I. Czakó-Nagy

Mössbauer spectroscopic study of the formation of Fe(III) oxyhydroxides and oxides by hydrolysis of aqueous Fe(III) salt solutions

Abstract Mossbauer spectroscopy has been used to investigate the precipitates formed by hydrolysis of 0.1 M solutions of Fe(NO 3 ) 3 , FeCl 3 , Fe 2 SO 4 ) 3 , and NH 4 Fe(SO) 2 at 90°C. The isomer shifts, electric quadrupole splittings, and nuclear magnetic splittings were used for the qualitative and quantitative identification of the hydrolysis products. Proposals were made concerning the mechanism of formation of the oxides and hydroxyoxides of iron. Hydrolysis in the nitrate and chloride solutions proceeds by the formation of monomers and dimers of iron III) ions, followed by the formation of polymeric species. The polymers formed in the nitrate solution are not presumed to include the nitrate ion in the polymer chain, whereas the polymers formed in the chloride solution contain some chloride ions in place of the hydroxyl ion. The next step in the precipitation process is the formation of oxybridges and the development of α-FeOOH and β-FeOOH structures. This step is followed by loss of water and internal crystallization of α-FeOOH to α-Fe 2 O 3 in nitrate solution or by dissolution of β-FeOOH and growth of α-FeOOH in chloride solution. In sulfate solutions the formation of an FeSO 4 + complex suppresses the polymerization process and the formation of oxyhydroxides and oxides. Basic Fe(III) sulfates are formed instead.

Mössbauer spectroscopy studies of Sn-Pt/Al2O3 catalysts prepared by controlled surface reactions

Abstract Mossbauer spectroscopy was used to study the chemical state of tin in a new type of Sn-Pt/Al2O3 catalysts. Controlled surface reaction of tin tetraethyl with hydrogen preadsorbed on platinum resulted in the exclusive formation of a Pt-SnEt3 surface complex, which decomposed in hydrogen at 773 K to PtxSn alloy (3

Formation of nanocrystalline NiFe2O4

Coprecipitates, Ni(OH)2 + 2Fe(OH3, were milled in a planetary mill, using an agate bowl and balls (99.9% SiO2), and then heated at a maximum temperature of 573 or 773 K. The Mossbauer spectra at 298 K of the ball-milled samples (not heated) showed the superposition of two quadrupole doublets. After heating at 573 K, the superposition of two doublets was preserved at 298 K with a slight increase in the quadrupole splitting for the sample ball-milled for 64 h. After heating at 573 K, the ball-milled samples showed at 80 K a central quadrupole doublet and a sextet with very broad lines. All the samples heated to 573 K were amorphous for according to X-ray diffraction while those heated to 773 K exhibited crystalline NiFe2O4 with broadened diffraction lines. At 298 K, superposition of two sextets in the Mossbauer spectra was observed for the samples obtained after heating at 773 K, indicating the presence of nickel ferrite as a single phase. The average sizes of NiFe2O4 crystallites were in the nanosize range...

Formation of α-Fe2O3 particles in aqueous medium and their properties

Abstract The formation of α-Fe 2 O 3 particles by forced hydrolysis of partially neutralized FeCl 3 solutions with tetramethylammonium hydroxide (TMAH) was investigated. The effect of full neutralization of FeCl 3 solution with TMAH on the phase composition of the precipitate was also monitored At low pH values the hydrolytical product was β-FeOOH which transformed to α-Fe 2 O 3 . The shape of the FT-IR spectrum of α-Fe 2 O 3 depended on the size and morphology of the particles. The presence of polycrystalline particles, consisting of much smaller oriented α-Fe 2 O 3 crystallites, was considered. The surface magnetism and magnetic coupling between these crystallites was supposed to be responsible for the inner broadening of the Mossbauer spectroscopic lines. In alkaline medium (pH ~ 11), a mixture of α-FeOOH and α-Fe 2 O 3 was obtained. α-FeOOH and α-Fe 2 O 3 obtained at high pH showed pronounced intraparticle porosity.

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (CrxFe1−x)2O3, 0 ⩽ x ⩽ 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, α-(CrxFe1−x)2O3, which is isostructural with α-Fe2O3 and phase C, which is closely related to Cr2O3. Phase F exists in samples heated up to 900°C, for 0 ⩽ x ≲ 0.95. Phase C exists from x≳0.27 to x=1 for samples heated up to 900°C and from x≳0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr2O3 in α-Fe2O3 and 4.0 ± 0.5 mol % of α-Fe2O3 in Cr2O3. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. 57Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x≳0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at ∼ 460 and 370 nm (22 000 and 27 000cm−1) for x⩾ 0.5, ∼500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the 6A1 → 4T1 Fe3+ ions in all spectra and the development of the absorption at 13000 cm−1 due to a metal-metal charge transfer (Cr3+ → Fe3+) transition, might be explained by exchange coupling which has been observed in some spinel compounds.

Structural properties of precipitates formed by hydrolysis of Fe3+ ions in Fe2(SO4)3 solutions

The structural properties of the solid phase, formed by the hydrolysis of Fe3+ ions in Fe2(SO4)3 solutions at 90 or 120 °C, were investigated using X-ray diffraction,57Fe Mössbauer spectroscopy, Fourier transform-infrared spectroscopy (FT—IR) and transmission electron microscopy. The concentration regions of Fe2(SO4)3 were determined for the precipitation of goethite, α-FeOOH, or hydronium jarosite, H3OFe3(OH)6(SO4)2′ as a single phase. Superparamagnetic behaviour of α-FeOOH particles was observed. Hydrolysis of Fe3+ ions in 0.1 M Fe2(SO4)3 solutions at 120 °C produced H3OFe3(OH)6(SO4)2 and basic sulphate, Fe4(OH)10SO4. The interpretation of57Fe Mössbauer and FT—IR spectra is given.

Structural properties of the systemm-ZrO2-α-Fe2O3

Abstract The existence of solid solutions in the system m -ZrO 2 -α-Fe 2 O 3 was studied by X-ray powder diffraction. A series of samples were prepared from aqueous solutions of corresponding nitrales and subsequent heating of the obtained precipitate up to 1100°C. Diffraction patterns were taken by step-scanning and determination of unit-cell parameters was performed by means of the powder-pattern-fitting methods. In the whole concentration range there exist two types of solid solution, Z and F, of the form Zr 1 , Fe 2 , O 2 , very closely structurally related to m -ZrO 2 and α-Fe 2 mO 3 respectively. Exclusively the Z phase exists for0 ⩽ × ⩽0.02. Its unit-cell parameters are different and its crystal lattice is asymmetrically distorted in relation to those of m -ZrO 2 . Exclusively the F phase exists for0.99 ⩽ × ⩽ 1, having unit-cell parameters very similar to those of α-Fe 2 O 3 . In the two-phase region,0.02 ⩽ × ⩽ 0.99, the unit-cell parameters of both the Z and F phases did not change, while the fraction of the F phase increased and the fraction of the Z phase decreased as x increased. The terminal solid solubility limits at room temperature. (2.0 ± 0.3) mol% of α-Fe 2 O y in m -ZrO 2 and (1.0 ± 0.3) mol% of m -ZrO 2 in α-Fe 2 O 3 , were found from the dependence of diffraction line intensities of both Z and F phases on x by extrapolation to zero intensity. The existence of solid solutions in the limiting concentration ranges were confirmed by Mossbauer spectroscopy.

Effect of urea on the hydrolysis of Fe3+ ions in aqueous solutions at elevated temperature

Abstract The hydrolysis of 0.1 M FeCl3 or 0.1 M Fe(NO3)3 solutions containing urea was investigated at elevated temperature. β-FeOOH was the principal hydrolytical product of 0.1 M FeCl3 hydrolysis at 90 °C. Its structural stability was preserved up to pH 7.55 for initial concentration of 2 M urea. After the formation of α-FeOOH, the phase transformation β-FeOOH → α-Fe2O3, via dissolution, was accelerated. For the initial concentration of ≥ 3 M urea, the formation of α-Fe2O3 and a small amount of α-FeOOH was observed. In the presence of nitrate anions, the mixtures of α-Fe2O3 and α-FeOOH were produced with tendency for the formation of α-Fe2O3 as the end product. In some samples, the presence of a small amount of γ-FeOOH could be detected on the basis of IR bands at 1024 and 745 cm−1. The 57Fe Mossbauer spectra indicated that α-FeOOH and α-Fe2O3 particles were very small and probably not well-crystallized. The samples were also characterized with transmission electron microscopy.

Mössbauer study of electrodeposited Fe−Cr−Ni alloys

Mössbauer spectroscopy was used to compare Fe−Cr−Ni alloys prepared by different electrochemical as well as thermal methods. The main phase of the electrodeposits was ferromagnetic contrary to the thermally prepared alloys of same composition, which were paramagnetic. Obvious differences can be observed among the spectra of electrodeposited samples of different composition. Narrower line width and smaller paramagnetic contribution can be observed in samples electrodeposited onto A1 substrate compared to those in samples plated onto graphite with modified electrochemical parameters. Changes in hyperfine field distribution of as-deposited and aged samples indicate a precipitation process due to the heat treatment.

Ferritization of Y3+ and Nd3+ ions in the solid state

Abstract The ferritization of Y3+ and Nd3+ ions in the solid state was investigated by X-ray powder diffraction, 57Fe Mossbauer and FT-IR spectroscopies. Magnetization measurements were performed with selected samples between 4.2 and 300 K in magnetic fields up to 20 kOe. Incorporation of Nd3+ ions into a garnet-type structure and the formation of solid solutions of Y3−xNdxFe5O12 were shown. The unit-cell parameter measured for Y3Fe5O12 was 12.359(3) A, whereas for the initial molar ratio Fe2O3:Y2O3:Nd2O3=5:1:2 a value of a unit-cell parameter of 12.512(3) A was measured for the garnet-type solid solution. Hyperfine magnetic fields (HMF) of 487 and 394 kOe at the a- and d-sites, respectively, were measured at 300 K for Y3Fe5O12, while HMF of 509 kOe was measured at 300 K for NdFeO3. The formation of solid solutions of Y3−xNdxFe5O12, caused a gradual shift to lower wave numbers of IR bands defined for Y3Fe5O12. The magnetization measurements of the compounds with nominal composition (Y2Nd)Fe5O12 and (YNd2)Fe5O12 showed that the magnetic moments were higher than in Y3Fe5O12. The magnetic moments at 20 kOe, corrected for presence of some orthoferrite impurity, were 5.75 μB and 6.45 μB, respectively, whereas the moment of Y3Fe5O12 was 4.81 μB, and even in saturation it is only 5 μB. This proves that the Nd sublattice magnetization is coupled ferromagnetically to the total iron magnetization, in agreement with theoretical expectations for light rare earth garnets. The increase in magnetic moment of (YNd2)Fe5O12 relative to Y3Fe5O12 is less than twice of the increase for (Y2Nd)Fe5O12. This result is probably due to the magnetic anisotropy in the Nd-rich compound. (Y2Nd)Fe5O12 seems to exhibit a spin reorientation transition around 60 K, previously not noticed.