Structure and properties of iron borophosphate glasses

Thermal stability, structure and aqueous chemical durability of glasses of nominal composition 60P2O5-40Fe2O3 (mol%) doped with up to 20% B2O3 have been investigated. B2O3 was substituted for either P2O5 or Fe2O3, or was added on a pro rata basis. No substantial melt volatilisation was detected, and refractory corrosion was minimal. Iron cations of both redox states occupied distorted octahedral sites; B2O3 additions did not affect iron coordination, although Tg and the Fe2+/ΣFe redox ratio increased. Addition of up to 10% B2O3 did not significantly affect durability but it improved thermal stability and increased liquidus temperature. Chemical durability (as measured by the product consistency test, PCT) decreased by approximately one order of magnitude for every further 5% B2O3 addition above 10% when substituted for Fe2O3. Thermal analysis indicated increasing glass stability with increasing B2O3 content. X-ray diffraction showed that one sample of nominal composition 50P2O5-40Fe2O3-10B2O3 developed crystalline B0·57Fe0·43PO4 or a similar phase during cooling. There was no evidence that this significantly affected chemical durability. The addition of B2O3 at low levels (<10%) may be useful in improving the performance of iron phosphate glasses for applications such as waste immobilisation or sealing glasses.