Sources of Configurational Entropy versus Compositional Trends in Fragility of Inorganic Glass‐Forming Liquids

Abstract

The effect of the average network connectivity ⟨r⟩ on the configurational entropy S conf plays a key role in governing the compositional evolution of the fragility index m for chalcogenide, silicate, and phosphate glass‐forming liquids (GFLs). The effect of ⟨r⟩ on S conf, and thus on m in chalcogenide and phosphate GFLs can be explained using the self‐avoiding walk model for a chain of either chalcogen atoms (e.g., Se) or PO4 tetrahedra, with various degrees of cross‐linking. It is shown that while a disjointed chain is suitable for modeling the inverse relationship between m and ⟨r⟩ in chalcogenides, semiflexible chains with a higher degree of bending stiffness are more appropriate for modeling the phosphate liquids. In contrast, the silicate and aluminosilicate GFLs exhibit a fundamentally different pattern, which is indicative of S conf being controlled by the structural relaxation of a dynamically percolative network via the chemical exchange between bridging and non‐bridging oxygen. In contrast to silicates and phosphates, in the case of borate and germanate GFLs, the temperature dependence of the relative fractions of B or Ge atoms in multiple coordination states becomes the predominant contributor to the , which is consistent with their apparently anomalous trend of m increasing with ⟨r⟩.