Structure Of Iron Phosphate Glasses Research Articles

Effect of molybdenum oxide addition on the durability and structure of iron phosphate glasses

Effect of molybdenum oxide addition on the durability and structure of iron phosphate glasses

Structural investigations on Mo, Cs and Ba ions-loaded iron phosphate glass for nuclear waste storage application

Borosilicate glass is considered to be the standard matrix for immobilization of fission products obtained from the reprocessing of nuclear fuel. However, nuclear waste containing constituents like molybdenum oxide (MoO3) and cesium oxide (Cs2O) are less soluble in borosilicate glass. Hence iron phosphate glass (IPG) is being contemplated as an alternate vitrification matrix since it shows better solubility for these constituents. However, radioactive 137Cs undergoes β− decay to yield 137Ba. In the present work, iron phosphate glass co-loaded with Mo/Cs/Ba ions has been synthesized using the melt quench technique to study their effect on the structure of IPG. The homogeneity, structure and composition of IPG and Mo/Cs/Ba ions co-loaded IPG has been investigated using powder X-ray diffraction, scanning electron microscopy (SEM), electron diffraction spectroscopy (EDS), Mössbauer, FTIR and Raman spectroscopy. A gradual structural modification of the undoped IPG is indicated by the FTIR and Raman spectra with increase in Mo/Cs/Ba ions loading in IPG samples. Detailed thermal analysis of all the glass samples have been done to study their glass stability and glass forming ability which has been estimated in terms of the characteristic temperatures obtained from thermogravimetric analysis (TGA) and differential thermal analysis (DTA) data. The correlation between glass forming ability and the evaluated critical cooling rate of the glasses has been analyzed. Presence of Fe2+ ions has been observed in all the IPG samples as confirmed from Mössbauer spectroscopy. The structure at the atomic scale has been studied in detail using Fe/Mo K-edge X-ray absorption spectroscopy (XAS). The IPG glass co-loaded with Mo/Cs/Ba ions exhibits higher glass transition temperature, comparable glass stability parameters, superior glass forming ability compared to unloaded IPG. Also, a slight modification in the glass structure in Mo/Cs/Ba ions co-loaded IPG as indicated by FTIR, Raman, EXAFS and XANES studies, suggests the potential application of IPG as a suitable vitrification matrix for nuclear wastes having these constituents.

Laboratory Based X-ray Absorption Spectroscopy of Iron Phosphate Glasses for Radioactive Waste Immobilisation: A Preliminary Investigation.

We report the application of laboratory based X-ray absorption spectroscopy to the speciation of Fe in iron phosphate glasses prepared by conventional and microwave melting. Analysis of the weak pre-edge features in Fe K-edge XANES data demonstrated glasses produced by microwave melting to have a higher fraction of reduced Fe2+ species, since microwave melts do not have sufficient time to equilibrate with the prevailing oxygen partial pressure, compared to counterparts produced by conventional melting. Furthermore, our laboratory XANES data are consistent with the formation of octahedral Fe2+ at the expense of tetrahedral Fe3+ species, with increasing Fe2+ content. These findings are consistent with the previous findings of our 57Fe Mossbauer study, synchrotron XANES data, and current understanding of the structure of iron phosphate glasses, and demonstrate the utility of laboratory based XANES for routine speciation of Fe in these and other materials.

BaO-Fe2O3-P2O5 glasses: Understanding the thermal stability

The effect of barium oxide on the structure of iron phosphate glass was investigated by Raman spectroscopy to understand the better thermal stability of barium loaded iron phosphate glasses compared to iron phosphate glass containing caesium oxide. Glasses with composition (mol %) of 36.1 Fe2O3-54.1 P2O5- 9.8BaO and 32.2 Fe2O3-48.3 P2O5- 19.5BaO revealed the existence of higher concentration of meta phosphate linkage, which could be a reason for their enhanced thermal stability. In addition, the enthalpy increment data were obtained by isoperibol drop calorimetry on these glasses. These data of barium oxide loaded iron phosphate glasses were compared with those of caesium loaded iron phosphate glasses. The specific heat of pristine IPG is always higher than that of IPG containing either BaO or Cs2O, showing the effect of modifiers (Ba2+ or Cs+) in glass. Depending on the concentration of the modifier, the density of the glass changes and higher the density lower was the specific heat of the glass.

Iron phosphate glasses: Bulk properties and atomic scale structure

Bulk properties such as glass transition temperature, density and thermal expansion of iron phosphate glass compositions, with replacement of Cs by Ba, are investigated as a surrogate for the transmutation of 137Cs to 137Ba, relevant to the immobilisation of Cs in glass. These studies are required to establish the appropriate incorporation rate of 137Cs in iron phosphate glass. Density and glass transition temperature increases with the addition of BaO indicating the shrinkage and reticulation of the iron phosphate glass network. The average thermal expansion coefficient reduces from 19.8 × 10−6 K−1 to 13.4 × 10−6 K−1, when 25 wt. % of Cs2O was replaced by 25 wt. % of BaO in caesium loaded iron phosphate glass. In addition to the above bulk properties, the role of Ba as a network modifier in the structure of iron phosphate glass is examined using various spectroscopic techniques. The FeII content and average coordination number of iron in the glass network was estimated using Mössbauer spectroscopy. The FeII content in the un-doped iron phosphate glass and barium doped iron phosphate glasses was 20, 21 and 22 ± 1% respectively and the average Fe coordination varied from 5.3 ± 0.2 to 5.7 ± 0.2 with increasing Ba content. The atomic scale structure was further probed by Fe K-edge X-ray absorption spectroscopy. The average coordination number provided by extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure was in good agreement with that given by the Mössbauer data.

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FTIR spectra and thermal properties of TiO2-doped iron phosphate glasses

Structure and thermal properties of xTiO2·(90−x) (60P2O5–40Fe2O3)⋅10CaF2 (x=0, 5, 10, 15, 20 and 25mol%) glasses are investigated in detail by Fourier Transform Infrared Spectrum (FTIR) and Differential Thermal Analysis (DTA), respectively. It is found that incorporation of TiO2 increase the density and glass transition temperature of iron phosphate system glass. The increment of doped-TiO2 can also strengthen phosphate network chains due to increasing O/P ratios and more orthophosphate (Q0) units formed in the glass structure at expense of pyrophosphate (Q1) units and metaphosphate (Q2) groups. Moreover, the structure of iron phosphate glass with TiO2 content contain distorted octahedral [TiO6] linked to phosphate unit through POTi bonds, thus enhanced structure cohesion and increased density obtained. The knowledge provides an improved understanding of the role of TiO2 in the structure of iron phosphate glasses.

Structure of iron phosphate glasses modified by alkali and alkaline earth additions: neutron and x-ray diffraction studies

The structure of iron phosphate glasses modified by additions of K2O and BaO, with nominal molar compositions [(1 − x)(0.6P2O5–0.4Fe2O3)]xMeyO, where x = 0–0.4 in increments of 0.1; Me=K or Ba; and y = 1 or 2, has been investigated using neutron diffraction and x-ray diffraction techniques. Fitted coordination numbers for P–O and Fe–O showed a notable change in the P–ONBO and P–OBO contributions at greater than 20 mol% modifier addition, with barium producing a markedly larger increase in P–ONBO contribution than potassium. Fitting of TN(r) and TX(r) provided average nBaO = 9 and nKO = 6. Iron occurs in a range of coordination sites in these glasses: [6]Fe2+, [4]Fe3+, [5]Fe3+ and [6]Fe3+. The partitioning between these sites gives average nFeO = 5.2–5.8, with barium-doped glasses exhibiting higher average nFeO than potassium-doped glasses. The stronger depolymerizing effect of Ba2+ than K+ on the phosphate network, coupled with its greater ionic charge and higher Me–O, Fe–O and O⋯O coordination numbers, explain previously observed divergences in physical properties between the barium-doped and the potassium-doped glasses.

Chemical durability and Structural approach of the glass series (40-y) Na2O-yFe2O3-5Al2O3-55P2O5-by IR, X-ray diffraction and Mössbauer Spectroscopy

The relationship between the composition and structure for the glasses of general composition (40-y)Na2O-yFe2O3-5Al2O3-55P2O5 (5≤y≤20), has been studied. The chemical durability and density of these glasses increase with increasing Fe2O3 content. The dissolution rate (DR), calculated from the weight loss in distilled water at 90°C for up to 20 days was ≈ 3.10−9 g/cm2/mn which is 30 times lower than that of window glass. The structure and valence states of the iron ions in the glasses were investigated using, X-ray diffraction, 57Fe Mössbauer spectrometry, potentiometric analysis, and infrared spectroscopy. Both Mössbauer spectrometry and potentiometric analysis allow to estimate both Fe2+ and Fe3+ contents in all these glasses. X-ray diffraction indicates that the local structure of iron phosphate glasses is related to the short range structures of NaFeP2O7. Infrared spectra indicate the formation of P–O–Fe bonds in the pyrophosphate glasses that replace P–O–Na bonds. The presence of a small content of Al2O3 in the glass seems to play a role as a network modifier. The addition of Fe2O3 to Al2O3 in phosphate glasses favours the enhancement of the formation of pyrophosphate units because iron ions have stronger effect on the depolymerization of metaphosphate chains than the aluminium ions. Finally, the I.R spectra indicate that the presence of P-O-Fe bands of these glasses containing more than 15 mol%Fe2O3 is consistent with their good chemical durability.

Effects of modifier additions on the thermal properties, chemical durability, oxidation state and structure of iron phosphate glasses

Modified iron phosphate glasses have been prepared with nominal molar compositions [(1−x)·(0.6P2O5–0.4Fe2O3)]·xRySO4, where x = 0–0.5 in increments of 0.1 and R = Li, Na, K, Mg, Ca, Ba, or Pb and y = 1 or 2. In most cases the vast majority or all of the sulfate volatalizes and quarternary P2O5–Fe2O3–FeO–RyOz glasses or partially crystalline materials are formed. Here we have characterized the structure, thermal properties, chemical durability and redox state of these materials. Raman spectroscopy indicates that increasing modifier oxide additions result in depolymerization of the phosphate network such that the average value of i, the number of bridging oxygens per –(PO4)– tetrahedron, and expressed as Qi, decreases. Differences have been observed between the structural effects of different modifier types but these are secondary to the amount of modifier added. Alkali additions have little effect on density; slightly increasing Tg and Td; increasing α and Tliq; and promoting bulk crystallization at temperatures of 600–700 °C. Additions of divalent cations increase density, α, Tg, Td, Tliq and promote bulk crystallization at temperatures of 700–800 °C. Overall the addition of divalent cations has a less deleterious effect on glass stability than alkali additions. 57Fe Mössbauer spectroscopy confirms that iron is present as Fe2+ and Fe3+ ions which primarily occupy distorted octahedral sites. This is consistent with accepted structural models for iron phosphate glasses. The iron redox ratio, Fe2+/ΣFe, has a value of 0.13–0.29 for the glasses studied. The base glass exhibits a very low aqueous leach rate when measured by Product Consistency Test B, a standard durability test for nuclear waste glasses. The addition of high quantities of alkali oxide (30–40 mol% R2O) to the base glass increases leach rates, but only to levels comparable with those measured for a commercial soda-lime-silica glass and for a surrogate nuclear waste-loaded borosilicate glass. Divalent cation additions decrease aqueous leach rates and large additions (30–50 mol% RO) provide exceptionally low leach rates that are 2–3 orders of magnitude lower than have been measured for the surrogate waste-loaded borosilicate glass. The P2O5–Fe2O3–FeO–BaO glasses reported here show particular promise as they are ultra-durable, thermally stable, low-melting glasses with a large glass-forming compositional range.

A high energy x-ray and neutron scattering study of iron phosphate glasses containing uranium

The atomic structure of iron phosphate glasses containing uranium has been studied by complementary neutron and x-ray scattering techniques. By combining x-ray and neutron structure factors, detailed information about different pair interactions has been obtained. Most of the basic structural features such as coordination numbers and O–O and P–O distances in uranium containing glasses are the same as those in the base glass of batch composition 40Fe2O3–60P2O5 (mol %). However, the Fe–O distances change slightly with the addition of uranium. The observed structural parameters support a structural model in which the waste elements occupy voids in the Fe–O–P network, hence, not altering the basic structure of the parent iron phosphate glass.

X-ray photoelectron and Mössbauer spectroscopic studies of iron phosphate glasses containing U, Cs and Bi

The atomic structure of iron phosphate glasses and those containing common high level nuclear waste components such as UO 2, Cs 2O and Bi 2O 3 has been investigated using X-ray photoelectron spectroscopy (XPS) and Mössbauer spectroscopy. Oxygen 1s spectra indicate that only 16–26% of the oxygen ions are bridging oxygens (BOs). The addition of waste components such as UO 2, Cs 2O and Bi 2O 3 does not change the BO/(BO + NBO) ratio appreciably. Some structural similarities in the short range order was found between the glass and crystalline Fe 3(P 2O 7) 2 which crystallizes from the glass upon heat treatment. The Mössbauer hyperfine parameters, isomer shift and quadrupole splitting, showed that the environment around iron ions was independent of the composition. In general, the addition of waste elements does not alter the basic features of the parent glass. The measured isomer shifts and quadrupole splittings indicate that the Fe 3+ and Fe 2+ ions in these glasses are in octahedral or distorted octahedral coordination. The fraction of Fe 2+ in the glass increases with the melting temperature. However, most physical properties of these glasses do not depend appreciably on the valence state of iron ions.

The Effects of Uranium on the Structure of Iron Phosphate Glasses

AbstractBecause of their high chemical durability and waste loading capacity, iron phosphate glasses are a natural candidate for a nuclear waste disposal medium. We have studied the effects of uranium on the structure of iron phosphate glasses with both neutron and high-energy x-raydiffraction. The results of neutron scattering, which is mostly sensitive to pair correlations involving light atoms i.e. O-O, Fe-O and P-O, indicate the main structural features of the base glass are largely unaffected by the addition of UO2. The nearest-neighborbour P-O, Fe-O and O- O peaks remain at the same position in real space and their intensities scale approximately with concentration. These findings are consistent with earlier results using Raman scattering and EXAFS on the Fe-K edge, where in both cases the spectra remain similar to the base glass. The results of high-energy x-ray scattering, which is sensitive to correlations involving the heavier atoms and thus complements the neutron measurements, are also consistent with the overall picture of uranium occupying interstitial sites in the relatively undisturbed base glass structure. Combining the neutron and x-ray data for a 10 mol% UO2 glass suggests the intriguing possibility of a U6+ uranyl ion configuration although further work is needed to establish the precise local structure and valence state of uranium in these glasses.