Mössbauer evidence for Fe2+−Fe3+ ordering in magnesioferrochromite

Mössbauer absorption spectra of a natural chromite from Shanxi province of China were measured in a temperature range from 12K to 800K. Each spectrum at low temperature can be fitted to three doublets; the first two are attributed to tetrahedral (T) site Fe ions and the third one to octahedral (M) site Fe ions. As a main result. our data strongly supported an ordered distribution with Fe²⁺ in T-site and Fe³⁺ in M-site for chromite studied.     

Mössbauer spectra of gadolinite and aeschynite

The Mössbauer spectra of the Natural gadolinite and aeschynite at 297K and 81K were measured. The Mössbauer spectrum of the gadolinite consists of one Fe³⁺ doublet and two Fe²⁺ doublets, and the multiple Fe²⁺ doublets in the spectrum of the gadolinite were relating to the next nearest neighbour effect. The Mössbauer spectrum of the aeschynite is composed of one Fe³⁺ doublet and one Fe²⁺ doublet, and this result is in agreement with crystal structure determination.     

Quadrupole Splitting Distributions in Purpurite and Related Minerals

The quadrupole splitting distributions (QSDs) from the Mössbauer spectra of triphylite, ferrisicklerite and purpurite at 298 K and 80 K were obtained by the use of the Voigt-based quadrupole splitting distribution (QSD) method for the first time. QSDs of Fe²⁺ and Fe³⁺ are attributed to Fe²⁺ and Fe³⁺ at the corresponding octahedrally coordinated sites in the crystal structures of the three phosphate minerals. The influence on the distortion of the M2 site by different next-nearest neighbor (NNN) configurations was discussed based on the Jahn–Teller effect in purpurite, and the authors propose two M2 subsites with different distortions in purpurite. Two QSDs of Fe³⁺ in the Mössbauer spectra of purpurite are tentatively assigned to Fe³⁺ at the two M2 subsites, and next-nearest neighbor (NNN) effects were used to interpret the Mössbauer spectra of purpurite.     

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.     

Iron distribution in chevkinite

In this paper Mössbauer spectra of chevkinite at 283K and 83K are presented for the first time. The spectra consist of two Fe²⁺ and two Fe³⁺ doublets, and the assignments were made according to the structure of chevkinite. Iron distribution obtained from the spectrum at 83 K is consistent with that at 283 K.     

Mössbauer study of amphiboles originated from the Carpathian region

Mössbauer spectroscopy was used in order to get information about the differences in the geological occurences at the different areas by investigating the distribution of iron in various crystallographic positions in amphiboles originated from the Carpathian region. The spectra of amphibole samples can be decomposed into doublets corresponding to Fe²⁺ and Fe³⁺ in different crystallographic sites. In the case of spectra recorded up to very high counts a distinction of the M1, M2, M3 and M4 positions occupied by Fe²⁺ was achieved. In contrast to previous investigations, two different doublets of Fe³⁺ were found in each spectrum. These can be associated with Fe³⁺ substituted tetrahedrally as well as in M1, M2, and M3 sites. Dramatic changes in Fe²⁺/Fe³⁺ ratio were observed among the amphiboles (found to be similar by microscopic studies) originated from Mecsek Mts, Balaton Highland and other localities. Consequently, conclusion were drawn for the differences in formation of amphiboles.     

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.     

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.     

57Fe Mössbauer study of YBa4Cu2.95Fe0.05O8.5+y

⁵⁷Fe Mössbauer measurements were performed on the Fe-doped YBa₄Cu₃O_8.5+y compound as a function of annealing temperature in vacuum. Mössbauer spectra were analyzed using three different quadrupole split doublets. The resistance measurements showed that the compound is not superconducting. The possible identification of the Cu sites with their nearest neighbor oxygen sites in this compound is suggested. Comparison of the results with data of the presence of the superconductor YBa₂Cu₃O₇ allows us to suppose that the presence of the Cu-chain sites alone does not result in superconductivity.     

57Fe Mössbauer investigation of naturally oxidised chlorite

The green chlorite, occurring as monomineralic mass in the Sukinda (India) area has been investigated by⁵⁷Fe Mössbauer spectroscopy together with IR, XRD and XRF analyses. The Mössbauer spectra (at RT and 100 K), split into four symmetric doublets, show hyperfine parameters suggesting Fe²⁺ atcis and brucite sites and Fe³⁺ attrans andcis sites. The results show that susceptibility to oxidation (Fe²⁺→Fe³⁺) is highest in thetrans site, moderate in thecis site, and least in the brucite site.     

The electronic structure of ilvaite and the pressure and temperature dependence of its 57Fe Mössbauer spectrum

⁵⁷Fe Mössbauer measurements have been performed on four, chemically analyzed samples of the mixed-valence mineral ilvaite, Fe³⁺Fe₂²⁺(Si₂O₇)O(OH), as polycrystals and oriented-single-crystals between 300 and 475 K and between 1 and 60,000 bar. At 1 bar and T < 400 K, four absorption features are necessary to account for the spectral envelopes: Three are quadrupole doublets which are assigned to the three crystal/chemical species of iron but not without some ambiguities regarding the appropriate electronic structure. Only the doublet assigned to Fe²⁺ on 4c (space group Pnma) corresponds to that of an orthodox, high-spin Fe²⁺ ion. The remaining two doublets are assigned to nominal Fe²⁺ and Fe³⁺ on the 8d site but have hyperfine parameters that exhibit anomalous values and temperature dependences, which lead to a coalescence at ～400K into a single “Fe³⁺-like” doublet. These phenomena are interpreted as arising from a delocalization primarily of the d-electron from the 8d Fe²⁺ ion into a band-like, itinerant state. For (001) oriented crystals, the influence of pressure is remarkable and features associated with extensive electron delocalization in the high temperature spectra are highly developed even at 30kb and 298 K, indicating enhanced electron delocalization as a result of decreased internuclear separations parallel to the infinite ribbons of 8d sites. The spectra of polycrystalline and oriented-(100) and -(010) absorbers change more slowly with pressure and 60 kb pressure is required to achieve the effects of 30 kb on a (001) absorber. The fourth feature in the spectrum consists of a broad absorption at ～1.5mms^?1 and is tentatively assigned to an electronic state of minor importance due to electron transfer between localized Fe²⁺ and Fe³⁺ centers. The low intensity of this absorption feature, ～10% of the total, probably accounts for its neglect by other investigators.     

NGR study of iron molybdates and iron-cobalt molybdates catalysts

Iron molybdates FM and cobalt-iron molybdates CF _x M (which means Co_1?y Fe _y MoO₄ withy=x/100 6≤x≤67), used as matrix of catalysts for propene mild oxidation, were studied by Mössbauer spectroscopy at 295 K and 4 K. All the spectra exhibit three Fe²⁺ doublets, two of them correspond to β-FeMoO₄ and the third one to α-FeMoO₄. Enhancement of quadrupole splittings of Fe²⁺ in the spectra of the CF _x M catalysts is ascribed to the occurrence of a solid solution of either iron in CoMoO₄ or cobalt in FeMoO₄. In each spectrum at 295 K one or several Fe³⁺ doublets were present, which low temperature spectra allow to be assigned to Fe₂(MoO₄)₃, small particles of Fe₂O₃, solid solution of Fe³⁺ in ferrous molybdates and even Fe₂MoO₄ in some cases. The behaviour of all the iron species of these solids during reduction by hydrogen is described.     

A study of the cobalt sites in cobalt and iron molybdate catalysts by emission Mössbauer spectroscopy

Molybdates samples containing 74MBq (2mCi) of cobalt 57 have been studied by emission Mössbauer spectroscopy. Spectra of⁵⁷CoMoO₄ exhibited the three usual Fe²⁺ doublets corresponding to α and β ferrous phases, and two Fe³⁺ doublets accounting for about one third of the absorption area. The Co²⁺ sites of β⁵⁷CoMoO₄ phase exhibit higher quadrupole splittings, i.e. are more dissymmetrical than Fe²⁺ sites of βFeMoO₄ phase. Cobalt and iron molybdates Fe_x ⁵⁷Co_1?xMoO₄ provided spectra where only one Fe³⁺ doublet appeared and occupied less than 7% of the spectral area. It is concluded that a part of cobalt was present in⁵⁷CoMoO₄ as Co³⁺.     

Mössbauer study of glasses in meteorites: the D'Orbigny angrite and Cachari eucrite

Mössbauer spectroscopy measurements at room temperature (RT) and at liquid helium temperature (4.2 K) were carried out on bulk and glass samples from the D'Orbigny (angrite) and Cachari (eucrite) meteorites. The RT Mössbauer spectrum of the bulk sample of D'Orbigny shows the presence of Fe²⁺ in olivine and pyroxene and that of bulk Cachari contains only pyroxene. Very small amounts of Fe³⁺ are also present in the bulk samples, but are attributed to surface contamination. The RT spectra of the D'Orbigny and Cachari glasses are fitted with three doublets, which are assigned to Fe²⁺ at three different octahedral positions. No Fe³⁺ was detected in the glass samples. The spectra of the glasses measured at 4.2 K show the presence of relaxation effects. The results suggest a certain degree of structural ordering in these glasses.     

A polyhedron model for the spinel systems LI x Fe1−x CR2O4

Mössbauer spectra of the system Li⁺ _xFe²⁺ _1?2xFe³⁺ _x[Cr₂]O₄ measured at 150 K consist of one Fe(III) absorption and several Fe(II) doublets. An explanation was able by use of a statistical model of the Fe(II) environments.     

Temperature dependence of the anomalous emission line intensities in57Co:LiTaO3-initial population and relaxation

Mössbauer spectra of⁵⁷Co: LiTaO₃ single crystals were recorded in an external longitudinal magnetic field of 6 T at different temperatures between 4.2 K and 150 K. The spectra were taken at two different orientations of the crystallographicc-axis relative to the magnetic field. The line intensities of the Fe³⁺-subspectra show a temperature dependent anomalous population of the three Kramers doublets of the Fe³⁺ spinS=5/2 ground state. A relaxation broadening is observed at higher temperatures, which cannot successfully be reproduced within a relaxation model taking into account only the six lowest lying electronic states.     

Mössbauer effect study of natural chromites of Brazilian and Philippine origin

Mössbauer spectroscopy has been used to study nine natural chromite samples from deposits throughout Brazil and one sample from Philippine. It was possible to identify three different groups of chromite spinels, depending on the iron distribution over tetrahedral and octahedral sites. Using the Mössbauer Fe³⁺/Fe²⁺ ratio, chemical analysis and x-ray data a chemical formula was proposed for each group.     

Studies of the state of iron contained in α- and β-silicon nitride by Mössbauer and Raman effects

The Mössbauer spectra of β- and α-Si₃N₄ containing Fe have been measured at 300 and 93 K. The former consists of a singlet and a doublet, whereas the latter exhibits paramagnetic hyperfine patterns and a doublet. The single line and hyperfine patterns arise from isolated high-spin Fe³⁺ ions which are bonded equivalently with 4 nitrogen neighbors. Both doublets arise from high-spin Fe³⁺ ion-pairs each of which is strongly bonded by a kind of superexchange force. In β-Si₃N₄ the Fe³⁺ ions occupy the same sites regardless of whether they are doped into the crystal lattice before or after its preparation, while in α-Si₃N₄ the Fe³⁺ ions are located at different sites depending on the doping procedure. From the Raman spectra, the difference in the Mössbauer spectra for the two compounds is attributable to different relaxation by lattice phonons of the host crystals.The Mössbauer spectral lines for β-Si₃N₄ alone have been found to decrease in intensity with tune even at room temperature, such an ageing being observed to be entirely suppressed by γ-irradiation. From different behaviors of the three lines in the ageing and quenching from 1150°C down to room temperature, the ageing is interpretable based on a model that the Fe³⁺ ions jump from substitutional sites into interestitial sites and take place thermal vibration with a large amplitude there.     

The Unusual Mössbauer Spectrum of Beryl

The Mössbauer spectra of several aquamarine samples have been obtained in the temperature range of 4.2–500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe²⁺ doublet (ΔE _Q～2.7 mm/s, δ～1.1 mm/s), with a very broad low-velocity peak. This asymmetry is not caused by preferred orientation since the spectrum collected under the magic angle did not show any difference in the line intensities, nor is it caused by the superposition of a Fe³⁺ doublet. At 4.2 K the spectrum of a deep-blue beryl could be well fitted with three symmetrical doublets, with the major Fe²⁺ doublet accounting for 87% of the total spectral area. At 14 K the symmetry remains, but at 30 K the low-velocity peak is again broad. Surprisingly, the spectrum at 500 K also shows a broad, but symmetrical doublet, with a clear splitting of the lines indicating the presence of at least two Fe²⁺ components. The room-temperature spectrum obtained after the 500 K run shows the same features as before the heating. A meaningful fit for the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed.     

High-temperature cation ordering in olivine: an in situ Mössbauer study of synthetic (Mg0.55Fe0.45)2 SiO4

High-temperature in situ Mössbauer spectroscopy measurements (300–950°C) were done on synthetic olivine of composition (Mg_0.55Fe_0.45)₂ SiO₄ (=Fa45) in order to study the distribution of Fe^2?+? over the M1 and M2 octahedral sites. The spectra are fit with two doublets, which are assigned to Fe^2?+? at the M1 (smaller splitting) and M2 sites. The Fe^2?+? site-occupancies at M1 and M2, obtained from the Mössbauer relative areas, suggest that Fe^2?+? has a slight preference for the M1 site at temperatures below ~500°C, with a tendency of disordering around this temperature. At higher temperatures, Fe^2?+? again prefers to occupy the M1 site, where it shows a considerable order at this site up to 750°C. At still higher temperatures, the spectra indicated partial reduction of the Fa-component into metallic iron and the resolution of the doublets was severely deteriorated.