Quadrupole Splitting Distributions in Grandidierite and Kornerupine from Antarctica

The Mössbauer spectra of grandidierite and kornerupine at 298 and 90 K were measured. The quadrupole splitting distributions from the Mössbauer spectra were obtained by using the Voigt-based method, and the assignments for QSDs in the Mössbauer spectra of both minerals are presented. Site occupancies of iron in the crystal structures of two minerals were determined, and the chemical formulas of grandidierite and kornerupine were rewritten based on the relative absorption areas and Mössbauer fraction f for Fe³⁺ and Fe²⁺.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Mixed valence of iron in natural vonsenite

The exchange of electrons between adjacent ions in different oxidation states in vonsenite was observed using Mössbauer spectroscopy. The Mössbauer spectra of a series of naturally occurring vonsenite were recorded over a temperature range of 120–773 K. Four quadrupole doublets were resolved by computer fitting and assigned to Fe²⁺(Fel), Fe²⁺(Fe3), Fe³⁺(Fe2, Fe4) and Fe²⁺-Fe³⁺(Fe2–Fe4). The percentage of iron sites participating in an electron exchange process increases from 17% between 120 and 298 K to 27% between 573 and 773 K.     

Next-nearest neighbour effects in the Mössbauer spectra of Cr-spinels: An application of partial quadrupole splittings

Mössbauer spectra of Fe(Cr, Al)₂O₄ spinels are reported and interpreted using partial quadrupole splitting theory. Fe²⁺T site quadrupole splittings are calculated for different next-nearest neighbour configurations. Assuming random distribution of the Cr³⁺ and Al³⁺ in the octahedral M sites, good semiquantitative agreement between calculated and observed quadrupole splittings and absorption intensities is obtained.     

Mössbauer study on M1 and M2 sites of Fe2+ in natural hypersthene under dynamic crystal field potential approach

⁵⁷Fe Mössbauer spectra of hypersthene, a natural silicate mineral belonging to the orthopyroxene group, have been taken over the temperature range 77–292 K. At temperatures above 77 K, they show asymmetric quadrupole peaks. This asymmetry arises from the overlapping of two quadrupole doublets from Fe²⁺(3d⁶,⁵D) ions in two different sites (M1 and M2). The quadrupole splitting, isomer shift and their temperature dependence are appreciably different for Fe²⁺ ions in the two sites. The Fe²⁺ quadrupole splitting in the M1 site decreases linearly with temperature, which can be explained quite satisfactorily by using a very simple model of the orbit-lattice interaction.     

Mössbauer analysis of the firing process of the sky-green glaze of the imitative ancient Chinese Ru porcelain

The variation of the Mössbauer parameters of the imitative ancient Ru porcelain skygreen glaze with the firing conditions is studied in detail in the present paper. The Mössbauer spectra show that the sky-green glaze contains three kinds of iron minerals, i.e. the structural iron (Fe²⁺ and Fe³⁺); Fe₂O₃ and Fe₃O₄. The relative intensity of the paramagnetic peak Fe²⁺ increases and the magnetic ratio of the magnetic peak decreases with increasing temperature. Based on the variation of the quadrupole splitting (QS) of the paramagnetic peak Fe²⁺, the phase transformation characteristics of the sky-green glaze in the firing process is discussed. The coloring mechanism of the sky-green glaze and the variation of its magnetism in the firing process are also investigated in the present paper.     

Mössbauer and magnetic studies of orthorhombic FeSiO3

Magnetic properties of orthoferrosilite FeSiO₃ have been examined using susceptibility, magnetization measurements and Mössbauer spectroscopy. From magnetic and Mössbauer measurements, one obtains close values of the magnetic ordering temperature, T_N=39±1 K and T_N=41±1 K, respectively. The magnetic order is characterized by strong ferromagnetic coupling of Fe²⁺ moments within the ribbons and a weak antiferromagnetic coupling of the moments between adjacent ribbons. The 4.2 K Mössbauer spectra can be fitted with two different hyperfine magnetic fields H_hf=68 kOe and H_hf=314 kOe which can be assigned to Fe²⁺ in the octahedrally coordinated M1 and M2 sites, respectively, of the FeSiO₃ structure.     

Electron delocalization observed in the Mössbauer spectrum of ilvaite

Mössbauer spectroscopy is used to detect species associated with delocalized electrons undergoing Fe²⁺ → Fe³⁺ electron delocation in ilvaite. Mössbauer spectra of a suite of naturally occurring ilvaites were recorded from 80K to 575K and fit to five quadrupole doublets assigned to Fe²⁺ (A), Fe²⁺ (B), Fe³⁺ (A), and Fe²⁺ (A) → Fe³⁺ (A)6c and ⊥c. These assignments disagree with earlier interpretations of temperature dependent quadrupole splitting and isomer shift in ilvaite.     

Mössbauer study of NH4FeCl3: A quasi one-dimensional magnet

NH₄FeCl₃ is a quasi one-dimensional magnet. Mössbauer spectra of powder samples of NH₄FeCl₃ have been recorded in the temperature range 1.3 to 250 K. The structural phase change between 10 and 30 K was not accompanied by changes in the Mössbauer spectra. Apparently, the changes in the local environment of the Fe²⁺ ions are not sufficient to affect the quadrupole splitting. The Mössbauer spectrum at 1.3 K, belowT _N=1.7(2) K, is magnetically split, but cannot be fitted to just one iron site. It appears to be a superposition of at least two broadened spectra, one with an internal fieldB=4.7(1) T perpendicular to the principalz-axis of the electric field gradient, the other with zero field. The intensity for these sites was found to be 3.31. The fit is improved if slow relaxation is allowed between the two sites. Broadening of the lines starts to occur at 6 K, well aboveT _N. This has not been reported for the other members of the AFeX₃ group.     

Characterisation of synthetic trioctahedral micas by Mössbauer spectroscopy

Trioctahedral potassium micas |K}[M₃]〈T₄〉O₁₀(OH)₂ have been synthesized by hydrothermal techniques with various cationic substitutions in the octahedral and the tetrahedral sheet. Taking annite |K}[Fe ₃ ²⁺ ]〈AlSi₃〉O₁₀(OH)₂ as the reference mineral, [Fe²] was replaced by [Mg²] and [Ni²], 〈Al³⁺〉 by 〈Fe³⁺〉 and finally [Fe²⁺] + 〈Si⁴⁺〉 by [Al³⁺] + 〈Al³⁺〉. Mössbauer spectra were evaluated in terms of quadrupole splitting distributions (QSDs) using three generalized sites for 〈Fe³⁺〉, [Fe³⁺] and [Fe²]. Annites, nominally free of 〈Fe³⁺〉, show a lower limit of [Fe³⁺]/Fe _tot of 0.10, which stabilizes the structure. The ferrous iron, [Fe²], QSD consists of two main components. In some of the solid solution series, there is strong experimental evidence for a third ferrous component, particularly at higher [Al³⁺] contents. This third component is centered at low quadrupole splittings and may be assigned to a defect [Fe²] site, forming 1:2 structures with two neighbouring trivalent octahedral cations. For charge compensation one OH^? is replaced by O^2? for each [M³⁺] cation. The ferrous QSDs vary systematically with chemical composition. Compared to those of annite, the QSD parameters (mean quadrupole splitting 〈QS〉 and quadrupole splitting with maximum probability, QS _peak ) are shifted towards higher values with increasing [Mg²] and [Ni²] contents, and decrease slightly with increasing content of trivalent cations. These trends can be interpreted in terms of changes in the local environment around the Fe probe nucleus, i.e., in terms of decreasing or increasing distortions from the ideal octahedral configurations.     

Nanophase mixed-valence iron minerals in meteorites identified by cryogenic Mössbauer spectroscopy

Determination of oxidation states and the crystal chemistry of iron-bearing minerals in carbonaceous chondrites by Mössbauer spectroscopy is complicated by thermally-induced electron-hopping in cronstedtite, superparamagnetism of hydrous ferric oxides and ill-defined contributions from an incommensurate layered iron sulfide phase believed to be tochilinite. Mössbauer spectra measurements at 30 K of several terrestrial cronstedtite and tochilinite specimens have enables modal proportions of these minerals, as well as Fe³⁺/Fe²⁺ ratios, to be determined quantitatively in a suite of CM-type meteorites.     

Mössbauer study of the firing process of the imitative ancient sky-blue Jun porcelain

Mössbauer spectra of the imitative ancient Jun porcelain indicate that the glaze and body materials contain Fe₂O₃, Fe₃O₄ and structural iron. It is clear that during the firing process, the glaze undergoes dehydration, dehydroxylation, vitrification and recrystallization. The Fe²⁺ quadrupole splitting value of the paramagnetic peak of the body material is high even at low firing temperatures. For the body material, the distinction between dehydration and dehydroxylation is not clear. The changes of magnetism of the glaze and body materials in the firing process and the coloring mechanism of the sky-blue Jun porcelain are analyzed in the present paper.     

Mössbauer study of illitic clays containing iron-rich impurities

The iron mineralogy of nineteen illitic clays from eastern Bavaria was studied by Mössbauer spectroscopy and X-ray powder diffraction. Mössbauer spectra of the <2 μm fraction were taken at RT, 120 K and 4.2 K. The clays contain both paramagnetic Fe³⁺ and Fe²⁺. Superparamagnetic oxides are frequently present. The Fe²⁺ quadrupole splitting and the ratio of Fe³⁺ at 4.2 K to Fe²⁺ at 120 K are correlated and define two groups distinguished by their mineral content. The samples were heated systematically for 48 h up to 1250°C in steps of 50°C. One clay which is rich in chlorite and Fe(II) was studied in detail after firing in air and following a reduction for 3 h at 800°C with charcoal. The transformations of the mineral phases with temperature as shown by X-ray diffraction are also evident in the Mössbauer spectra.     

Mössbauer study of cobalt ferrite nanocrystals substituted with rare-earth Y ions

Mössbauer spectroscopy is used to characterize the crystallite size and structure of CoFe_2−xY_xO₄ (x=0, 0.1, 0.3, 0.5) ferrite nanocrystallites synthesized by the sol-gel auto-combustion method. The effect of the substitution of Fe³⁺ ions by Y³⁺ ions on the structure of cobalt ferrite nanocrystallites is investigated. The Mössbauer spectra showed two sets of six-line hyperfine patterns for all the samples, indicating the presence of Fe in both A and B-sites. On increasing the concentration of doped Y, the hyperfine field strength and the isomer shift first increase and then decrease, whereas the quadrupole splitting continuously increases. The superparamagnetism was observed for all the samples and the change of ratio of the superparamagnetism component reflects the size of crystal grain.     

Frequency Spectra of Quantum Beats in Nuclear Forward Scattering of 57Fe: The Mössbauer Spectroscopy with Superior Energy Resolution

Frequency spectra of quantum beats (QB) in nuclear forward scattering (NFS) are analysed and compared to Mössbauer spectra. Lineshape, number of lines, sensitivity to minor sites, and other specific properties of the frequency spectra are discussed. The most characteristic case of combined magnetic and quadrupole interactions is considered in detail for ⁵⁷Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of QB, six of which show the same energy separation as the six lines in Mössbauer spectra. Two other lines (called 2′ and 3′) are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction E _Q appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the 6th (largest frequency) lines. Each of the lines 3 and 5 generates a doublet split by 2E _Q, and the lines 2 and 4 generate triplets. In QB frequency spectra (QBFS) of thin absorbers of GdFeO₃ we demonstrate the enhanced spectral resolution compared to Mössbauer spectra. Small particle size in an antiferromagnet (Fe₂O₃) was found to affect the QBFS via enhancement of the intensity around zero-frequencies. An asymmetric hyperfine field distribution mixes up into the hybridization with dynamical beats, which enlarges the frequencies of the low-lying QBFS lines and makes their shifts relatively large compared to the shift of the highest-frequency line.     

Mössbauer spectroscopy as a probe of magnetic states in La0.5Ca0.5FeO3−δ perovskite

Charge disproportionation in La_0.5Ca_0.5FeO_3−δ perovskite has been detected by zero-field Mössbauer spectra from 20 K to room temperature. On the basis of the parameters of center shifts and hyperfine fields, Mössbauer spectra identified that the iron ionic states are Fe³⁺ and Fe⁵⁺ below 150 K, Fe³⁺, Fe⁴⁺ and Fe⁵⁺ in the intermediate temperature region, as well as Fe³⁺ and Fe⁴⁺ above 220 K. At low temperatures, the system exhibits a cluster-glass-like state resulting from competition between antiferromagnetic interaction of Fe³⁺–Fe³⁺ and ferromagnetic interaction of Fe³⁺–Fe⁵⁺.     

Determination of the quadrupole splitting distributions of the A- and B-site ferrous ions in Fe2TiO4

Fe₂TiO₄ has been investigated using⁵⁷Fe Mössbauer spectroscopy with the aim of determining the shape and the temperature dependence of the separate A- and B-site quadrupole splitting distributions. This has been impossible hereto because both contributions are strongly overlapping and the normal doublet spectra do not contain enough information to differentiate between both. It is found that, through th use of external magnetic fields, the necessary information to perform the separation is unveiled. Furthermore, the change in shape as a function of temperature of the distributions thus obtained is well understood from the known behaviour of Fe²⁺.     

Effect of the water of crystallization on the Mössbauer spectra of hexacyanoferrates (II and III)

Mössbauer spectra were recorded on hydrated, dehydrated and rehydrated hexacyanoferrate (II and III) samples including alkaline, alkaline-earth, transition metal and rare-earth salts. The location of the water molecules around the complex hexacyanoferrate anion has a decisive role on the quadrupole splitting of the low-spin Fe^III cation.     

Structure-property relationships in the R-type hexaferrites: Cation distributions and magnetic susceptibilities of MX2Fe4O11

The strong, and heretofore unexplained, dependence of the magnetic properties of the R-type hexaferrites MX₂Fe₄O₁₁ on heat treatment has been investigated by means of⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopy at 297 K.⁵⁷Fe Mössbauer spectra of samples of BaTi₂Fe₄O₁₁, BaSn₂Fe₄O₁₁, and SrSn₂Fe₄O₁₁, annealed at several temperatures consist of three partially resolved quadrupole doublet patterns. The pattern with the very large quadrupole splitting, i.e. 1.3 mm s^–1 E _Q 1.7 mm s^–1, is readily assigned to the trigonal bipyramidal site, and exhibits no dependence of its parameters, including the relative intensity, on heat treatment. The remaining two patterns are assigned to Fe³⁺ ions on octahedral sites, and exhibit relative intensities that are strongly dependent on heat treatment. The¹¹⁹Sn spectrum of BaSn₂Fe₄O₁₁ is a broad, apparently single line that, with the exception of verifying the presence of tin as Sn⁴⁺, provides little insight into the details of the structure. The distributions of the cations over all the lattice sites have been deduced from the Mössbauer spectra and compared with the magnetic susceptibility data. It is concluded that the cation distributions of the Sn-containing phase are not strongly dependent on heat treatment, and the magnetic properties exhibit a similar insensitivity to heat treatment. The Ti analogs exhibit a strong dependence of cation distribution on heat treatment, which is reflected in widely varying magnetic ordering temperatures, a variation that is confirmed by the incipient magnetic hyperfine splitting despite the strong magnetic frustration in the R-type hexaferrites.     

An Fe Mössbauer effect study of metastable Al86Fe14prepared by rapid solidification

Icosahedral Al₈₆Fe₁₄ has been prepared by rapid quenching from the melt. Mössbauer effect spectra are quadrupole split doublets. The temperature dependence of the isomer shift is consistent with the second order doppler shift and the quadrupole splitting decreases linearly with temperature.