Neutron, X-ray diffraction, DSC, Raman, Mössbauer and leaching studies of iron phosphate glasses and crystalline phases

Abstract

xFe₂O₃–(100 − x)P₂O₅ glasses were synthesized by melt quenching and structure–property correlation studies were carried. Glasses containing 25 to 40 mol% Fe₂O₃ were prepared while the sample with 50 mol% Fe₂O₃ formed a crystalline sample containing Fe₃²⁺Fe₄³⁺[PO₄]₆³⁻ and Fe₂²⁺[P₂O₇]⁴⁻ phases on melt-quenching. Glass density increases from 2.98 to 3.20 g cm⁻³, ionic packing fraction is in the range of 0.63–0.65 and the glass transition temperature decreases from 500 °C to 493 °C on increasing Fe₂O₃ concentration from 25 to 40 mol%. Pair distribution function analysis and Reverse Monte Carlo simulations of neutron diffraction datasets were used to calculate the atomic pair distributions, interatomic distances and co-ordination environments. The P–O co-ordination is essentially tetrahedral and is in the range: 3.9–3.7 (±0.1), the Fe–O co-ordination number decreases steadily from 4.8 to 4.2 (±0.1) with an increase in Fe₂O₃ concentration in the phosphate network, while O–O co-ordination is in the range: 6.6–6.3(±0.1), the decrease in these co-ordination numbers are due to an increase in oxygen deficiency in the glass network with an increase in Fe₂O₃ mol%. Fe–O and P–O pair distributions are asymmetrical indicating short-range disorder due to the existence of a wide range of bond-lengths with maxima at 1.79 Å and in the range: 1.45–1.51 Å respectively. Mössbauer studies carried out at room temperature and 80 K found that Fe exists in 2+ and 3+ valence states, and the glass and crystalline samples contained Fe²⁺ at least at three different sites. Raman studies found that the meta and pyrophosphate structural units are dominant species up to 35 mol% Fe₂O₃ concentration, while the orthophosphate units are in majority at 40 mol% of Fe₂O₃. The crystalline sample is a two phase material and contained both orthophosphate and pyrophosphate units with the former being the dominant species. Leaching studies on two iron phosphate glasses carried out in purified water at 90 °C found that dissolution of glasses decreases and the chemical durability increases drastically with an increase in Fe₂O₃ mol%.