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.     

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.     

Short range antiferromagnetic ordering in vitreous Fe2O3P2O5

An outline is given of the theory of neutron magnetic scattering from amorphous materials, and data are presented for vitreous 0.79 Fe₂O₃ · P₂O₅. The magnetic correlation function shows that at low temperatures the glass exhibits short-range antiferromagnetic ordering with Fe³⁺?Fe³⁺ interionic distances similar to those found in crystalline FePO₄. The neutron data are not consistent with a previous suggestion that the material is microcrystalline.     

Mössbauer spectroscopic study of gamma-ray irradiated potassium phosphate glasses

A Mössbauer spectroscopic study was performed to investigate the effect of γ-ray irradiation on the formation of non-bridging oxygens in the potassium phosphate glasses denoted by the formula x K₂O · (100 ? x)P₂O₅ · 7 Fe₂O₃ (0 ? x ? 50 mol.%). Mössbauer spectra for these glass samples consisted of two kinds of doublets due to Fe²⁺ and Fe³⁺ ions of octahedral symmetries. Only small changes occurred in the Mössbauer parameters as a result of irradiation at room temperature in a dry nitrogen atmosphere, except for the decrease in the absorption area for the Fe²⁺ ions. The decrease in the absorption area was attributed to the electron transfer from the Fe²⁺ ions to the neighboring oxygens. Thermal annealing experiments for a few non-irradiated glass samples indicated that the decrease in the absorption area was confirmed to be due to γ-rays rather than heating during the irradiation.     

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.     

Composition and morphology control of Fex(PO4)y(OH)z·nH2O microcrystals

A series of Fe_x(PO₄)_y(OH)_z·nH₂O microcrystals were prepared by the hydrothermal reaction at 150 °C. The ratio of Fe²⁺/Fe³⁺ in Fe_x(PO₄)_y(OH)_z·nH₂O microcrystals can be adjusted by using Na₂S₂O₃·5H₂O as a reducing agent. The morphology control of Fe_x(PO₄)_y(OH)_z·nH₂O microcrystals was realized through regulating the molar ratio of LiAc·2H₂O/FeCl₃. Further, the morphology, structure and composition of Fe_x(PO₄)_y(OH)_z·nH₂O microcrystals were also investigated by x‐ray diffraction (XRD), x‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The small anomaly in the temperature dependence of the electrical conductivity of iron containing phosphate glasses

The dc conductivities, σ,of 2 CaO·K₂O·x Fe₂O₃·(5 - x)P₂O₅ glasses (1.4 ?x?2.2) [Fe³⁺/(Fe²⁺ + Fe³⁺)= constant (0.88±0.015)] have been measured in the temperature range from room temperature to 300°C. It has been found that the glass transition temperature increases rapidly with increasing Fe/P ratio and that mixed-valence hopping type electronic conduction takes place in these glasses and every log σ versus 1/T curve obtained shows a change in slope near 100°C, which produces a small increase, 0.14 (for x=2.2)?0.18 eV (for x=1.4), in activation energy at higher temperatures. Both the activation energy and pre-exponential factor for conduction display different dependences on the Fe/P ratio in the different temperature ranges on both sides of the anomaly temperature. These results are discussed in terms of the Mott's equation for small polaron hopping.     

A spectroscopic determination of ferrous iron content in glasses

A comparison is given between a Mössbauer spectroscopy determination and the classical chemical analysis of the Fe²⁺ content in SiO₂Na₂OFe₂O₃ glass systems, for different iron concentrations, and for 16.0 and 24.0 wt% Na₂O respectively. The agreement between Mössbauer analysis and chemical analysis is good only for large Fe₂O₃ contents, while for small Fe₂O₃ contents the Mössbauer analysis gives a much larger Fe²⁺/(Fe²⁺ + Fe³⁺) ratio than the chemical analysis: a physical reason is given for this fact. The Fe₂O₃ content below which Fe²⁺ becomes predominant in glass is rather sharply defined, and a qualitative explanation is proposed to account for this rapid transition from Fe³⁺ to Fe²⁺.     

EPR study of disorder processes in the ternary vanadate Zn2FeV3O11−x compound

Two samples of the Zn₂FeV₃O₁₁ compound were synthesized using different preparation routes: a solid state reaction of the ZnO–Fe₂O₃–V₂O₅ oxides and a direct reaction of the Zn₂V₂O₇ and FeVO₄ vanadates. In both cases the temperature dependences of the EPR spectra revealed a dominant contribution of antiferromagnetically coupled Fe³⁺ ions in the high-spin state. Specific differences in the EPR spectra of the two samples were recorded at all the investigated temperature intervals. A short range, or less likely, partial magnetic order involving a fraction of the Fe³⁺ distributed on the two octahedral crystallographic sites appeared to take place below 50 K. The differences observed between the EPR spectra of the two Zn₂FeV₃O₁₁ samples might suggest the existence of small variations in the cation disorder phenomena of Fe³⁺ and Zn²⁺ ions or the presence of an oxygen deficiency process.     

Square-wave voltammetry and impedance spectroscopy in iron-doped melts of the system Li2O/Al2O3/SiO2

Glass melts with the basic compositions xLi₂O · 15Al₂O₃ · (85 − x)SiO₂ (x = 8.5, 11, 13.5, 16 and 18.5) doped with 0.25 mol% Fe₂O₃ were studied by square-wave voltammetry and impedance spectroscopy at temperatures in the range from 1100 to 1600 °C. The square-wave voltammograms show a pronounced peak attributed to the reduction of Fe³⁺ to Fe²⁺. The attributed peak potentials which are equal to the standard potentials of the redox pair decrease linearly with the temperature. Impedance spectra measured could be simulated using an equivalent circuit attributed to a simple electron transfer reaction controlled by diffusion.     

Mössbauer spectra in the xNa2O·(1 − x − y)B2O3·yFe2O3 glass system

A Mössbauer spectroscopic study is made on xNa₂O·(1 ? x ? 7)B₂O₃·yFe₂O₃; 0.045 ? x ? 0.405 and y = 0.05, 0.10 and 0.15 samples quenched from the melt. The presence of two quadrupole doublets indicates that an amphoteric cation like Fe³⁺ occupies both the tetrahedral “network forming” and the octahedral “network modifying” sites in the glass structure. The observed variations of isomer shift and quadrupole splitting with the values of x are similar to the ones found earlier in lead borates but different from those observed earlier in soda borates and other alkali alumino borates. In samples with smaller values of x, an additional devitrified magnetic phase of Fe₂O₃ is seen. The distribution of Fe³⁺ ions in different sites and phases is also discussed.     

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.     

The effect of fluoride on the Fe2+/Fe3+-redox equilibrium in glass melts in the system Na2O/NaF/CaO/Al2O3/SiO2

Liquids with the base compositions (16 − x/2)Na₂O · xNaF · 10CaO · 74SiO₂ (x = 0, 1, 3, and 4) and (10 − x/2) · Na₂O · xNaF · 10CaO · yAl₂O₃ · (80 − y)SiO₂ (x = 0, 1, 3, 5 and y = 5 and 15) doped with 0.25 mol% Fe₂O₃ were studied by means of square-wave voltammetry in the temperature range from 1000 to 1500 °C. With increasing temperature, the redox equilibria were shifted to the reduced state. Also while increasing the alumina concentration, the Fe²⁺/Fe³⁺-redox equilibrium is shifted to the reduced state. In the soda-lime–silica melt the addition of fluoride shifts the equilibrium to the oxidized state, while in the aluminosilicate melts with 15 mol% Al₂O₃, the equilibrium is shifted to the reduced state. In the aluminosilicate melts with 5 mol% Al₂O₃, the equilibrium was not affected by the fluoride concentration. This is explained by the structure of the respective glass compositions.     

Untersuchungen zur Unstöchiometrie von Magnetit in Abhängigkeit von den Bildungsbedingungen

This paper reports on investigations on the chemical composition and structure of magnetite depending on the conditions of chemical reactions. Magnetites were prepared according to a method described by GIOVANOLI and BRÜTSCH which, originally, had been a means for the preparation of γ-FeOOH. — After investigations on lattice parameters and relation Fe²⁺:Fe³⁺ we found that the chemical composition and structure of magnetites is a function of the conditions under which reaction takes place. The composition of the reaction product more and more approaches the state of an ideal stoichiometric magnetite (Fe₃O₄) with increasing basicity during the reaction. With increasing acidic conditions, the composition of the reaction product approximates to that of maghemite (γ-Fe₂O₃); the crystal structure is disturbed. Likewise, some other reaction products will originate, their chemical composition being FeOOH.     

X-ray photoemission study of fluorozirconate glass and related crystals

Silicate glasses with the composition (in per cent by weight) of 74 SiO₂, 20 Na₂O, 6 CaO and different Fe₂O₃ concentrations were subjected to X-ray irradiation and the behaviour of the iron as well as that of the radiation-induced defects was investigated systematically using ESR. The defects which are abundant in iron-free glasses (they produce a strong signal at g = 2.01) and which lead to colouring of the glass, decrease with rising Fe₂O₃ concentration and cease to exist at 2% Fe₂O₃. These glasses are radiation-resistant.Because of the X-rays a valence change of the iron takes place. This process is initiated by a radiation-induced redox interaction between Fe³⁺ and Fe²⁺ according to:

$\text{Fe}{}^{\mathrm{2+}}\text{+}\text{Fe}{}^{\mathrm{3+}}\text{→}\text{hv}\text{Fe}{}^{\mathrm{3+1}}\text{+}\text{Fe}{}^{\mathrm{2+1}}$

Because of the different structures (coordinations) of Fe²⁺ and Fe³⁺ the process leads to a decrease in Fe³⁺ at g = 4.2 in glasses with low Fe₂O₃ concentration. This decrease is reduced with increasing concentration of Fe₂O₃.     

Influence of the temperature and the oxygen partial pressure on the phase formation in the system Cu – Fe – O

Copper iron oxides, Cu_1‐xFe_2+xO₄ (0 ≤ x ≤ 0.5), have been synthesized by thermal oxidation of copper ‐ iron mixtures. In this process, the phase formation and the phase stability were investigated as function of the temperature (800°C – 1200°C) and the oxygen partial pressure (1.013 x 10¹ – 1.013 x 10⁵ Pa). The phase formation starts with the reaction of the metallic components to simple oxides (Fe₃O₄, Fe₂O₃, CuO). From these simple oxides, the formation of complex oxides requires a minimum temperature of 800°C. The synthesis of single phase spinel compounds Cu²⁺_1‐2x Cu¹⁺_xFe_2+xO_4±δ is realized for 0.1 ≤ x ≤ 0.5, using specific temperature – p(O₂) – conditions for a given value of x. Remarkably, to achieve our goal, we found that the increase of x implies that of the reaction temperature and/or a decrease of the p(O₂) in the reaction gas stream. Besides, a single phase spinel CuFe₂O₄ does not exist in the temperature / p(O₂)‐field investigated. Using the results of XRD ‐ phase analysis, T ‐ p(O₂) – x – diagrams were constructed. These diagrams allow the prediction of phase compositions expected for different synthesis conditions. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

A mössbauer study of hematite dissolution and devitrification in float glass

The effect of 5% hematite on the devitrification behaviour of float glass has been measured as functions of time and temperature by Mössbauer spectroscopy and X-ray diffraction (XRD). The devitrification products are devitrite (Na₂O·3CaO·6SiO₂), cristobalite, quartz and an iron-rich pyroxene, aegirine ((Na Ca)(Fe, Mg) Si₂O₆) for which XRD data are tabulated. The Mössbauer parameters for glassy and crystalline materials are interpreted in terms ofthe aegirine structure, whose site assignment is discussed in terms of next-nearest neighbour effecs. The bulk dissolution data obtained by Mössbauer and XRD are used to generate diffusion coefficients for Fe³⁺ in the glass melt, using a moving boundary model for diffusion controlled dissolution. The diffusion coefficients at 800°C are D_{Fe³⁺ (XRD)} = 3.52 × 10^?15 cm²/s and D_{Fe³⁺ (MS)} = 1.27 × 10^?15 cm²/s.     

Thermodynamische Berechung des Systems Ge/H2O/H2 und experimentelle Untersuchung mit der Nahabstandsanordnung

In the temperature range from 950 to 1200 K K_p-values result from 5 · 10⁻⁵ to 6,9 · 10⁻³ for the heterogeneous reversible reaction Ge_(s) + H₂O_(g) ⇋ GeO_(g) + H₂O_(g), the average reaction enthalpy being 46,5 ± 05 kcal/mol. Etching rates calculated with these equilibrium constants for closed systems are 20-25% larger than the experimental etching rates for (110)-Ge in the sandwich device. The ratio of the etching rates is for (111)-, (100)- and (110)-Ge 1:1,4:1,8; the average reaction enthalpy calculated experimentally is 46 ± 1 kcal/mol for the temperature range 1000-1200 K. The dependence of transport rate on distance between source and substrate characterizes the sandwich device as a quasi-closed system with the diffusion as the rate controlling step of the material transport.     

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.