Abstract
Iron phosphate glass is a versatile matrix for the immobilisation of various radioactive elements found in high-level nuclear waste (HLW). Quenched glass structures of iron phosphate glasses with Fe/P ratios of 0.33, 0.67 and 0.75 and with a composition of 40mol% Fe2O3 and 60mol% P2O5, with 4% and 17% Fe2+ ion concentrations were generated using molecular dynamics and the threshold displacement energies calculated. In the minimum energy structures, we found that in nearly all cases the P atoms were 4-fold coordinated. The potential energy per atom increased with increasing concentration of Fe2+ ions with similar Fe/P ratio, suggesting that decreasing the Fe2+ content is a stabilising factor. The average bond distances between Fe2+–O, Fe3+–O, P–O and O–O were calculated as 2.12, 1.88, 1.5 and 2.5Å respectively. The threshold displacement energy (Ed) was found to be dependent upon the ion specie, less for Fe2+ ions compared to Fe3+ ions, and was overall slightly lower than that determined for borosilicate glass.
Highlights
Phosphate glasses, due to their favourable properties such as: reasonably low liquid and glass transition temperatures, low viscosity, high thermal expansion coefficient, high electrical conductivity and high ultraviolet transmission, find application in a wide range of fields
The better chemical durability of iron phosphate glass is attributed to the presence of more hydration resistant Fe–O–P bonds compared to P–O–P bonds available in other phosphate glasses [16]
The structures of iron phosphate glasses with varying Fe/P atomic ratios were modelled as a function of density using the molecular dynamics simulation method
Summary
Due to their favourable properties such as: reasonably low liquid and glass transition temperatures, low viscosity, high thermal expansion coefficient, high electrical conductivity and high ultraviolet transmission, find application in a wide range of fields. According to the literature [6], in spite of the good glass forming characteristics of phosphate glasses, their relatively poor chemical durability limits their application, especially in the field of nuclear waste immobilisation. Better chemical durability and better corrosion resistance [13, 14] are certain promising features of iron phosphate glass compared to other phosphate glasses. The better chemical durability of iron phosphate glass is attributed to the presence of more hydration resistant Fe–O–P bonds compared to P–O–P bonds available in other phosphate glasses [16]. The available literature on the structural modelling of iron phosphate glasses is limited [21,22]. This is because, the structure of iron phosphate glasses, depends on composition, quenching temperature of the melt and quenching technique [23], it depends on the concentration of Fe2+ and Fe3+ ions in the glass
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