Tetrahedral Fe3 Research Articles

Quadrupole Splitting Distribution of Fe2+ in Synthetic Trioctahedral Micas

About 80 different synthetic trioctahedral micas {K}[Fe2+ 3−x Me x ]〈Si,Al,Fe〉4O10(OH)2 with Me = Ni2+, Mg2+, Co2+, Al3+ and Fe3+ have been synthesized by hydrothermal methods and subsequently investigated by 57Fe Mossbauer spectroscopy. Mossbauer spectra were refined in terms of a quadrupole splitting distribution (QSD) with at least 2 components for Fe2+ and additional components for Fe3+ on octahedral and tetrahedral positions. Three Fe2+ components have to be used in all samples containing distinct amounts of trivalent cations (Fe3+, Al3+). A rough positive correlation between the intensity of this third Fe2+ QSD component and the content of trivalent octahedral cations has been found. Substitution of Fe2+ by smaller divalent cations causes a distinct increase of ferrous quadrupole splitting. This suggests the remaining Fe2+O4(OH)2 octahedra to become more regular. This holds true for micas with 〈AlSi3〉 and with 〈FeSi3〉 composition of the tetrahedral sheet. In micas with Fe3+ in tetrahedral coordination there is a distinct increase of the quadrupole splitting of tetrahedral Fe3+ with decreasing size of the octahedral sheet. This can be explained by an increasing distortion of the second oxygen coordination sphere around tetrahedral Fe3+ due to increasing ditrigonal distortion of the tetrahedral sheet.

Mica Crystal Chemistry and the Influence of Pressure, Temperature, and Solid Solution on Atomistic Models

The 2:1 mica layer is composed of two opposing tetrahedral (T) sheets with an octahedral (M) sheet between to form a “TMT” layer (Fig. 1a⇓). The mica structure has a general formula of A M2–3 T4 O10 X2 [in natural micas: A = interlayer cations, usually K, Na, Ca, Ba, and rarely Rb, Cs, NH4, H3O, and Sr; M = octahedral cations, generally Mg, Fe2+, Al, and Fe3+, but other cations such as Li, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn can occur also in mica species; T = tetrahedral cations, generally Si, Al and Fe3+ and rarely B and Be; X = (OH), F, Cl, O, S]. Vacant positions (symbol:□) are also common in the mica structure (Rieder et al. 1998). In the tetrahedral sheet, individual TO4 tetrahedra are linked with neighboring TO4 by sharing three corners each (i.e., the basal oxygen atoms) to form an infinite two-dimensional “hexagonal” mesh pattern (Fig. 1b⇓). The fourth oxygen atom (i.e., the apical oxygen atom) forms a corner of the octahedral coordination unit around the M cations. Thus, each octahedral anion atom-coordination unit is comprised of four apical oxygen atoms (two from the upper and two from the lower tetrahedral sheet) and by two (OH) or F, Cl, O and S anions [the X anions, usually indicated as the OH or O(4) site]. The OH site is at the same level as the apical oxygen but not shared with tetrahedra. In the octahedral sheet, individual octahedra are linked laterally by sharing octahedral edges (Fig. 1c⇓). The smallest structural unit contains three octahedral sites. Structures with all three sites occupied are known as trioctahedral, whereas, if only two octahedra are occupied [usually M(2)] …

Poisoning Effect of SO2 on NO Reduction by i-Butane over Fe/ZSM-5 Prepared by Sublimation Method

X-ray photoelectron spectroscopy (XPS) in situ electron paramagnetic resonance (EPR), in situ Fourier transform infrared spectroscopy (FT-IR), SO2 temperature-programmed desorption (TPD), and reaction tests were performed to investigate the poisoning effect on the selective catalytic reaction of NO by i-butane over Fe/ZSM-5. Kinetic studies at 623 K, showed that the presence of SO2 in the reaction mixture poisoned Fe/ZSM-5 irreversibly. XPS studies of Fe/ZSM-5 after reaction with SO2 at 623 K revealed the formation of sulfate species, which corresponds well to the SO2 TPD result. in situ EPR spectra showed that the lines arising from the distorted tetrahedral Fe3+ ions at g≈5.8 and 6.5, which were known to be reactive species, disappeared and a new and sharp one at g≈1.9978 assigned to paramagnetic coke species appeared. The preadsorption of SO2 suppressed the formation of the IR bands arising from Fe–NO2 (1623 cm−1) and Fe–NO3 (1573 cm−1). On the basis of these results, SO2 deactivated Fe/ZSM-5 by suppressing the formation of Fe–NOy (y=2, 3) complexes and promoting the formation of carbonaceous deposits, which resulted in consumption of the active Fe3+ sites.

Cordierite III: the site occupation and concentration of Fe3+

Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18.x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mossbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mossbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000 °C for 2 days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000 °C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000 cm−1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.

Mössbauer, EPR, and MCD studies of the C9S and C42S variants of Clostridium pasteurianum rubredoxin and MDC studies of the wild-type protein.

Rubredoxins contain a mononuclear iron tetrahedrally coordinated by four cysteinyl sulfurs. We have studied the wild-type protein from Clostridium pasteurianum and two mutated forms, C9S and C42S, in the oxidized and reduced states, with Mössbauer, integer-spin EPR, and magnetic circular dichroism (MCD) spectroscopies. The Mössbauer spectra of the ferric C42S and C9S mutant forms yielded zero-field splittings, D = 1.2 cm(-1), that are about 40% smaller than the D-value of the wild-type protein. The 57Fe hyperfine coupling constants were found to be ca. 8% larger than those of the wild-type proteins. The present study also revealed that the ferric wild-type protein has delta=0.24+/-0.01 mm/s at 4.2 K rather than delta = 0.32 mm/s as reported in the literature. The Mössbauer spectra of both dithionite-reduced mutant proteins revealed the presence of two ferrous forms, A and B. These forms have isomer shifts delta = 0.79 mm/s at 4.2 K, consistent with tetrahedral Fe2+(Cys)3(O-R) coordination. The zero-field splittings of the two forms differ substantially; we found D = -7+/-1 cm(-1), E/D = 0.09 for form A and D = +6.2+/-1.3 cm(-1), E/D = 0.15 for form B. Form A exhibits a well-defined integer-spin EPR signal; from studies at X- and Q-band we obtained g(z) = 2.08+/-0.01, which is the first measured g-value for any ferrous rubredoxin. It is known from X-ray crystallographic studies that ferric C42S rubredoxin is coordinated by a serine oxygen. We achieved 75% reduction of C42S rubredoxin by irradiating an oxidized sample at 77 K with synchrotron X-rays; the radiolytic reduction produced exclusively form A, suggesting that this form represents a serine-bound Fe2+ site. Studies in different buffers in the pH 6-9 range showed that the A:B ratios, but not the spectral parameters of A and B, are buffer dependent, but no systematic variation of the ratio of the two forms with pH was observed. The presence of glycerol (30-50 % v/v) was found to favor the B form. Previous absorption and circular dichroism studies of reduced wild-type rubredoxin have suggested d-d bands at 7400, 6000, and 3700 cm(-1). Our low-temperature MCD measurements place the two high-energy transitions at ca. 5900 and 6300 cm(-1); a third d-d transition, if present, must occur with energy lower than 3300 cm(-1). The mutant proteins have d-d transitions at slightly lower energy, namely 5730, 6100 cm(-1) in form A and 5350, 6380 cm(-1) in form B.

The crystal and magnetic structure of nonstoichiometric K + β-ferrite

The crystal and magnetic structure of K + β-ferrite, K 1.33Fe 11O 17, has been studied with neutron powder diffraction ( λ=1.47) at 10, 295 and 923 K. Parameters describing the crystal and magnetic structure were refined with the Rietveld method. A magnetic phase transition at 713 K occurs from an antiferromagnetic to a paramagnetic state. The nuclear structure (space group P6 3/mmc, Z=2) consists of spinel blocks of iron and oxygen atoms interleaved by conduction layers where the K + ions reside. The net magnetic structure is antiferromagnetic consisting of two ferrimagnetic spinel blocks with opposite spins in the unit cell. At 10 K, the derived magnitudes of the moments for the four crystallographically independent iron atoms are: 4.37(4) μ B for the octahedral site Fe1, 3.82(10) μ B and 4.09(11) μ B for the tetrahedral sites Fe2 and Fe3, respectively, and 3.89(15) for Fe4, the second octahedral site. The K + ions in the conduction planes are refined in two 6h-sites (0.70, −x, 1 4 and 0.83, −x, 1 4 ) with an occupation 0.92 and 0.41 for a total of 1.33 K + ions at both 10 and 295 K. Above the Neél temperature an ordering of the column oxygen ion O5 occurs together with an ordering of the K + ions into the 6h-positions (0.71, −x, 1 4 ) and (0.84, −x, 1 4 ) with the occupations 0.64 and 0.73. Magnetic disordered diffuse scattering is observed at 923 K.

Local order and magnetic behavior of amorphous and nanocrystalline yttrium iron garnet produced by swift heavy ion irradiations

Thin epitaxial films of gallium or scandium-doped and undoped yttrium iron garnet (Y3Fe5O12 or YIG) on nonmagnetic Gd3Ga5O12 substrates were irradiated with swift heavy ions (50 MeV 32S, 50 MeV 63Cu, and 235 MeV 84Kr) in the electronic slowing down regime. The mean electronic stopping power in the films was always larger than the threshold for amorphous track formation in YIG which is around 4.5 MeV/μm in this low ion-velocity range. The local order and magnetic properties of the damaged films were then studied at room temperature by Fe57 conversion electron Mössbauer spectroscopy (CEMS) and x-ray absorption spectroscopy (XAS) at the iron K edge in the fluorescence mode. In the case of paramagnetic gallium or scandium-substituted films (YIG:Ga, YIG:Sc) irradiated with S32 or Cu63 ions, the CEMS data show that the tetrahedral Fe3+ sites are preferentially damaged, while the octahedral sites are conserved. This is confirmed by the decrease of the pre-edge peak in the XAS data of the ferrimagnetic undoped YIG films showing that the number of tetrahedral iron sites is decreased in the amorphous phase obtained with Kr84 ion irradiation, due to the formation of fivefold-coordinated pyramidal sites, as already found in a previous study on undoped YIG sinters amorphized by 3.5 GeV Xe132 ion irradiation. In the case of the nanophase induced by ion-beam recrystallization of the tracks with S32 or Cu63 irradiations, a further decrease of the pre-edge peak is found. This is interpreted by (i) an increase of the fivefold-coordinated pyramidal sites and/or (ii) a probable decomposition of the garnet into orthoferrite (YFeO3) and haematite (α-Fe2O3) under the high-pressure and high-temperature conditions in the thermal spike generated by the ions. The CEMS data of irradiated undoped YIG also show that both the amorphous and nanocrystalline phases have a paramagnetic behavior at room temperature. The nanophase magnetic behavior is analyzed on the basis of a superparamagnetic relaxation above the blocking temperature, whereas the amorphous phase behavior is ascribed to a speromagnetic state.

Mineralogical and chemical characteristics of five nontronites and Fe-rich smectites

AbstractFive Fe–bearing dioctahedral smectites (three nontronites and two Fe-rich smectites) were purified using a variety of physical and chemical procedures. The structural formulae indicate one nontronite and one Fe-rich smectite to be montmorillonitic, whereas the other three smectites are beidellitic. Mössbauer spectra showed Fe to be exclusively trivalent and were fitted with three doublets, two of which had quadrupole splittings characteristic of Fe3+in octahedral coordination, whereas the third had a distinctly lower quadrupole splitting. Although the position of the Si–O stretching band in the infrared spectra could reflect tetrahedral Fe3+, the lack of distinctive features prevented a definitive attribution of this component to tetrahedral Fe3+. The18O/16O data suggest that fractionation of nontronite-water at ambient temperatures (1000 lnα = 23 ± 2‰) is lower than that of Fe-rich smectite (1000 lnα = 27 ± 2‰). The estimated formation temperatures of the samples are below 70°C.

Origin of exchange field reductions in diluted magnetic garnets

New theory on the source of magnetic frustration that causes canting of spin alignments in diluted ferrimagnetic systems is presented, providing a fundamental basis for the widely used semiempirical relations between molecular field coefficients and sublattice dilution fractions in the ferrimagnetic garnets. The theory explains the remarkable linearity and symmetry found in the original relations between the coefficients and the sublattice dilution fractions. It also distinguishes between the dilution and canting contributions to the weakening of the exchange fields, which has remained an unresolved issue. As part of the analysis, a probability is defined for each sublattice to characterize the canting effect on a particular spin caused by the removal of a neighboring magnetic ion from the opposing sublattice. The canting probabilities are found to be almost identically 0.07 for both the octahedral and tetrahedral garnet Fe3+ sublattices, thereby confirming the overall reciprocity of the garnet magnetic system.

Nuclear magnetic relaxation in ferrimagnetic Y3Fe5−x SixO12 films

The effect of silicon impurities on the damping of spin-echo signals from the 57Fe nuclei of tetrahedral Fe3+ ions in epitaxial yttrium-iron-garnet films was investigated. It was found that for silicon concentrations 0.015⩽x⩽0.037 the damping of the spin echo is a two-component process, which made it possible to separate nuclei into two types, differing by both the longitudinal and transverse magnetic relaxation times. For silicon concentrations 0.044⩽x⩽0.073 the decay of the echo can be described by one exponential and all nuclei in the sample have the same transverse relaxation times and the same longitudinal relaxation times. The experimental results are interpreted on the basis of the supposition that impurity “macromolecules” form around the Si4+ ions. The relaxation times of the iron nuclei in a “macromolecule” are much shorter than the relaxation times of iron nuclei belonging to the matrix ions. The radius of a “macromolecule” is estimated on the basis of percolation theory.

Crystallization and the electrical conductivity of vanadate glass

The 57Fe Mossbauer spectrum of 25K2O·65V2O5· 10Fe2O3 glass showed a doublet due to tetrahedral Fe3+. The linewidth showed a gradual decrease from 0.49 to 0.27 (±0.02) mm s-1 after heat treatment at 340°C for 10--600 min, reflecting an increased uniformity of Fe--O lengths and O--Fe--O angles. The quadrupole splitting showed a gradual decrease from 0.61 to 0.50 (±0.02) mm s-1 after the heat treatment, reflecting a decreased distortion of FeO4. A jump of the electrical conductivity (σ) was observed from the order of 10-8 S cm-1 to 10-4 S cm-1 after treatment at 380°C for 10--30 min because of the structural relaxation which probably caused an increase in the 3d-electron (polaron) hopping from V4+ to V5+. The value of σ decreased gradually after heat treatment of 60--600 min and became comparable to that of original glass when the precipitation of KV3O8 phase became pronounced.

Refined Relationships between Chemical Composition of Dioctahedral Fine-Grained Micaceous Minerals and Their Infrared Spectra Within the OH Stretching Region. Part II: The Main Factors Affecting OH Vibrations and Quantitative Analysis

AbstractA new model for the interpretation of dioctahedral mica infrared (IR) spectra in the OH stretching vibration region is proposed. It is based on the analysis of the main factors responsible for the observed sequence of the OH frequencies. In terms of this model, the simple analytical dependence between the OH frequencies and the mass and valency of cations bonded to OH groups has been found. The specific character of the interaction between octahedral Al and OH groups in the mica structures is assumed.Integrated optical densities of the OH bands determined by the decomposition of the studied mica IR spectra are used for the quantitative analysis, that is, for the determination of a number of each type of octahedral cations per unit cell of the sample under study. A good agreement between the octahedral cation contents of 2:1 layers found from the IR spectra decomposition and the chemical analysis has shown that this technique may be used to study the local order-disorder of isomorphous cation distribution in mica structures.The essential result obtained by the quantitative analysis of the mica IR spectra is the determination of Fe3+ cations in the tetrahedral sites of some samples. This means that the conventional presentation of structural formulae for Al-Fe3+-containing 2:1 layer silicates is unacceptable without consideration of tetrahedral Fe3+ by spectroscopic techniques and by the quantitative analysis of the IR spectra, in particular.

Synthesis and characterization of ferrisilicate analogs of ferrierite (Fe-FER) zeolites

Ferrisilicate analogs of ferrierite (FER) type zeolite having SiO2/Fe2O3 ratios ranging between 30 and 200 have been synthesized using pyrrolidine as the templating agent. On calcination, the samples develop a typical buff color indicating conversion of some tetrahedral Fe3+ into octahedral Fe3+. Correspondingly, a doublet in the Mossbauer spectrum with isomer shift at 0.15 and 0.68 mm s-1 was observed. The Fe-FER samples were further characterized by UV-vis, IR and TG/DTA. Fe-FER samples were found to exhibit catalytic activity for n-hexane oxidation reactions with hydrogen peroxide as oxidizing agent.

Iron Distribution in High P-T Garnets from Sittampundi Granulites, and its Geochemical Implications

Garnets from high P—T granulite field of Sittampundi, Tamil Nadu were investigated by 57Fe Mössbauer spectroscopy. The room temperature (298 K) three-band spectra, which have been splitted to two doublets, reveal Fe2+ ions to be exclusively located at dodecahedral site (IS = 1.303–1.389 mm/s; QS = 3.423–3.822 mm/s) and Fe3+ at tetrahedral site (IS = 0.0060.163 mm/s; QS = 0.520–0.663 mm/s). The Fe3+/∑Fe ratios vary between 0.073 and 0.150, which indicate its formation at high P—T field. The high values of QS of tetrahedral Fe3+ suggest that the occupied site is very regular with little distortion. The fairly broad line-width (FWHM, 0.455–0.596 mm/s) of Fe3+ doublet may arise from a substitution of some tetrahedral Si by (OH)− group. The strong partition and ordering of Fe2+ and Fe3+ in dodecahedral and tetrahedral sites, having octahedral sites totally free from iron, suggest in a strong measure of slow cooling and recrystallization of garnet through decompression. The presence of tetrahedral Fe3+ strongly depicts the existence of octahedral Si, amounting to an excess of saturation value for tetrahedral sites by Si + Fe3+ ions together at these sites.

Octahedral-site Fe2+ quadrupole splitting distributions from M�ssbauer spectroscopy along the (OH, F)-annite join

We have performed a detailed Mossbauer study of synthetic annites on the (OH, F)-join. Recently developed data treatment and spectral analysis methods were used to extract true intrinsic Fe2+ quadrupole splitting distributions (QSDs) that represent the most information that can be resolved from the spectra. The overall room temperature (RT) QSDs can be consistently interpreted in terms of four QSD contributions (or populations) centered at: QSHH∼2.55 mm/s for Fe2+O4(OH)2 octahedra (cis and trans not resolved), QSHF ∼ 2.35 mm/s for Fe2+O4(OH)F octahedra (cis and trans not resolved), QScFF∼2.15 mm/s for cis-Fe2+O4F2 octahedra, and QStFF∼ 1.5 mm/s for trans-Fe2+O4F2 octahedra. Each such contribution has a width (σΔ ∼ 0.2 mm/s) caused by distortions of the octahedra. Minor contributions due to Fe2+O5(OH) and Fe2+O5F octahedra probably also contribute to the overall Fe2+ QSDs. The ferric iron spectral components were also characterized. Here, two distinct types of octahedral Fe3+ contributions are seen and interpreted as being due mainly to Fe3+O5OH and Fe3+O5F octahedra, respectively. Tetrahedral Fe3+ is seen only in the OH-annite end-member and the total Fe3+ content drops significantly on addition of F.

Faraday effect in garnets with tetrahedral Fe3+ ions

We present original data on Faraday rotation and MCD in the garnet Ca3ZrSbFe3O12 with magnetic Fe3+ ions situated in the tetrahedral sites. The magneto-optical measurements were performed in the wavelength region 0.42–1.15 μm at temperatures of 6–300 K. The analysis of the spectral and temperature dependences of the one-ion-based FR model enabled us to identify the individual contributions to the magneto-optical effects.

Temperature and Composition Dependence of the Cation Distribution in Synthetic ZnFeyAl2-yO4 (0 ≤ y ≤ 1) Spinels

Members of the spinel solid solution series ZnFeyAl2-yO4 (y = 0, 0.2, 0.4, 0.6, 1.0, 1.5, and 2) were synthesized by direct solid state reaction of the simple metal oxides in air at 1400 K. Two aliquots of the single-phase spinels obtained for each composition were annealed at 1350 and 1000 K for 48 hr and then quenched in water.The structural study of the quenched samples was performed by powder XRD, using the Rietveld method of structure refinement. The inversion degrees estimated for ZnFe2O4 up to 1350 K are in good agreement with those published by O'Neill (Eur. J. Mineral. 4, 571 (1992)) in the range 773-1223 K. The nonlinear enthalpy of disordering model of O'Neill and Navrotsky (Am. Mineral. 68, 181 (1983); Am. Mineral. 69, 733 (1984)), recently applied to several studies of simple spinel oxides, could successfully describe the cation arrangement in spinels of the binary solid solution series prepared in this work, with parameters αZn-Al = +82 kJ mole-1, αZn-Fe3 = +46 kJ mole-1, β = -20 kJ mole-1, and σ=0.57Fe Mössbauer spectra collected at 300 and 80 K consisted of only one doublet. Although the contribution of tetrahedral Fe3+ could not be resolved, the variation of the estimated average hyperfine parameters with composition and quenching temperature could be correlated with the cation arrangement deduced from XRD data.

Magneto-optical properties of Al and In-substituted CeYIG epitaxial films grown by sputtering (abstract)

Cerium substitution to iron garnet strongly enhances magneto-optical (MO) effect in the visible region around hν=1.4 and 3.1 eV. We have previously proposed a Ce3+(4f)-Fe3+ (tet.) charge transfer model as an electronic transition inducing the strong MO effect. In this study, we have measured Faraday spectra of CeYIG films in which Fe ions were diluted with Al or In ions. These ions, respectively, are known to preferentially occupy tetrahedral and octrahedral iron sites in the garnet structure, giving us some information on the role of Fe3+ in the MO enhancement. The Y2Ce1Fe5−xMxO12 (M=Al, In; x=0–5) films were epitaxially grown in situ on (111)-oriented Gd3Ga5O12 (GGG) single crystal substrates by conventional rf diode sputtering. With the amounts of substitutions increasing, Faraday rotation and ellipticity of the films at hν=1.4 eV reduced at nearly the same rates for both ions of Al and In. We found from the analysis using molecular field theory that these reductions are in proportion to the magnetic moment of the tetrahedral iron sublattice. This indicates that the tetrahedral Fe3+ contributes to the electronic transition at 1.4 eV, supporting the proposed charge transfer model.

The Mössbauer Spectrum of Illite

AbstractEight illites were studied by Mössbauer spectroscopy at room temperature and 4.2 K. On the basis of their Fe contents, these illites can be divided into an Fe-rich (>5 wt% Fe) and an Fe-poor (<1 to ∼3 wt% Fe) group. Mössbauer spectroscopy showed that the Fe-rich illites had lower proportions of divalent Fe than their Fe-poor counterparts. Slow paramagnetic relaxation was observed in the Fe-poor illites and must be accounted for when fitting the spectra. Two samples contained iron oxides that were superparamagnetic and thus, although contributing to the Fe3+ doublets, escaped detection at room temperature. Structural Fe3+ had a temperature-independent quadrupole splitting that was lower for the Fe-rich illites (0.59 mm/s) than that of the Fe-poor illites (0.73 mm/s). The Fe2+ quadrupole splittings were higher at 4.2 K than at room temperature, but the Fe-rich illites again had lower Fe2+ quadrupole splittings both at room temperature (2.69 vs. 2.88 mm/s) and at 4.2 K (2.96 mm/s vs. 3.08 mm/s). Distinction of Fe sites in the illites with cis- and trans-OH coordination was not possible. The presence of tetrahedral Fe3+ was observed only in the most Fe-rich sample (8.4 wt% Fe).

Mössbauer spectral invetigations of ultrafine rare-earth iron garnets

Ultrafine rare-earth iron garnets (RIG) in the crystalline size range 1.0–35 nm, prepared by the citrate precursor method have been investigated by Mossbauer spectroscopy at room temperature. The crystallites of 10–35 nm size show two six-line spectra corresponding to tetrahedral and octahedral Fe3+ with isomer shifts, δ, and magnetic hyperfine fields, H hf, comparable to the bulk polycrystalline garnets. However, 1.0–1.5 nm crystallites (X-ray amorphous) show a quadrupole split doublet due to superparamagnetic nature with isomer shifts (0.3 mms−1) internediate between tetrahedral and octahedral values of bulk garnets. The relative increase of tetrahedral δ and the decrease in the octahedral δ indicate, respectively, an increase of tetrahedral Fe-O bond distance and a decrease of octahedral Fe-O bond distance in the ultrafine state, in complete agreement with the observations made earlier in the spectrochemical investigations. The lines are broad (linewidth=0.65 mms−1) and the quadrupole splittings are relatively larger (ΔE Q = 1.0 mms−1) indicating large deviation from cubic symmetry at the Fe3+ sites.