M. Karabulut

Structural features of iron phosphate glasses

The structures and valence states of iron ions in several iron phosphate glasses with batch compositions similar to 40Fe2O3-60P2O5 (mol%) have been investigated using Mossbauer spectroscopy, X-ray absorption fine-structure spectroscopy (XAFS), X-ray photoelectron spectroscopy (XPS), differential thermal (DTA) and thermo-gravimetric (TGA) analysis and X-ray and neutron diffraction. Mossbauer spectra show that a redox equilibria corresponding to an Fe(II)/[Fe(II) + Fe(III)] ratio of 0.2–0.4 is reached under processing conditions described in this paper. Even though the valence state of iron ions in the glass appears to be insensitive to the oxygen content in the melting atmosphere, the Fe(II) content can be increased within the observed range of redox equilibria by increasing the partial pressure of a reducing gas in the melting atmosphere. Large amounts of Fe(II), Fe(II)/[Fe(II) + Fe(III)] ≥ 0.4, appear to be detrimental to the glass-forming ability of the iron phosphate melts. The local structure of the iron phosphate glasses appears to be related to the short range structure of crystalline Fe3(P2O7)2 which consists of a network of (Fe3O12)−16 clusters. These clusters consist of one iron(II) ion and two iron(III) ions in sixfold coordination with near-neighbor oxygen ions. The (Fe3O12)−16 clusters are interconnected via (P2O7)−4 groups. Compared to other phosphate glasses, the proposed structure for iron phosphate glasses contain a smaller number of POP bonds, a feature which is believed to be responsible for the unusually good chemical durability of iron phosphate glasses.

Iron Redox Equilibrium, Structure and Properties of Zinc Iron Phosphate Glasses

Abstract Iron redox equilibrium, structure and properties were investigated for 40Fe2O3–60P2O5 (mol%) glasses melted at different temperatures. The Fe2+/(Fe2++Fe3+) ratio increased from 17% to 50% as the melting temperature changed from 1150°C to 1400°C. The equilibrium constant, K, for the reaction of Fe3+ being reduced to Fe2+ varied with temperature as lnK=9.40–1.58×104/T. The Raman and infrared spectra indicated that the basic iron pyrophosphate structure of the 40Fe2O3–60P2O5 (mol%) glasses did not change as the Fe2+/(Fe2++Fe3+) ratio changed. All of the properties did not change to any major degree with increasing the melting temperature. The molar volume decreased while the density increased with increasing Fe2+/(Fe2++Fe3+) ratio. It was found by DTA and XRD that two phases, Fe3(P2O7)2 and Fe4(P2O7)3, crystallized from the glass when the glass was heated in nitrogen. The crystallization behavior suggested that the amount of the crystal, Fe3(P2O7)2, may increase with increasing Fe2+/(Fe2++Fe3+) ratio, which supported the opinion that there are some structural similarities between the iron phosphate glass and the crystalline Fe3(P2O7)2 in terms of the iron coordination number and bonding of the phosphate groups. The decrease in dc resistivity and increase in dielectric constant and dielectric loss tangent, which occurred with increasing the Fe2+/(Fe2++Fe3+) ratio, were attributed to the increase of the electronic hopping from Fe2+ ions to Fe3+ ions.

Chemical Durability and Structure of Zinc-iron Phosphate Glasses

Abstract The chemical durability of zinc–iron phosphate glasses with the general composition (40−x)ZnO–xFe2O3–60P2O5 has been measured. The chemical durability and density of these glasses increase with increasing Fe2O3 content. Glasses containing more than 30 mol% Fe2O3 had an excellent chemical durability. The dissolution rate (DR), calculated from the weight loss in distilled water at 90 °C for up to 32 days, was ∼ 10 −9 g / cm 2 / min which is 100 times lower than that of window glass and 300 times lower than that of a barium ferro, aluminoborate glass. The structure and valence states of the iron ions in these glasses were investigated using Mossbauer spectroscopy, X-ray diffraction, infrared spectroscopy and differential thermal analysis. X-ray diffraction indicates that the local structure of the zinc–iron phosphate is related to the short range structures of crystalline Zn2P2O7, Fe3(P2O7)2 and Fe(PO3)3. Both Fe(II) and Fe(III) ions are present in all of these glasses. The presence of an Fe–O–P related band in the infrared (IR) spectra of the glasses containing more than 30 mol% Fe2O3 is consistent with their excellent chemical durability.

Effect of Melting Temperature and Time on Iron Valence and Crystallization of Iron Phosphate Glasses

The effect of melting temperature and time on iron valence, dissolution rate (DR) in deionized water, and crystallization of iron phosphate glasses was investigated using a 40Fe2O3–60P2O5, mol%, batch composition. The concentration of Fe2+ ions in these glasses increased from 17% to 57% as melting temperature increased from 1150°C to 1450°C, but remained nearly constant at about 20% for melting times longer than 1 h at 1200°C. Measurements by differential thermal analysis (DTA) combined with X-ray diffraction (XRD) and thermogravimetric analysis (TGA) showed that these glasses crystallized to Fe3(P2O7)2 and Fe4(P2O7)3 when heated in nitrogen between 600°C and 820°C, but with continued heating in air at 820°C the Fe3(P2O7)2 changed to Fe(PO4), which produced a weight gain in the sample associated with the oxidation of Fe2+ to Fe3+ ions. The DR (in deionized water) of these glasses was generally very low (∼10−9 g cm−2 min−1) and nearly independent of the relative concentration of Fe2+ or Fe3+ ions, but decreased with total iron content.

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

Abstract The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe 2 O 3 –60P 2 O 5 (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mossbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mossbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mossbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron–oxygen coordination is in the 4–5 range indicating that iron ions have mixed tetrahedral–octahedral coordination.

Mechanical and Structural Properties of Phosphate Glasses

Abstract Mechanical and structural properties of sodium (NAFP) and zinc (ZAFP) iron–aluminum–phosphate bulk glass and fibers have been investigated. Youngs modulus of the fibers was measured by a three-point bending method while the strength was measured by a two-point bending method. In general, the tensile strength of the ZAFP fibers (4.2–7.2 GPa) was higher than the tensile strength of the NAFP fibers (2.8–4.2 GPa). After exposing the fibers to air for 10 days, the strength decreased by 15–34%. The structure of bulk glass as well as fibers, studied by Mossbauer and IR spectroscopy, was very similar for all the compositions studied.

Structural features and properties of lead–iron–phosphate nuclear wasteforms

The structure and properties of vitreous and crystalline lead–iron–phosphate glasses containing up to 21 wt% of a simulated high level waste have been investigated using Fe-57 Mossbauer, X-ray diffraction and Raman spectroscopies. The Mossbauer spectra indicated that both Fe(II) and Fe(III) ions were present in all the samples. The Raman spectra for the glasses contained two dominant bands, which were characteristic of pyrophosphate groups, (P–O) stretching mode of P–O nonbridging oxygen at 1074 cm−1 and sym stretching mode of bridging oxygen at 760 cm−1, respectively. The chemical durability of glassy and crystallized samples was investigated by measuring their weight loss in distilled water at 90 °C for up to 32 days. The weight loss of the lead–iron–pyrophosphate wasteforms was up to 100 times less than that for window glass. X-ray diffraction patterns showed that FeSiO3 and SiP2O7 phases are present in samples containing more than 14 wt% of the simulated nuclear waste.

Structure and properties of lanthanum-aluminum-phosphate glasses

Abstract Glass-forming characteristics, properties and structural features of glasses in the La 2 O 3 –Al 2 O 3 –P 2 O 5 system have been investigated. The glass-forming region is small compared to the Na 2 O–Al 2 O 3 –P 2 O 5 system. Glass transition temperature increases and thermal expansion coefficient, refractive index, and density all decrease as the alumina content of the glass is increased. Infrared (IR) spectroscopy indicates that the glass network is dominated by bridging P-tetrahedra with terminal tetrahedra present in glasses with O/P ratios >3. 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy indicates that Al ions are incorporated in these glasses in 4-, 5-, and 6-coordinated sites. The average Al-coordination number (CN) increases with an increase in the Al/La ratio and decreases with an increase in the O/P ratio. Both trends can be explained by the avoidance of Al–O–Al bonds in the glass structure.

Oxygen and Phosphorus Coordination Around Iron in Crystalline Ferric Ferrous Pyrophosphate and Iron-Phosphate Glasses with UO2 or Na2O

Fe {ital K}-edge x-ray absorption fine-structure (XAFS) measurements were performed on glass samples of (Fe{sub 3}O{sub 4}){sub 0.3}(P{sub 2}O{sub 5}){sub 0.7} with various amounts of Na{sub 2}O or UO{sub 2}. Near-edge and extended XAFS regions are studied and comparisons are made to several reference compounds. We find that iron in the base glass is {approximately}25{percent} divalent, and that the Fe{sup 2+} coordination is predominantly octahedral, while Fe{sup 3+} sites are roughly split between tetrahedral and octahedral coordinations. Also, we measure roughly one Fe{endash}O{endash}P link per iron. Substitution of Na{sub 2}O or UO{sub 2} up to 15 molh{percent} primarily affects the first Fe{endash}O shell. The results are compared to data from the related material Fe{sub 3}(P{sub 2}O{sub 7}){sub 2}. {copyright} {ital 1999 Materials Research Society.}

The Fe–O coordination in iron phosphate glasses by x-ray diffraction with high energy photons

Structures of (FeO)x(P2O5)1−x glasses with 0.2 ≤ x ≤ 0.5 are studied by x-ray diffraction using high energy photons from a synchrotron. Scattering intensities are obtained up to Qmax of 250 nm−1. P–O, Fe–O and O–O first-neighbour peaks are well resolved in the pair distribution functions. Constant P–O coordination numbers of 4.0 ± 0.1 with distances of 0.155 ± 0.001 nm are found as expected for PO4 tetrahedra. Mean Fe(II)–O coordination numbers of ~5 with distances of ~0.210 nm are extracted from fitting the Fe–O peaks where the oxygen coordination number of the minor fraction of 5–18% Fe(III) was set to six with Fe–O distances of 0.200 nm. The existence of FeO5 or strongly distorted FeO6 polyhedra instead of densely packed FeO6 octahedra for the Fe(II) sites is attributed to the mixed oxide effect. Fe3+ cations in rigid FeO6 octahedra hinder the Fe2+ cations in forming well defined octahedral environments.