The excellent chemical durability of iron phosphate glasses makes them potential hosts to vitrify high-level nuclear waste (HLW).^1–3 The properties of iron phosphate glasses depend on glass structure, which is determined by the type and concentration of metal cations and the resulting phosphate chain length distributions.^4–8

The structural complexity of iron phosphate glasses is increased by the variation in the Fe-redox conditions and by disproportionation reactions of phosphate anions in the melts.⁴ The relative fraction of Fe²⁺/Fe_tot is typically about 20% for the glasses obtained by conventional melting (in air) and quenching techniques.^2,9,10 Disproportionation reactions produce the wider distributions of phosphate chains, which are related to the glass formation energy.^11,12 The Flory distribution model describes the distribution of chains in phosphate melts:¹³