Gel-glass transformation in the SiO2Fe2O3 system

Gel-glass transformation has been studied by Mössbauer spectroscopy, DTA-TG analyses and X-ray diffractometry for four compositions in the SiO₂Fe₂O₃ system (A: 5 wt% Fe₂O₃, B: 10 wt% Fe₂O₃, C: 20 wt% Fe₂O₃, D: 40 wt% Fe₂O₃).The gels were prepared by the hydrolysis of silicon tetraethoxide and iron triethoxide and successively dried and heated in oxygen in the temperature range 40–1000°C.Samples A and B gave typical amorphous X-ray patterns up to 700°C; heating at higher temperature yielded the precipitation of quartz, cristobalite and hematite in sample A, cristobalite and hematite in sample B. Crystallization was also detected by DTA in sample A for which X-ray diffraction exhibited a larger effect.In samples C and D crystallization took place starting from 300°C with the precipitation of hematite, which remained the only crystalline phase up to 1000°C.The presence of hematite was confirmed by the obtained Mössbauer spectra which showed the characteristic sextet. The apportion of iron ions in the Fe³⁺ and Fe²⁺ oxidation states was also determined, together with the attribution of the probable coordination states for Fe³⁺ ions.Complex magnetic structure appeared in samples treated above 800°C.     

High-iron ferric glass

Iron commonly exists as an equilibrium mixture of ferrous ions, Fe²⁺, and ferric ions, Fe³⁺, in glass. Fe³⁺ is a strong absorber of ultraviolet light and imparts a yellow color to glass, while Fe²⁺ has an absorption band in the near-infrared, resulting in a blue coloration. This study synthesized soda-lime-silicate glasses with high iron contents (1–3 wt%) in which virtually all the iron was fully oxidized to the ferric redox state, resulting in a UV-absorbing, yellow glass. The effectiveness of three common oxidizing agents (cerium, manganese, and antimony) to react with Fe²⁺ in these high-iron glasses was determined as a function of the processing temperature (1200–1400 °C). Cerium was the best oxidizer at the highest iron contents (3 wt%), while manganese was more effective at the lowest iron contents (1 wt%).     

Iron redox equilibrium, structure and properties of zinc iron phosphate glasses

Iron redox equilibrium, structure and properties were investigated for the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses melted at different temperatures. The structure and valence states of the iron ions in these glasses were investigated using Mössbauer spectroscopy, Raman spectroscopy and differential thermal analysis. Mössbauer spectroscopy indicated that the concentration of Fe²⁺ ions increased in the 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glass with increasing melting temperature. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio increased from 0.18 to 0.38 as the melting temperature increased from 1100 to 1300 °C. The measured isomer shifts showed that both Fe²⁺ and Fe³⁺ ions are in octahedral coordination. It was shown that the dc conductivity strongly depended on Fe²⁺/(Fe²⁺ + Fe³⁺) ratio in glasses. The dc conductivity increases with the increasing Fe²⁺ ion content in these glasses. The conductivity arises from the polaron hopping between Fe²⁺ and Fe³⁺ ions which suggests that the conduction is electronic in nature in zinc iron phosphate glasses.     

Silica encapsulated hemoglobin and myoglobin powders prepared by an aqueous fast-freezing technique

Hemoglobin and myoglobin were encapsulated in silica gels and powders. Protein encapsulated powders were fabricated via the condensation of silicic acid around the protein, followed by a fast freezing with liquid nitrogen, and subsequent thawing. The fast-freezing technique led to high surface area stable silica encapsulated protein powders. Transmission UV-vis spectroscopy techniques were used to verify that neither protein was damaged during gelling or freezing processes. Both hemoglobin and myoglobin gels and powders retained their biological activity and were able to bind cyano ligands while in the oxidized Fe³⁺ state and carbon monoxy ligands while in the reduced Fe²⁺ state. Kinetics experiments showed that the rates of binding of CO and CN⁻ to the proteins in the silica gel versus a buffer solution are decreased by 30-45%. This result was likely due to mass transfer effects associated with diffusion through the gel network. Hemoglobin/silica powders were successfully stabilized in the Fe²⁺ oxidation state by addition of the amino acid l-cysteine.     

Redox kinetics of iron in alkali silicate glasses exposed to ionizing beams: Examples with the electron microprobe

We report herein on changes in the Fe³⁺/ΣFe ratios induced by the electromigration of alkali ions (Alk⁺s) in natural alkali silicate glasses by measuring the shift of the Fe Lα emission peak at the electron microprobe. The Fe³⁺ reduction to Fe²⁺ classically occurs by electron transfer (Fe³⁺ + e^? → Fe²⁺). The inward diffusion (to the bulk) of Alk⁺s is correlated with the outward diffusion (to the surface) of electrons transferred from a Fe²⁺ site to a neighboring Fe³⁺ site. This reduction process is somewhat different when iron is found at low amounts in glasses. In the latter case, Fe³⁺ is an efficient electron trap and its reduction to Fe²⁺ occurs by direct capture of a free electron. The Fe²⁺ oxidation is induced by the formation and the outward diffusion of O^2? interstitial ions produced at the sites of paired nonbridging oxygens after the departure of the charge compensating Alk⁺s. The accumulation of free oxygens beneath the surface makes Fe³⁺-rich oxide phases to precipitate as separate nanometer sized particles. Outgassing of atomic oxygens as bubbles is also observed.     

Mössbauer study in the glass system PbO · 2B2O3 · Fe2O3

The Mössbauer technique has been employed to study the structure and crystallite formation in the glass system PbO · 2B₂O₃ containing upto 30 wt% Fe₂O₃. Like alkali borate glasses, this glass system also exhibits a broadened quadrupole doublet and iron ions are present in Fe³⁺ state. Above about 20 wt%, the crystallites of magnetically ordered states have been identified. Susceptibility variation with concentration suggests the formation of a superparamagnetic state.     

Colorimetric study on the redox number of P2O5–PbO–Fe2O3 glasses

Semi-conducting property of lead iron phosphate glasses is assumed to be due to the hopping process. Redox number of glass is usually determined by Mössbauer spectroscopy. However, the technique is limited by low sensitivity in lower concentration of iron ions. Neither is suitable for the traditional method since Fe²⁺ in the glass is apt to oxidize and results in the uncertainty of the concentration. In this paper, a colorimetric method was performed to study the redox number of the glasses. Sample solutions have been prepared by using a vanadate as an oxidizing inhibitor for the equilibrium of Fe²⁺/Fe³⁺ pairs. The V⁵⁺/V⁴⁺ couple, which is strongly acid-dependent, oxidizes the Fe²⁺ to Fe³⁺ in the highly acidic solution, and it is regenerated when the solution was adjusted to pH 5 from V⁴⁺ preservative. The measurement of the electric conductivity of the glasses has indicated the fact that, other than the concentration of the electric conducting carriers, redox number of the glass plays a significant role for the semi-conducting phosphate glasses. The method was validated by analyzing a mixture of ammonium iron (II) sulfate and/or ammonium iron (III) sulfate solutions and has been proved to be simple, accurate and reliable.     

Surface nature of nanoparticle gold/iron oxide aerogel catalysts

The aerogel catalysts investigated are constituted by two chemically different nanoparticle systems consisting of the gold phase and the iron oxide support. High-resolution transmission electronic microscopy (HRTEM) showed an increased surface roughness for aerogel particles with higher gold loading. X-ray photoelectron spectroscopy (XPS) revealed increases in the surface coverage of hydroxyl groups and the Fe²⁺/Fe³⁺ ratio due to the addition of gold, and showed the transition of gold from oxidized to metallic states due to calcination. In the presence of gold species, the Fe³⁺ satellite structure in XPS was not produced. The crystallinity of maghemite as the support was found quite stable with respect to gold addition and thermal treatment. The aerogels were evaluated for methanol oxidation carried out in an ambient flow reactor. The oxidation activity enhanced with decreasing catalyst pretreatment temperatures and with increasing gold loadings up to 5 wt%. A wide selectivity pattern formed between dimethyl ether and carbon dioxide products. The size of gold particles and the status of surface gold species played a crucial role in the catalytic conversion of methanol. The oxidized gold was more active than the metallic gold towards the total combustion to carbon dioxide. The surface nature has been proven to transform from strong Lewis acidic to high basic characters due to the formation of reactive hydroxyl groups near by the gold sites.     

Characterization of iron phosphate glasses prepared by microwave heating

Phosphate glasses have been prepared by melting batch materials in electric furnaces, induction furnaces, and in microwave ovens. In the present work mixtures of (NH₄)₂HPO₄ and Fe₃O₄ or Fe₂O₃ were exposed to microwave energy, heated to 1200 °C, and cast to produce iron phosphate glasses. Glasses were also produced in electric furnaces for comparison. The material was analyzed by X-ray diffraction, Mössbauer spectroscopy, and differential thermal analysis. For magnetite-based glasses produced in an electric furnace, the Fe²⁺/(Fe²⁺ + Fe³⁺) ratio is compatible with the value in the batch material. The Fe²⁺/(Fe²⁺ + Fe³⁺) ratio is higher for glasses produced in a microwave oven. Glasses with nominal composition 55Fe₃O₄–45P₂O₅ (mol%) produced in an electric furnace present an arranged magnetic phase with hyperfine field that could be associated to hematite (estimated to be 21%). All the glasses submitted to heat treatments for crystallization present the following crystalline phases: FePO₄, Fe₃(PO₄)₂, Fe(PO₃)₃, Fe(PO₃)₂ and Fe₇(PO₄)₆. The amount of these phases depends on the glass composition, and glass preparation procedure. Microwave heating allows to reach melting temperatures at high heating rates, making the procedure easy and economical, but care should be taken concerning the final Fe²⁺/(Fe²⁺ + Fe³⁺) ratio.     

The solubility and activity determination of NiO in the SiO2-NiO-FeO system

A structural model for the liquid phase of silicate systems was used to obtain the SiO₂-NiO-FeO phase diagram and the NiO activities at 1573 and 1673 K. The NiO activities were determined experimentally by equilibrating a pure nickel crucible with an iron-silicate slag and CO-CO₂ gas mixture to produce pO₂ = 10⁻⁶ and 10⁻¹⁰ atm. The NiO activities obtained by the structural model are in good agreement with those obtained experimentally for strongly reduction conditions (10⁻¹⁰ atm), due to the structural model consider only the iron in the divalent state. The Fe³⁺/Fe²⁺ ratio and the nickel oxide solubility were increased when the oxygen partial pressure and the temperature were increased.     

State of Fe3+ ion and Fe3+ −F− interaction in calcium fluorosilicate glasses

The state of Fe³⁺ ions and Fe³⁺ ?F^? interaction in calcium fluorosilicate glasses xCaF₂·(1- x)CaO·SiO₂) (0 ≤ x ≤ 0.3) containing a small amount of iron were investigated by ESR spectroscopy. Two resonances observed near g = 2.0 and g = 4.3 were assigned to dipole-dipole interacted Fe³⁺ ions and Fe³⁺ ions in a rhombic crystal field, respectively. The fraction of Fe³⁺ ions in a rhombic crystal field decreased and that of dipole-dipole interacted Fe³⁺ ions increased with increasing Fe₂O₃ content. It was found that the quantity of dipole-dipole interacted Fe³⁺ ions depends on the negative partial charge of fluorine ions and shows a maximum at 10 mol% CaF₂ (x = 0.2). The maximum is attributed to the largest difference between absolute values of the ionic potentials of ferric and fluorine ions which is caused by the smallest negative partial charge of flourine at 10 mol% CaF₂.     

The study of interactions between iron and vanadium ions in semiconducting barium-vanadate glasses doped with Fe2O3

Semiconducting barium-vanadate glasses doped with Fe₂O₃ ranging from 0.1 to 10 wt% were studied. We made attempts to understand features of an incorporation of the impurity ions into the host matrix. EPR, magnetic susceptibility, dc-conductivity and the Mössbauer effect were investigated.It was established that iron entered into the host as Fe³⁺·Fe³⁺ and V⁴⁺ ions formed associates coupled by dipole-dipole interactions for low Fe₂O₃ contents in the glass. The V⁴⁺?Fe³⁺ and Fe³⁺?Fe³⁺ pairs co-existed for all glasses. The contribution of Fe³⁺?Fe³⁺ interactions increased with increasing Fe₂O₃ content. The deviation from paramagnetic behaviour was observed for glasses with 8–9 wt% Fe₂O₃. It was attributed to presence of fine crystalline magnetic particles.The iron impurity induces no considerable changes in the dc-conductivity of the glass. The concentration dependence of dc-conductivity exhibits a minimum of about 5–6 wt% Fe₂O₃.     

The effect of initial oxidation state on crystallization of basaltic glass

Iron-rich silicates mined as basalt may be processed as glass fibers and, woven to textile mats, it may be used as heat insulation. High-temperature stability is however a limiting factor because at least three process of alteration occur upon heat treatment that affect mechanical and chemical properties, micron to nano-crystallization, oxidation and cation-enrichment at the glass surface with a few microns depth. We here evaluate the crystallization behavior of synthetic Fe³⁺-rich basalt (SB) heat treated in air (negligible oxygen potential) in comparison with previously studied Fe²⁺-rich natural basalt (NB) heat treated in air and in Ar (i.e., high and low oxygen potential, respectively). Initial crystals growing in Fe²⁺-rich basalt are micron-sized dendrites, and with temperature of heat treatment they become increasingly granular and are identified as pyroxene. Pyroxene in SB form smaller dendrites; crystallization textures and nucleation and growth rates, derived from crystal size distribution, are independent of the temperature of heat treatment in the range between 850 and 1030 °C. Here, the activation energy of pyroxene vanishes, though crystallization rates are smaller than those of NB. Whereas in NB pyroxene growth occurs diffusion limited, in SB it is likely to be limited by the attachment of ions to new crystal surface.     

Surface oxidation of basalt glass/liquid

To evaluate surface oxidation of Fe²⁺-rich multi-component silicate glass, powder and pieces of natural basalt glass are heat-treated in Ar and subsequently investigated by Mössbauer spectroscopy to monitor the increase of Fe³⁺. Glass pieces show no increase in oxidation with time or temperature, suggesting that the oxygen potential between glass and Ar is insufficient to cause volume oxidation. In contrast, glass powder oxidizes readily to a degree comparable with that of powder oxidation in air, suggesting that surface oxidation does not depend on the oxygen potential. No cation diffusion to the glass surface is detected in Ar, though it is observed upon heat treatment in air; cation diffusion is therefore unlikely to be involved in oxidation. We suggest the following mechanism for surface oxidation: (1) adsorption of water on the glass surface as OH⁻, by exposure to air and (2), a concomitant reaction, i.e., oxidation with the glass, upon heating (chemisorption). Hereby, either oxygen of air or residual oxygen in Ar would react with the hydrogen of the -OH, liberating the oxygen for oxidation of iron. Heat treatment in vacuum of 10⁻⁸ mbar does not result in any oxidation, and we assume that the adsorbed OH is exhausted from the glass surface.     

Effect of trace metals on reactive crystallization of gypsum

The effect of Fe²⁺, Fe³⁺, and Cr³⁺ ions on crystallization of calcium sulfate dihydrate (gypsum) produced by the reaction between calcium hydroxide suspension and sulphuric acid solution was investigated at 3.5 pH and 65°C in the absence and presence of 2500 ppm citric acid concentration. Crystal size distributions, filtration rates, and morphology of gypsum were determined and discussed as a function of ion concentration. Average particle size of gypsum was not affected significantly by the presence of Fe²⁺, Fe³⁺, and Cr³⁺ ions individually. Variation of gypsum morphology depending on ion concentration affected the filtration characteristics. The presence of Fe³⁺ or Cr³⁺ ions besides 2500 ppm citric acid influenced both average particle size and filtration characteristics. The effect of citric acid on gypsum morphology was suppressed at high Fe³⁺ and Cr³⁺ ion concentrations. The change of morphology is related to the complex formation between Fe³⁺ or Cr³⁺ ions and citric acid at high ion concentrations. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the nanosiderite from ferruginous quartzites of Kursk magnetic anomaly by transmission electron microscopy

Biogenic siderite consisting of equally crystallographically oriented disklike nanoparticles 5–20 nm in size has been found (using transmission electron microscopy) in oxidized ferruginous quartzites (jaspilites) of the Lebedinsky field of the Kursk magnetic anomaly. Based on microdiffraction data and highresolution images, lowering of the siderite structure symmetry from \(R\overline 3 c\) to \(R\overline 3 \) has been established for the first time. A siderite structure model is proposed to explain this fact. Within this model, vacancies formed as a result of oxidation of some part of Fe²⁺ cations to the Fe³⁺ state are ordered in one of two nonequivalent octahedral sites. Identical crystallographic orientation and nanoparticle morphology have been established for coexisting siderite and hematite. It is suggested that the revealed specific features of nanosiderite are related to its biogenic origin.     

Iron environment in calcium-soda-phosphate glasses and vitroceramics

The structure of calcium-soda-phosphate glasses and vitroceramics with relatively high iron content was investigated by X-ray diffraction, electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. The X-ray diffraction analysis proves the vitreous state of the as prepared samples and the development of crystalline phases in the annealed samples. The ferric ions disposed in sites that give rise to the absorption line with g_ef ≈ 4.3 in the EPR spectra of vitreous samples are not more evidenced in the spectra of the annealed samples, from which only of a symmetric and narrowed line with g_ef ≈ 2 is recorded. The room temperature ⁵⁷Fe Mössbauer spectra both of glass and vitroceramic samples consist of two quadrupole doublets characteristic for octahedral sites of Fe²⁺ and Fe³⁺ ions. The isomer shift for glass samples decreases and for vitroceramic samples increases with the iron oxide content.     

Hole defects in the crystal structure of synthetic lipscombite (Fe 4.7 3+ Fe 2.3 2+ )[PO4]4O2.7(OH)1.3 and genetic crystal chemistry of minerals of the lipscombite-barbosalite series

The crystal structure of a synthetic analog of the mineral lipscombite (Fe _2.3 ²⁺ Fe _4.7 ³⁺ )[PO₄]₄O_2.7(OH)_1.3 obtained under hydrothermal conditions in the LiF-Fe₂O₃-(NH₄)₂HPO₄-H₂O system is resolved (R = 0.040) by X-ray diffraction analysis (Bruker Smart diffractometer with a highly sensitive CCD detector, MoK _α radiation): a = 14.776(3) Å, b = 14.959(3) Å, c = 7.394(1) Å, β = 119.188(4)°, sp. gr. C2/c, Z = 4, ρ_exp = 3.8 g/cm³, ρ_calcd = 3.9 g/cm³. Fe²⁺ and Fe³⁺ cations are statistically distributed in each of four crystallographically independent positions, while occupying the corresponding octahedra with probabilities of 60, 90, 100, and 91%. The ratio Fe²⁺/Fe³⁺ in the composition of the crystals was established by Mössbauer spectroscopy. Lipscombite is interpreted as a mineral of variable composition described by the formula (Fe _x ²⁺ Fe _n?x ³⁺ )[PO₄]₄O_y(OH)_4?y . The field of stability is determined as a function of the iron content and the ratio Fe²⁺/Fe³⁺. It is shown that at n = 6 iron cations are ordered in octahedra and barbosalite structure is formed. An interpretation of genetically and structurally related members of the lipscombite family within a unified polysomatic series is proposed.     

Synthesis and crystal structure of new phosphate NaFe 4 2+ Fe 3 3+ [PO4]6

New sodium iron orthophosphate NaFe ₄ ²⁺ Fe ₃ ³⁺ [PO₄]₆ was synthesized by the hydrothermal method. The crystal structure (sp. gr. $P\bar 1$ ) was established by the heavy-atom method, with the exact chemical formula of the compound being unknown; R _hkl = 0.0492, R _whkl = 0.0544, S = 0.52. The new compound is analogous to iron phosphate Fe ₃ ²⁺ Fe ₄ ³⁺ [PO₄]₆ studied earlier. However, these two compounds differ in the Fe²⁺ and Fe³⁺ contents, because Na⁺ ions in the new compound are located at the centers of symmetry not occupied earlier.     

An easy and economic method for preparing completely reduced basalt glass

Four basalt glass samples were prepared by fusing basalt rock (powder) with different amounts of sulfur in a platinum crucible at 1550 °C for 30 min. Each melt was quenched in air. Sulfur addition to the basalt powder was changed from 0 to 5, 10 and 15 wt%. The prepared glass samples were pulverized for measuring the Mössbauer spectra by the constant acceleration method. The basalt rock spectrum can be analyzed into four peaks; two sites due to Fe³⁺ with octahedral (Oh) and tetrahedral (Td) symmetry, and the other two due to Fe²⁺ with Oh and Td symmetry. Pure basalt glass (sulfur-free) consists of four doublets; two of them represent Fe²⁺(Oh) sites and the third represents Fe²⁺(Td); while the fourth doublet belongs to Fe³⁺(Td). The sample containing 5 wt% sulfur has four iron sites also, although there is a slight difference in the relative absorption area when compared with sulfur-free sample. The fraction of Fe³⁺ in the 5% sulfur sample was estimated to be only 7.1%; i.e., the fraction of Fe²⁺ was 92.9%. Three iron sites present in the 10% sulfur sample, two of them represent Fe²⁺ with (Oh) symmetry, while the third one represents Fe²⁺(Td) site. Mössbauer spectrum of 15 wt% sulfur sample is essentially the same as that of the sample which contains 10 wt%. It is noteworthy that the sulfur content shows a linear relationship with the Fe²⁺ fraction which is calculated from the Mössbauer spectra of basalt glasses. 7.5 wt% of sulfur is large enough to completely reduce the iron in basalt glass. The reduction of glasses could occur easily and economically using sulfur as a reducing agent. This method is a very easy and economic method for the preparation of completely reduced oxide glass.