Fe-dependent structural evolution of peralkaline soda aluminosilicate glasses: Iron speciation vs. glass transition

The incorporation of iron into peralkaline silicate glasses significantly impacts their structural and thermal properties. Here we investigate how addition of iron influences the network connectivity (short- and medium-range order) and glass transition temperature (T_g) with particular regard to the iron speciation and the Fe²⁺ and Fe³⁺ coordination state. We also found a sort of tipping point in iron concentration beyond which the short-range structures evolve linearly with density while the medium-range structure deviates from linearity. This behavior seems related to a re-enrichment of tetrahedral units triggered by iron self-compensation effect explaining the observed jump in T_g.