Mössbauer study of some Fe-containing M6 and M5 clusters

Isomer shift (δ) and quadrupole splitting (Δ) parameters have been assigned to the iron sites in [FeRh₅(CO)₁₆]⁻, trans- and cis-[Fe₂Rh₄(CO)₁₆]²⁻, [Fe₃-Rh₃(CO)₁₇]³⁻, [FeRh₄(CO)₁₅]²⁻, [Fe₃Pt₃(CO)₁₅]²⁻ and [Fe₄M(CO)₁₆]²⁻ (M = Pd or Pt) from ⁵⁷Fe Mössbauer spectra recorded at 78 K. The data for the closo compounds [FeRh₅(CO)₁₆]⁻ and [Fe₂Rh₄(CO)₁₆]²⁻ are compared with those for [Fe₆(CO)₁₆C]²⁻. In [Fe₃Rh₃(CO)₁₇]³⁻, the three major Fe sites were identified. For both [Fe₄M(CO)₁₆]²⁻ compounds two isomers were shown to be present in the solid state.     

57Fe Mössbauer spectroscopic studies of electron-hopping processes in vesuvianites

Room temperature and liquid-nitrogen temperature ⁵⁷Fe Mössbauer spectra of a number of vesuvianite crystals are reported. Certain of the spectra contain doublets whose parameters are intermediate between those normally associated with Fe²⁺ and Fe³⁺ ions in oxide environments. These doublets are attributed to electron-hopping processes between Fe²⁺ ions and adjacent Fe³⁺ or Ti⁴⁺ ion.     

57Fe Mössbauer spectroscopic and magnetic study of a spin-crossover polymer complex, Fe(3-chloropyridine)2Ni(CN)4

The coordination polymer Fe(3-chloropyridine)₂Ni(CN)₄ (2) has been prepared by a method similar to that for Fe(pyridine)₂Ni(CN)₄ (1). The complex (2) has been characterized by⁵⁷Fe Mössbauer spectroscopy and a SQUID technique.⁵⁷Fe Mössbauer and magnetic susceptibility data show that complex (2) exhibits spin-crossover behavior. The spin transition of (2) occurs between 120 and 80 K with very small hysteresis or without hysteresis. The temperature range of the spin transition in (2) is lower than that in (1). A residual high spin iron(II) fraction is observed at low temperatures in (2), being different from (1). SQUID data also show that samples treated differently yield different spin transition curves.     

A Mössbauer spectroscopic study of frozen solutions of FeCl3 — phenols

The production of blue, violet or red colourations by the addition of phenols to FeCl₃ solution is shown to be caused by the reduction of the Fe(III) to Fe(II) and the formation of an Fe(II)-phenol complex.     

Infrared and Mössbauer spectroscopic study of the metal-insulator transition in some oxides of perovskite structure

The IR spectra of some LaNi_1−xB_xO₃ (B = Cr, Fe, and Co) compounds having perovskite structure have been studied in the range 1000−300 cm⁻¹. An investigation of the changes in the metal-oxygen stretching frequency as x → x_c from the insulating side has been carried out. An important feature is that as x → x_c the vibrational features in the infrared spectra disappear when the resistivity is ∼10⁻¹ Ω cm which is of two orders of magnitude more than the value of ϱ₀ at which the temperature coefficient of resistance changes sign. Mössbauer studies on Fe-containing samples with various conductivities show that the isomer shift decreases as conductivity increases which is indicative of larger FeO overlap.     

Mössbauer spectroscopic study of an iron(III) complex with crown porphyrin

A complex of ⁵⁷Fe with 5-{4-[((4′-hydroxybenzo-15-crown-5)-5′-yl)diazo]phenyl}-10,15,20-triphenylporphyrin was studied by Mössbauer spectroscopy. The presence of signals of two types in the spectra (a doublet and an extended absorption band over a wide velocity range) suggests that the Fe atoms occupy two structurally different positions in this complex. The dependences of the doublet asymmetry on temperature and the angle between the normal to the sample plane and the γ-ray beam were studied. The isomer shift δ of the doublet in the temperature range from 360 to 5 K changes from 0.25 to 0.41 mm/s, while the quadrupole splitting remains virtually unchanged (Δ ≈ 0.65 mm/s). The relaxation-type absorption over a wide velocity range, the relative area of which strongly varies with temperature, can be described by a broad singlet with the following parameters: δ = 0.30–0.44 mm/s and Γ = 2.8–3.38 mm/s. According to the δ values, both signals are due to Fe(III) derivatives.     

57Fe Mössbauer spectroscopic characterisation of a ferromanganese nodule from the Central Indian Ocean

The iron bearing phases present in a ferromanganese nodule from the Central Indian Ocean have been determined using⁵⁷Fe Mössbauer spectroscopy. The Mössbauer results have been corroborated by XRD, IR and TG-DTA studies. The Mössbauer spectrum of a ferromanganese nodule shows a broad line width which indicates the presence of more than one iron bearing paramagnetic oxide or oxyhydroxide phases where iron is present as Fe³⁺. -FeOOH has been distinctly characterised as one of the iron bearing phases in the nodule. Other oxyhydroxide and oxide phases of iron in the nodule have been ruled out. A typical paramagnetic doublet persists even at very high temperature which has been proposed to be due to iron(III)phosphate. Formation of solid solution of Mn₂O₃–Fe₂O₃ has been observed in the heat treated nodule at 1073 K, which has been characterised by the Mössbauer technique.     

Mössbauer study on thermal decomposition of some hydroxy iron(III) carboxylates

Thermal decomposition of some hydroxy iron(III) carboxylates, i.e., iron(III) lactate, Fe(CH₃CHOHCOO)₃, iron(III) tartrate, Fe₂(C₄H₄O₆)₃ and iron(III) citrate, Fe(C₆H₅O₇) · 5H₂O has been studied in static air atmosphere in the temperature range 298–773 K employing Mössbauer, infrared spectroscopies and themogravimetric methods. The compounds directly decompose to -Fe₂O₃ without undergoing reduction to iron(II) intermediates. An increase in particle size of -Fe₂O₃ has been observed with increasing decomposition temperature. The thermal stability follows the sequence: iron(III) tartrate > iron(III)citrate > iron(III)lactate.     

57Fe Mössbauer and 31P NMR Spectroscopic Characterisation of Fe(CO)3L2 Complexes (L=Phosphite and Phosphine)

Abstract

A series of mixed ligand complexes of the type [Fe(CO)₃L¹L²] (L¹=tri-phenylphosphite and L²=phosphine or phosphite) have been prepared to study the Fe-P bond. The ⁵⁷Fe Mössbauer spectra of trans-[Fe(CO)₃L¹L²] showed a quadrupole splitting doublet characteristic of the disubstituted iron carbonyls in trigonal bipyramidal symmetry. The linear dependence of the quadrupole splittings on the isomer shifts with a positive slope has revealed that the iron-to-phosphorus σ-donation is offset by the phosphorus-to-iron π-back donation. The ³¹P{¹H} NMR spectra showed a couple of doublets assigned to the coordinated phosphite and the coordinated phosphine. The doublet of the phosphite site was generally observed at the down field compared with that of the phosphine site. The coordination shifts increase with the Mössbauer isomer shifts, suggesting that the iron-to-phosphorus π-back donation plays an important role in the Fe-P bond of trans-[Fe(CO)₃L¹L²]. The relatively large coupling constants due to ²J(P,P) have demonstrated that there exists a strong interaction between trans phosphorus ligands through the d_π orbitals of the central iron. The coupling constant is a measure of the bond strength between Fe-P, while the Mössbauer isomer shift reflects the electron density at the iron nucleus. Thus, a linear correlation has been established between these two spectroscopic parameters.     

Magnetic and Mössbauer effect study of YTi(Fe1−xNix)11

The magnetic properties and ⁵⁷Fe Mössbauer spectra of the compounds YTi(Fe_1−xNi_x)₁₁ were investigated. The compounds crystallize in a ThMn₁₂-type structure when x < 0.7. The substitution of nickel for iron results in an increase in Curie temperature with x < 0.5, and a peak is observed in the composition dependance of saturation magnetization at x = 0.07. The Mössbauer effect measurements were carried out at room temperature with x equal to 0.00, 0.04, 0.08 and 0.20. The best fits indicate that nickel atoms preferentially occupy the j and f sites. A maximum was found in the variation of hyperfine field as a function of nickel content, which is in good agreement with the results of magnetic measurements.     

237Np Mössbauer spectroscopic study on heptavalent neptunium compound

²³⁷Np Mössbauer spectrum of the heptavalent neptunium compound [Co(NH₃)₆][NpO₄(OH)₂]·2H₂O was remeasured for certification of the previous investigations. The isomer shift value of the Mössbauer spectrum showed a characteristic value for the heptavalent neptunium at 4.7K. Although the spectrum seemed to consist of two quadrupole-split peaks as previously reported byStone et al., the fitting with using two quadrupole-split peaks did not give a good agreement withStone's data.     

Mössbauer spectroscopic studies of thermal decomposition of substituted pentacyanoferrate(II) complexes

The thermal behaviour of substituted pentacyanoferrates(II) of the type Na₃[Fe(CN₅)L]·xH₂O, whereL=n-, sec-, tert- oriso-butylamine,di-iso-butylamine ortri-n-butylamine, was investigated with the aid of Mössbauer spectroscopy, XRD and TG-DTG-DTA. The Mössbauer spectra of these complexes exhibit a quadrupole doublet with E _Q=0.70–0.83 mm s^–1 at room temperature. The isomer shift, =0.00±0.03 mm s^–1 suggests that the iron atom is in the +2 low-spin state. The complexes start to decompose at 50°C, yielding a residual mass of 5.8 –21.3% in the temperature range 900–950°C. The Mössbauer spectra recorded after heating at 150 and 300°C exhibit an asymmetric doublet, suggesting partial decomposition. The Mössbauer spectra at higher temperature are complex. At different stages of the thermal process, the presence of -Fe₂O₃, -Fe₂O₃, -Fe, Fe₃C and Fe₃O₄ was demonstrated.On leave from A. N. College, Anandwan-442 914, IndiaWe are grateful to the Monbusho (Ministry of Education, Science, Sports and Culture) for the award of a fellowship to RBL and for financial assistance for the research work. Thanks are also due to Dr. T. Nakamoto for valuable cooperation.     

Applications of Mössbauer spectroscopy to the study of electronic materials

A brief introduction to the methodology of the Mössbauer spectroscopy will be given. The main applications to the study of the intrinsic properties of the materials employed in the electronic technology are reviewed.The reported examples prove the effectiveness of this techniques in the study of semiconducting and magnetic materials.Evidence will be given also to the studies of preparation processes of crystalline and amorphous compound.     

Mössbauer study of sodium ferrates(IV) and (VI)

Sodium ferrates(IV) and (VI) were synthesized by the reaction between Fe₂O₃ and Na₂O₂ in a dry oxygen stream. The Mössbauer data for the obtained samples are presented (for Na₂FeO₃–=0.18(2) mm/s; for Na₄FeO₅–=–0.54(2) mm/s). It was shown that pure K₂FeO₄ and Cs₂FeO₄ can be obtained by heating Fe₂O₃ with apropriate alkali metal peroxides.     

Mössbauer study of iron-containing atmospheric aerosols

The Mössbauer technique was applied to study the seasonal variations of iron concentration in atmospheric air. The concentration of iron in air was calculated by the area method from the experimental spectra obtained. From the shapes of the Mössbauer spectra it was concluded that iron appears as Fe₂O₃ in the form of ultrafine particles in the superparamagnetic state. The measurements as a function of temperature [from 300 to 75 K) made it possible to estimate the size of iron-containing aerosol particles. Correlation of the seasonal variations of iron concentration with meteorite activity was discussed. This method was applied also in investigations of iron concentration variations with air radioactivity due to nuclear explosions performed in the atmosphere. Attempts were made to find a relation between air pollution and the concentration of iron in the air.     

(57)Fe Mössbauer isomer shifts of heme protein model systems: electronic structure calculations

We report the results of density functional theory (DFT) calculations of the (57)Fe M?ssbauer isomer shifts (delta(Fe)) for a series of 24 inorganic, organometallic, and metalloprotein/metalloporphyrin model systems in S = 0, (1)/(2), 1, (3)/(2), 2, and (5)/(2) spin states. We find an excellent correlation between calculation and experiment over the entire 2.34 mm s(-1) range of isomer shifts: a 0.07-0.08 mm s(-1) rms deviation between calculation and experiment (corresponding to 3-4% of the total delta(Fe) range, depending on the functionals used) with R(2) values of 0.973 and 0.981 (p < 0.0001). The best results are obtained by using the hybrid exchange-correlation functional B3LYP, used previously for (57)Fe M?ssbauer quadrupole splittings and (57)Fe NMR chemical shifts and chemical shielding anisotropies. The relativistically corrected value of alpha, alpha(rel), converges with the large basis set used in this work, but the exact values vary somewhat with the methods used: -0.253 a(0)(3) mm s(-1) (Hartree-Fock; HF); -0.316 a(0)(3) mm s(-1) (hybrid HF-DFT; B3LYP), or -0.367 a(0)(3) mm s(-1) (pure DFT; BPW91). Both normal and intermediate spin state isomer shifts are well reproduced by the calculations, as is the broad range of delta(Fe) values: from [Fe(VI)O(4)](2-) (-0.90 mm s(-1) expt; -1.01 mm s(-1) calc) to KFe(II)F(3) (1.44 mm s(-1) expt; 1.46 mm s(-1) calc). Molecular orbital analyses of all inorganic solids as well as all organometallic and metalloporphyrin systems studied reveal that there are three major core MO contributions to rho(tot)(0), the total charge density at the iron nucleus (and hence delta(Fe)), that do not vary with changes in chemistry, while the valence MO contributions are highly correlated with delta(Fe) (R(2) = 0.915-0.938, depending on the functionals used), and the correlation between the valence MO contributions and the total MO contribution is even better (R(2) = 0.965-0.976, depending on the functionals used). These results are of general interest since they demonstrate that DFT methods now enable the accurate prediction of delta(Fe) values in inorganic, organometallic, and metalloporphyrin systems in all spin states and over a very wide range of delta(Fe) values with a very small rms error.     

Mössbauer study of the new pyrochlore form of FeF3

Mössbauer spectrometry has been undertaken as a function of temperature on a new form of FeF₃ with the modified pyrochlore structure, recently synthesized by topotactic oxidation. The Mössbauer data (in zero field and in external magnetic field) lead to a noncollinear magnetic structure, in agreement with previous neutron diffraction results. The low value of T_N is discussed in terms of magnetic frustration.