Experimental and Numerical Evidence of an Intermediate Phase in Sodium Phosphate Glasses

Alkali phosphate glasses are archetypical models for glass sealing materials, bioactive glasses, and solid-state electrolyte. The development of novel glasses with tailored properties requires an accurate knowledge of the linkages between composition, structure, and properties. Such relationships can be elegantly decoded within the framework of topological constraint theory, which reduces complex disordered atomic networks into simpler mechanical trusses. Here, based on modulated differential scanning calorimetry experiments performed on extremely dry sodium phosphate glasses, we report an intermediate phase with sharp boundaries, wherein glass transition is nearly reversible. Based on molecular dynamics simulations, we show that the glass compositions belonging to the intermediate phase exhibit an isostatic network, that is, their atomic network is topologically rigid but free of internal stress. These results shed new light into the origin of rigidity and stress transitions in oxide glasses.