Glassy materials with enhanced thermal stability

The nature of glass transitions in chalcogenides and modified oxides depends on the network mean coordination number < r >. These display systematic trends when spanning across the three topological phases: flexible, intermediate, and stressed-rigid. Trends in the glass-transition temperature T-g(< r >) show a monotonic increase with < r >, but the nonreversing enthalpy of relaxation at T-g, Delta H-nr(< r >), shows a deep-and square-well-like minimum with the walls representing the rigidity and stress transitions with increasing < r >, respectively. In the well, the. Delta H-nr (< r >) term remains minuscule (similar to 0) corresponding to the isostatically rigid intermediate phase (IP). The melt fragility index (m) shows rather low values, m(< r >) < 20 for IP compositions, but increases outside the IP. Glass compositions in the IP show absence of network stress, form compacted networks, possess thermally reversing glass transitions, and display high glass-forming tendency-functionalities that have attracted widespread interest in understanding the physics of glasses and applications of the new IP formed.