Variable-temperature 57Fe Research Articles

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe III 4 Fe II 4 } Cubes

Thermally Induced Reversible Metal-to-Metal Charge Transfer in Mixed-Valence {Fe ^III ₄ Fe ^II ₄ } Cubes

Nanocrystalline Transition-Metal Gallium Oxide Spinels from Acetylacetonate Precursors via Solvothermal Synthesis.

The synthesis of mixed-metal spinels based on substituted γ-Ga2O3 is reported using metal acetylacetonate precursors in solvothermal reactions with alcohols as solvents at 240 °C. New oxides of Cr, Mn and Fe have been produced, all of which are formed as nanocrystalline powders, as seen by high-resolution transmission electron microscopy (HR-TEM). The first chromium-gallium mixed oxide is thus formed, with composition 0.33Ga1.87Cr0.8O4 ( = vacant site). X-ray absorption near-edge spectroscopy (XANES) at the chromium K-edge shows the presence of solely octahedral Cr3+, which in turn implies a mixture of tetrahedral and octahedral Ga3+, and the material is stable on annealing to at least 850 °C. An analogous manganese material with average chemical composition close to MnGa2O4 is shown to contain octahedral Mn2+, along with some Mn3+, but a different inversion factor to materials reported by conventional solid-state synthesis in the literature, which are known to have a significant proportion of tetrahedral Mn2+. In the case of iron, higher amounts of the transition metal can be included to give an Fe:Ga ratio of 1:1. Elemental mapping using energy dispersive X-ray spectroscopy on the TEM, however, reveals inhomogeneity in the distribution of the two metals. This is consistent with variable temperature 57Fe Mössbauer spectroscopy that shows the presence of Fe2+ and Fe3+ in more than one phase in the sample. Variable temperature magnetisation and electron paramagnetic resonance (EPR) indicate the presence of superparamagnetism at room temperature in the iron-gallium oxides.

Iron oxide nanoparticles in NaA zeolite cages

Zeolite NaA samples with varying concentration of Fe3+ ions have been prepared by wet chemical method. Based on powder X-ray diffraction, 29Si and 27Al MAS NMR and Fe3+ EPR investigations, the formation of nano-sized ferric oxide particles inside the larger α-cages of zeolite NaA has been established. Both Mössbauer effect and magnetization measurements carried out down to 4.5 K established the superparamagnetic behaviour of these Fe2O3 particles with a blocking temperature of ≈20 K, where the magnetization values showed deviation for the zero field cooled and field cooled samples and the appearance of a very narrow magnetic hysteresis loop below this temperature. For all Fe3+ containing samples the room temperature Mössbauer spectrum is a broad quadrupole doublet with chemical shift, δ ≈ 0.33 mm/s and quadrupole splitting, ΔEq ≈ 0.68 mm/s. Variable temperature 57Fe Mössbauer effect measurements exhibited magnetic features below the blocking temperature and at 4.5 K, the observed spectrum is a broad magnetic sextet characterized by an internal hyperfine field value of ≈504 kOe along with a very weak central superparamagnetic quadrupole doublet.

Synthesis and magnetic properties of an iron 1,2-bisthienyl perfluorocyclopentene photochromic coordination compound

The coordination compound Fe(BM-4-PTP) 2(NCS) 2⋅2MeOH ( 1) including the photoisomerizable ligand BM-4-PTP (1,2-bis(2′-methyl-5′-(pyrid-4″-yl)thien-3′-yl)perfluorocyclopentene) was obtained as an orange powder. The powder turns blue upon photocyclization of the 1,2-bisthienyl photochromic ligand induced by UV light irradiation at room temperature. Photocycloreversion is obtained by visible light irradiation of the material in the solid state. The orange and blue powders were investigated over the temperature range (5–293 K) and pressure range (1 bar–12 kbar) by magnetic susceptibility measurements and variable temperature 57Fe Mössbauer spectroscopy. The photo-induced colour change is accompanied by a distinct magnetic variation at room temperature. Potentialities of this functional optical material for display and data recording are introduced.

Effects of in situ biostimulation on iron mineral speciation in a sub-surface soil

The in situ alteration of Fe redox states in subsurface soils by bacteria, otherwise known as bioreduction, may play a key role in the immobilization of hazardous redox active metals such as U, Tc, and Cr. The objective of this study was to characterize changes in Fe mineralogy occurring in a subsurface soil as a result of biostimulation in order to evaluate the bioremediation potential of this approach. Biostimulation was achieved by injecting glucose into the soil through a small well next to a sampling well. Cores taken from the sampling well were analyzed by variable-temperature 57Fe Mössbauer spectroscopy. Results revealed that biostimulation resulted in an overall loss of Fe from the system and major changes in the distribution of its oxide and oxyhydroxide mineral forms. Compared to the non-biostimulated soil, the spectral components assigned to goethite were greatly diminished in intensity in the samples that had been biostimulated, whereas the hematite component was appreciably increased. The Fe(II):Fe(III) ratio in the non-oxide phase (aluminosilicate clay minerals) also increased, indicating that the bioreduction processes in the soil also affected the redox state of Fe in the constituent clay minerals.

A new example of a tetranuclear iron(III) cluster containing the [Fe 4O 2] 8+ core: preparation, X-ray crystal structure, magnetochemistry and Mössbauer study of [Fe 4O 2(O 2CMe) 6(N 3) 2(phen) 2

Two synthetic procedures have been employed that allow access to the new tetranuclear cluster [Fe 4O 2(O 2CMe) 6(N 3) 2(phen) 2] ( 1), where phen is 1,10-phenanthroline. Complex 1 · 3MeCN displays an unusual structural asymmetry (observed for the second time) in its [Fe 4O 2] 8+ core that can be considered as a hybrid of the bent (butterfly) and planar dispositions of four metal ions seen previously in such compounds with transition metals. Complex 1 has been characterized by variable-temperature magnetic susceptibility studies, and by IR and variable-temperature 57Fe Mössbauer spectroscopies. Magnetochemical data reveal a diamagnetic ground state ( S=0) with antiferromagnetic body–body and body–wingtip interactions between the iron(III) ions of the butterfly core ( J bb=−11 cm −1, J wb=−70 cm −1). Magnetochemical and Mössbauer studies on 1 show that its structural asymmetry has practically no influence on these properties compared with the more symmetric types.

Mössbauer Spectroscopic Studies of Molecule-Based Magnets: Two-Dimensional Complexes Derived from Metal Schiff-Bases and Hexacyanoferrate(III)

Variable-temperature 57Fe Mössbauer measurements on cyanide-bridged 2-D bimetallic assemblies NEt4[Mn(salen)]2[Fe(CN)6] (1), K[Mn(3-MeOsalen)]2[Fe(CN)6] (2) and K[Mn(3- MeOsalen)]2[Fe(CN)6]2 DMF (3), where salen2- = N,N'-ethylenebis(salicylidene iminato) dianion, revealed that these compounds exhibit a long range magnetic ordering below ca. 10 K under zero applied field. The quadrupole splittings of 1 - 3 showed intermediate values between those of typical cyano-bridged 1-D and 3-D complexes. The paramagnetic state of K[Mn(5-Clsalen)]2- [Fe(CN)6] 4 H2O (4) turned to a bulk magnetic one with desolvation

The dynamics of 57Fe nuclei in Fe III–DNA condensates

The dynamics of iron nuclei in the condensates obtained by interaction of Fe III with DNA, Fe III(DNA monomer) 2, have been investigated by variable temperature 57Fe Mössbauer spectroscopy. Studies were effected on gel and freeze-dried samples, obtaining nearly coincident values of the parameters isomer shift and nuclear quadrupole splitting in T ranges 20–260 K. Functions ln( A T/ A 77.3) vs. T, here employed to investigate the dynamics of Fe nuclei, showed linear trends in the T ranges 20–150 and 150–260 K, respectively, the latter with larger slopes. Data coincided for gelled and freeze-dried specimens. No variation of δ or Δ E parameters occurred at the two T intervals, which suggests constancy of structure and bonding with the temperature changes. Functions 〈 x 2〉( T) showed trends analogous to the corresponding functions determined for iron proteins, which were attributed to the occurrence of ‘conformational substates’.

The characterization of the iron content of industrially cast aluminium

Pure intermetallic phases Al13Fe4, Al6Fe, AlmFe and AlxFe have been extracted from Bridgman grown model aluminium–iron binary alloys by dissolving the aluminium matrix in butanol. Each phase has a distinct Mossbauer spectrum and variable temperature 57Fe Mossbauer studies have enabled the Debye temperature $$\theta _D $$ of each phase to be determined. Hence, the variation of the recoil‐free fraction f with temperature is determined for each phase. From this information it is possible to measure the proportion of each phase, either when the phases are extracted or in situ in aluminium. The results obtained can be used in the characterisation of industrially cast aluminium.

Organometallic cation-exchanged phyllosilicates:57Fe Mössbauer spectroscopic study of organoiron complexes on a low-iron montmorillonite

Variable-temperature 57Fe Mössbauer spectroscopy, thermogravimetry and powder X-ray diffraction have been used to study the cations tricarbonyl(η5–2,4-dimethylcyclohexadienyl)iron(1+) and tricarbonyl(η5–2-methoxycyclohexadienyl)iron(1+) as both the hexafluorophosphate salts (A1 and A2, respectively) and when cation-exchanged onto the interlamellar sites of Westone-L (W-L), a low-iron montmorillonite (denoted A1-W-L and A2-W-L, respectively). A combination of thermogravimetry, total carbon analysis and X-ray fluorescence analysis for iron indicated that 40% of the available exchange sites on W-L were occupied by the cyclohexadienyl cations. The isomer shift, δ, and quadrupole splitting, Δ, of both A1 (δ= 0.02 mm s–1; Δ= 1.56 mm s–1) and A2 (δ= 0.02 mm s–1; Δ= 1.47 mm s–1) were unaffected by incorporation of the cations into the interlamellar environment. The hexafluorophosphate salts, A1 and A2, exhibited similar Debye temperatures, θD, and recoilless fractions, f, of 160 ± 2 K and 0.61, respectively, indicating that both iron moities enjoyed the same amount of freedom in the parent salt. In contrast, the values of θD and f for A2-W-L (140 K and 0.54, respectively) were both higher than for A1-W-L (131 K and 0.49), indicating that A2 was held more tightly in the interlamellar environment than A1.

Magnetic and mössbauer characterization of the discontinuous high-spin ( 6A 1)⇄low-spin ( 2T 2) transition in solid bis(pyridoxal 4-phenylthiosemicarbazonato)iron(III) chloride

The iron(III) complexes [Fe(HPhthsa) 2]Cl, [Fe(HEtthsa) 2]Cl and [Fe(HMethsa) 2]Cl were synthesized and the phenomenon of discontinuous spin transition between the high-spin ( 6A 1, S=5/2) and the low-spin ( 2T 2, S=1/2) states in [Fe(HPhthsa) 2]Cl was confirmed by the measurement of temperature dependent magnetism and Mössbauer spectra. The tridendate ligands used were pyridoxal 4-phenylthiosemicarbazone (H 2Phthsa), pyridoxal 4-ethylthiosemicarbazone (H 2Etthsa), and pyridoxal 4-methylthiosemicarbazone (H 2Methsa). The magnetic moments for the spin-crossover complex show a thermal hysteresis of width δ T≈9 K with the transition centered at T c ↑≈299 K for increasing temperature and at T c ↓≈290 K for decreasing temperature. Variable temperature 57Fe Mössbauer results also provide evidence for the presence of a first order phase transition in this complex. The first order (cooperative) nature of the spin-crossover transformation most likely occurs through the extended coupling of ferric complexes via intermolecular H bonds. The EPR results indicate that the ground state is a well separated Kramers doublet in the low-spin complexes.

Variable temperature57Fe Mössbauer studies on polymers of vinylferrocene and 1, 1′-divinylferrocene

Variable temperature57Fe Mossbauer spectra have been recorded in the range 80–300 K for a number of low molecular weight amorphous polymers containing ferrocene units. These include samples of polyvinylferrocene, polydivinylferrocene by anionic initiation and polydivinylferrocene by cationic initiation. A similar non-linear temperature dependence of InA was observed for all the polymers, although they contain ferrocene units in different structural arrangements. A Debye model was extended to include intramolecular vibrations, and a fit to the experimental data was obtained. However, the model did not yield satisfactory fitting parameters. The observed behaviour of the experimental data could be accounted for by assuming that each macromolecule vibrates within an anharmonic potential.