Mössbauer study and molecular orbital calculations on some bimetallic derivatives of ferrocene and ferricinium

We have studied by absorption Mössbauer spectroscopy, some bridged derivatives of biferrocene, unoxidized (Fe^II– Fe^II) and mono-oxidized (Fe^II – Fe^III). The mixed-valence species exhibits a quasi-delocalized behavior at the Mössbauer timescale (～ 10^?8 s), with a splitting of the lines which is interpreted in terms of a partial valence trapping. This in turn reveals intramolecular properties at variance from those of the unbridged cation. SCC-Xα MO calculations have been performed; they correctly reproduce the measured quadruple splittings and provide evidence that the HOMO of the mixed-valence complex is of \(d_{xy} ,d_{x^2 - y^2 } \) type. Some consequences on the mixed-valence properties are discussed.     

Mössbauer spectroscopic studies of ferrocene adsorbed on silica gel

Mössbauer spectra of ferrocene adsorbed on silica gel were measured in order to study the state of adsorption. Ferrocene adsorbed on silica gel tended to oxidize in air and form ferricenium ion. It is assumed that the oxidation of adsorbed ferrocene was caused by surface hydroxyls on the silica gel and O₂ in air. It was observed that ferrocene adsorbed weakly at 293 K, although the ferricenium ion adsorbed strongly at 293 K. Thus the adsorption states of ferrocene depend on the experimental condition.     

A Mössbauer spectroscopic study of the magnetocrystalline anisotropy inα-FeOOH

Mössbauer spectra are obtained for a single crystal of -FeOOH, with a magnetic field of up to 10T applied along the c axis. No spin reorientation is observed, indicating a lower limit of 6 × 10⁴ J/m³ for the anisotropy constant K. A theoretical estimate gives K > 1.1 × 10⁶ J/m³.     

A Mössbauer spectroscopic study of cobalt-iron molybdates

Fe _x Co_1–x MoO₄ compounds prepared by coprecipitation were studied by XRD, electrical conductivity and mainly by absorption and emission Mössbauer spectroscopy. FeMoO₄ and CoMoO₄ samples were shown to contain Fe³⁺ and Co³⁺, respectively, in solid solution. Three kinds of Fe _x Co_1–x MoO₄ solids can be described. Forx0.16: one has a -Co(Fe²⁺, Fe³⁺)MoO₄ solid solution. For 0.17x0.25: one has the same solid solution with its surface rich in Fe³⁺. Forx0.26: one has the same solid solution with only bulk Fe³⁺, and ferric molybdate. Studies of reduction by hydrogen and of catalytic reaction of mechanical mixtures of CoMoO₄ and ferric molybdate support these statements.     

Gamma-x ray coincidence Mössbauer spectroscopic study of the aftereffects in sulfate hydrates

The anomalous charge states formed after the electron capture decay, of⁵⁷Co in FeSO₄·H₂O and FeSO₄·7H₂O are investigated using the conventional Mössbauer emission spectroscopy and a gamma-X ray coincidence method. This method is based on the idea that a Mössbauer spectrum observed with the coincidence technique only when K-X rays are emitted is reflected by isolated events with a reduced influence of the Auger-electron self-irradiation. The formation of the anomalous electronic and structural configuration is attributed to the self-radiolysis of the H₂O and SO ₄ ^2? ligands in the nearest and the second nearest coordination shells around the decaying atom.     

Mössbauer and infrared spectroscopic studies of novel mixed valence states in cobaltous ferrocyanides and ferricyanides

Novel mixed valence states have been obtained by the treatment of cobaltous ferrocyanides (Co⁺²Fe^II) and ferricyanides (Co⁺²Fe^III) in an ozone flow. The CN stretching bands occur at 2085 cm^–1 for Co⁺²Fe^II and at 2160 cm^–1 for Co⁺²Fe^III. After the ozonization process of Co⁺²Fe^II, an intense band approximately at 2125 cm^–1 is detected. This intermediate band must correspond to a mixed valence state of the type: Fe^II–CN–Co²⁺–NC–Fe^III Mössbauer spectra recorded in situ during the ozonization of Co⁺²Fe^II show the presence of two components: a doublet with isomer shift and quadrupole splitting values close to the cobalti ferricyanide and a very broad line for the mixed valence state. From the Mössbauer and infrared spectra of the aged samples of the Co⁺²Fe^II after ozonization, a relaxation process to the initial state of the samples is observed but the mixed valence state is stable.     

Mössbauer spectroscopic study of iron fluoride surface layers

The reaction products formed during exposure of iron foils to hydrogen fluoride in the presence of atmospheric oxygen and water have been examined with conversion electron Mössbauer spectroscopy (CEMS) [1]. After exposure for several hours the product in the corrosion layer was found to consist mainly of the mixed-valence iron fluoride, Fe₂F₅·7H₂O. Although the products formed during the initial steps of the reaction could not be completely characterized, a model for the reaction process is suggested.     

Mössbauer study of the vibrational anisotropy and of the light-induced population of metastable states in single-crystalline guanidinium nitroprusside

M?ssbauer studies were performed on single crystals of guanidinium nitroprusside with different orientations of their principal crystallographic axes (a, b, c) with respect to the incident radiation. The markedly anisotropic Lamb-M?ssbauer factor f _LM , i.e. f _LM ^(a) = 0.118(8), f _LM ^(b) = 0.174(8), f _LM ^(c) = 0.202(8) is in contrast to that of nitroprussides with inorganic anions. The observed anisotropy is ascribed to the anisotropic vibrational mean-square displacement of the nitroprusside anions as a whole which is due to the specific packing of both, anions and cations, as well as the very weak chemical bonding between the ions, typical only for guanidinium nitroprusside. The vibrational anisotropy of iron atoms in barium nitroprusside that has been observed by X-ray structural investigations has a different origin and therefore does not result in an anisotropic Lamb-M?ssbauer factor. We have also investigated metastable states in guanidinium nitroprusside that have been populated by means of incoherent irradiation from light-emitting diodes. With a specific orientation of the guanidinium nitroprusside single crystal a population of the metastable states up to 26% could be achieved. Populations of comparable size on lithium, sodium and potassium nitroprussides have only been reached using coherent laser irradiation. Received 15 December 1998 and Received in final form 3 March 1999     

Mössbauer spectroscopic studies on mixed-valence states in trinuclear iron carboxylates

Mixed-valence states of iron atoms in newly synthesized mixed-valence trinuclear iron carboxylate complexes with -bonding ligands, such as acrylate, diphenylacetate, 1-naphthylacetate, and cyclohexylacetate which has no -bonding, were studied by means of Mössbauer spectroscopy. The cyclohexylacetate complex shows a temperature-dependent valence delocalization, while the 1-naphthylacetate complex shows temperature-independent mixed-valence states. The acrylate and diphenylacetate complexes show temperature-dependent mixed-valence states, although they do not reach an averaged-valence state at room temperature.     

Iron-57 Mössbauer spectroscopic study of fluorinated strontium orthoferrite

A nuclear spin maser of a new type, that employs a feedback scheme based on optical nuclear spin detection, has been fabricated. The spin maser is operated at a low static field of 30 mG by using the optical detection method. The frequency stability and precision of the spin maser have been improved by a highly stabilized current source for the static magnetic field. An experimental setup to search for an electric dipole moment (EDM) in ¹²⁹Xe atom is being developed.     

57Fe Mössbauer spectroscopic study of nanostructured zinc ferrite

Nanosize zinc ferrite particles, prepared by nitrate method, were investigated by XRD, TEM, ⁵⁷Fe Mössbauer spectroscopy and VSM. The average particle size in this system varies from 10 to 62 nm as the sintering temperature increases from 300°C to 1,000°C. The lattice parameters in this system are almost constant at a value of ～8.41 Å. The Mössbauer spectra of all the sintered samples show a single doublet. The Mössbauer hyperfine parameters show little change with the change of sintering temperature. The doublets are ascribed to the presence of superparamagnetism in this system, which is also corroborated by the VSM measurements.     

A197Au Mössbauer study of the adsorption of aurocyanide onto activated carbon

The chemical state of gold adsorbed on activated carbon from solutions of KAu(CN)₂ has been studied by¹⁹⁷Au Mössbauer spectroscopy as a function of the pH and concentration of the solution, the loading of the carbon, and the drying conditions of the samples. On freshly prepared specimens Au(CN) ₂ ^? is always the dominant species. Drying leads to the formation of other forms of gold, mainly in samples loaded from strongly acidic media. An oligomeric form of AuCN is, however, also formed when samples loaded at high pH values are dried at temperatures around 100°C.     

A Mössbauer spectroscopic study of the effect of molybdenum during preparation of Nd-Fe-B magnets

Elements in atomic ratios Nd₂(Fe_0.9Mo_0.1)₁₄B were melted in an induction furnace and annealed in order to examine the effect of molybdenum during the preparation of Nd-Fe-B magnets. A phase analysis has been made from ⁵⁷ Fe Mössbauer spectroscopic measurements in the temperature range of 100 to 700 K. It is found that two iron containing phases are formed, one Mo rich Fe alloy and the other Nd-Fe(Mo)-B isostructural to Nd₂Fe₁₄B. The Nd_1.1Fe₄B₄ phase usually found in Nd-Fe-B magnets is not observed in these samples. The Curie temperature, T_c, is found to be 605(5) K from the Mössbauer as well as vibrational sample magnetometer measurements on the same sample. At low temperatures, the average hyperfine field at Fe nuclei is found to show a decrease with respect to the value for Nd₂Fe₁₄B.     

Uses and perspectives of Mössbauer spectroscopic studies of iron minerals in coal

The processes involved in the utilization of coal are affected by the minerals contained in it. Due to the presence of iron as a major constituent of coal mineral matter, and to the fact that the iron minerals, especially pyrite, and their transformation products play an important role in coal uses, ⁵⁷Fe-Mössbauer spectroscopy appears as an attractive tool in coal research. Mössbauer studies related to the characterization of iron phases, coal oxidation and quantitative determination of pyritic sulphur are discussed in this work.     

Mössbauer characterization of iron and cobalt dinitrosyl derivatives of 1,1′-bis(diphenylphosphino)ferrocene

The interest in the synthesis and reactivity of organometallic compounds which have two or more different metals in the same molecule has grown in recent years, due to the new chemical properties that they can present. In this work, we report the Mössbauer characterization of dppfFe(CO)₃ and two novel compounds, dppfFe(NO)₂ and [dppfCo(NO)₂]⁺[SbF₆]^–, where dppf =1,1-bis(diphenylphosphino)ferrocene. The complexes were also characterized by IR and³¹P NMR. The Mössbauer parameters of the dppfFe(NO)₂ and dppfFe(CO)₃, show two iron sites in the same proportions while for [dppfCo(NO)₂ ⁺[SbF₆]^– only one site was observed.     

Mössbauer study of tektites

Room temperature ⁵⁷Fe Mössbauer spectroscopy has been used to investigate the structural and oxidation state of Fe in tektites from different strewn fields. Spectra have been analyzed in terms of two quadrupole splitting distributions corresponding to Fe^3?+? and Fe^2?+?. All tektites show similar distribution of quadrupole splitting. Each distribution has one peak. The Fe^2?+? sites show a narrow region of Mössbauer line shift (δ) and quadrupole splitting (ε), δ?= 1.02–1.10 mm/s and ε?= 0.85–1.00 mm/s relative to α-Fe. These values have been assigned to intermediate coordination between tetrahedral and octahedral. The Fe^3?+? sites show wider regions of hyperfine parameters: δ?= 0.25–0.45 mm/s and ε?= 0.65–0.90 mm/s. The Fe^3?+?/Fe^2?+? ratio was found to be 0.05–0.15.     

Mössbauer spectroscopic study of 57Fe metabolic transformations in the rhizobacterium Azospirillum brasilense Sp245

Preliminary ⁵⁷Fe transmission Mössbauer spectroscopic data were obtained for the first time for live cells of the plant-growth-promoting rhizobacterium Azospirillum brasilense (wild-type strain Sp245) grown aerobically with ⁵⁷Fe^III–nitrilotriacetate (NTA) complex as a sole source of iron. The results obtained have shown that live cells actively reduce part of the assimilated iron(III) to iron(II), the latter amounting up to 33 % of total cellular iron after 18 h of growth, and 48 % after additional 3 days of storage of the dense wet cell suspension in nutrient-free saline solution in air at room temperature (measured at 80 K). The cellular iron(II) was found to be represented by two quadrupole doublets of different high-spin forms, while the parameters of the cellular iron(III) were close to those typical for bacterioferritins.     

Mössbauer spectroscopic investigation of tin-doped thiospinels

The tin-doped sulphur-containing compound of composition Fe_1.05Cr_1.90Sn_0.05S₄, in which tin is located on the octahedral sites as Sn^4?+? in the spinel-related structure, has been examined by ¹¹⁹Sn Mössbauer spectroscopy. The data complement results obtained by ⁵⁷Fe Mössbauer spectroscopy and show that tin increases the magnetic ordering temperature of pure FeCr₂S₄.     

A Mössbauer spectroscopic study of simulated weathering of Didwana ordinary chondrites

A sample of an ordinary chondrite fall, Didwana-Rajod, Rajasthan, India which was shown to be largely unweathered, was exposed to weathering in laboratory conditions. Wet and dry cycle of different strengths (0.01 and 0.1 M) of sodium sulfate solution was applied for different periods of time (maximum of 140 days). An attempt was made to simulate meteorite weathering more or less analogous to environmental exposure in desert-like conditions. It is found that lower concentration of the attacking ions is more effective in weathering the meteorite. Kamacite is affected in early cycles of weathering more than any other mineral. The olivine to pyroxene ratio is much less affected.     

Mössbauer spectroscopic study of FeAl1−x Co x

We report the results of a Mössbauer study of the alloy sytem FeAl_1?x Co _x forx ≥ 0.3 at temperatures down to 83 K. Magnetic splitting is observed forx ≥ 0.35 at all temperatures. However, forx=0.3, no splitting is observed at room temperature, and superparamagnetic behavior occurs at LN₂ temperature. The magnetically split spectra are fitted each with a distribution of hyperfine fields and the average hyperfine field \(\bar B_{hf} \) as a function of temperature is obtained. The variation of \(\bar B_{hf} \) withT is explained using the model of magnetic clusters with collective magnetic excitations from which the saturation hyperfine field and the magnetic anisotropy energy for these clusters are obtained. Also, the results are discussed using the model of random atomic distributions, and the agreement between the calculated and the experimentally obtained distributions of hyperfine fields is found improve asx increases.