenide, sulfide, and modified oxide glasses can be traced to Isostatically Rigid Local Structures (ISRLSs) formed in the IP. On the other hand, the flexible and stressed-rigid phase glasses form non-compacted and stressed networks with increasing residual pressure as the count of ODF increases. Their melts are found to be fragile and display fragility indices m > 20 as the count of the ODF in the glass increases and the count of ISRLSs dissipates.