Investigation of Virgin Coals and Coals Subjected to a Mild Acid Treatment

A quantitative determination of the relative marcasite/pyrite contents in virgin coals is possible by means of ⁵⁷Fe Mössbauer spectroscopy. Complications arise, however, when iron-containing silicates, carbonates, or other salts are present. The application of a mild chemical treatment involving hydrofluoric acid has been employed to remove these Fe-containing phases while leaving the iron-disulfide phases unaffected. Several South African coal samples with non-iron disulfide, Fe-containing phases ranging from 18 to 30 weight percent have been subjected to a hydrofluoric acid leaching at room temperature. The loss of mineral matter with HF leaching correlates well with the mineral matter residue following low temperature ashing. The ⁵⁷Fe Mössbauer spectra of the resulting coal samples, collected at 297K, indicate that only FeS₂ phases are present. The Mössbauer parameters for these samples (0.619 ≤E_Q≤0.622 mm s^?1; 0.306≤δ≤0.309 mm s^?1) indicate the absence of appreciable quantities of marcasite in the coals. These Mössbauer parameters differ slightly, but systematically, from those of pyrite for which a quadrupole splitting of 0.6110 ± 0.0020 mm s^?1 has been established. On the basis of previous studies, these increased E_Q values suggest the presence of As substitution in the pyrite phases. ⁵⁷Fe Mössbauer spectra of virgin coals exhibit phase assemblages comparable to those observed by x-ray diffraction (XRD), e.g. pyrite and (Ca,Fe)CO₃, even though the presence of pyrite is less definite in the XRD data.     

Application of 57Fe Mössbauer spectroscopy as a tool for mining exploration of bornite (Cu5FeS4) copper ore

Nuclear resonance methods, including Mössbauer spectroscopy,are considered as unique techniques suitable for remote on-line mineralogical analysis. The employment of these methods provides potentially significant commercial benefits for mining industry. As applied to copper sulfide ores, Mössbauer spectroscopy method is suitable for the analysis noted. Bornite (formally Cu₅FeS₄) is a significant part of copper ore and identification of its properties is important for economic exploitation of commercial copper ore deposits. A series of natural bornite samples was studied by ⁵⁷Fe Mössbauer spectroscopy. Two aspects were considered: reexamination of ⁵⁷Fe Mössbauer properties of natural bornite samples and their stability irrespective of origin and potential use of miniaturized Mössbauer spectrometers MIMOS II for in-situ bornite identification. The results obtained show a number of potential benefits of introducing the available portative Mössbauer equipment into the mining industry for express mineralogical analysis. In addition, results of some preliminary ^63,65Cu nuclear quadrupole resonance (NQR) studies of bornite are reported and their merits with Mössbauer techniques for bornite detection discussed.     

Mössbauer study of pH dependence of iron-intercalation in montmorillonite

⁵⁷Fe Mössbauer spectroscopy and XRD have successfully been applied to show the incorporation of Fe ion into the interlayer space of montmorillonite via treatment with FeCl ₃ in acetone. The 78K ⁵⁷Fe Mössbauer spectra of montmorillonite samples reflected magnetically split spectrum part indicating the intercalation of iron into the interlayer of montmorillonite via the treatment with FeCl ₃+acetone and washed with water until the initial pH=2.3 increased to pH=4.14. It was found that the occurrence of intercalated iron in the form of oxide-oxihydroxide in montmorillonite increases with the pH. Intercalation was confirmed by the gradual increase in the basal spacing d₀₀₁ with pH.     

Mössbauer and XRD study of Al-Sn linished steel bimetal alloy

Aluminium alloy free CS1 type steel (0.06 wt% C, 0.45 wt% Mn) and samples of cold roll bonded steel bimetal alloys (MAS15 and MAS16) were fabricated and investigated by X-ray diffraction (XRD), ⁵⁷Fe conversion electron Mössbauer spectroscopy (CEMS) at room temperature. XRD has revealed only the existence of the alpha iron solid solution (steel) phase in the steel only sample, while identified steel and metallic Al and Sn constituent phases in the bimetallic alloys. ⁵⁷Fe Mössbauer spectroscopy revealed the presence of 4 % secondary iron-bearing phase attributed mainly to iron oxide/ oxyhydroxides (ferrihydrite) besides the steel matrix on the surface of the steel sample. A significant difference between the occurrences of the secondary phase of differently prepared bimetal alloys found in their ⁵⁷Fe CEM spectra allowed to identify the main phase of debris as different iron oxide/ oxyhydroxides.     

Thermal equilibrium defects in iron-based alloys

The Mössbauer spectra of ⁵⁷Fe were measured for the thermal equilibrium b.c.c. Fe_0.947V_0.053 and Fe_0.956Co_0.044 solid solutions being at temperature ranging from 300 to 1,000 K. The obtained data were analysed in terms of concentration of unoccupied sites in the 14-site surroundings of an ⁵⁷Fe Mössbauer probe in a b.c.c. sample. It turned out that the probe detects unoccupied sites in its neighbourhood when the temperature of the material studied does not exceed about 900 K. This result suggests that the Mössbauer spectroscopy “sees” the pre-vacancy effect revealed by the positron annihilation spectroscopy in the early 1960s.     

An57Fe Mössbauer effect study of the highT c superconductor GdBa2Cu3O7−y

Split source⁵⁷Fe Mössbauer effect spectroscopy has been performed between 4 K and 295 K on the superconducting perovskite GdBa₂Cu₃O_7?y. No evidence is seen for magnetic splitting at low temperatures as reported in some split absorder⁵⁷Fe Mössbauer experiments on this material. There is evidence for phonon mode softening, as observed for¹¹⁹Sn Mössbauer spectra of some other highT _c superconductors.     

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.     

Effect of FeCl3 and acetone on the structure of Na–montmorillonite studied by Mössbauer and XRD measurements

⁵⁷Fe Mössbauer spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and infrared spectroscopy were used to study the effect of FeCl₃ and acetone on the structure of a Na–bentonite. XRD indicated the incorporation of Fe³⁺ ions into the interlayer space since the basal lattice spacing of montmorillonite increased to 1.6 from 1.24 nm after treatment with FeCl₃ dissolved in acetone. Interlayer Na⁺ ions could be exchanged to Fe³⁺. Magnetically split Mössbauer subspectra with internal magnetic fields 41 and 46 T at 74 K, were associated with two main Fe³⁺ microenvironments within the interlayer regions. The resultant Fe–montmorillonite was successfully applied as a catalyst in the preparation of 1,1-diacetates from aromatic aldehydes and acetic acid anhydride.     

Effect of particle size and alloying with different metals on 57Fe Mössbauer spectra

Iron nanoparticles of various sizes have been synthesized using the chemical route which involves the preparation of iron bipyridine complexes in presence of different capping agents followed by thermal decomposition at 450°C in inert atmosphere. The bimetallic nanoalloys of Fe with Mg and Pd have also been prepared by following the same route. The resulting nanoparticles have been characterized by EDX-RF, XRD, AFM and ⁵⁷Fe Mössbauer spectroscopy. The appearance of quadrupole doublets in the Mössbauer spectra of Fe nanoparticles indicates the absence of magnetic interaction and variation in parameters is due to the varying particle size. The Mössbauer spectrum of Fe–Mg₂ bimetallic nanoalloy shows two doublets indicating the presence of superparamagnetism. The two doublets can be attributed to change in s-electron density of iron resulting from its two neighboring magnesium atoms. Fe–Pd nanoalloy Mössbauer spectrum is characterized by having a superparamagnetic doublet and a ferromagnetic sextet.     

Mössbauer study of FINEMET with different permeability

Stress field and magnetic field annealed FINEMET ribbons were investigated by ⁵⁷Fe Mössbauer spectroscopy, magnetic and XRD methods. The change in relative areas of the 2nd and 5th lines in the Mössbauer spectra indicated significant variation in magnetic anisotropy due to the different annealing. High velocity resolution Mössbauer spectroscopy was also used to control the model applied for the evaluation of Mössbauer spectra. A correlation was found between the permeability and the magnetic anisotropy of the annealed FINEMET samples. This can be applied to predict production parameters of FINEMET ribbons with more favorable soft magnetic properties for technological applications.     

An iron-57 Mössbauer spectroscopic study of titania-supported iron- and iron-iridium catalysts

⁵⁷Fe Mössbauer spectroscopy shows that titania-supported iron is reduced by treatment in hydrogen at significantly lower temperatures than corresponding silica- and alumina-supported catalysts. The metallic iron formed under hydrogen at 600°C is partially converted to carbide by treatment in carbon monoxide and hydrogen. In contrast to its alumina- and silica-supported counterparts, the remainder of the titania-supported iron is unchanged by this gaseous mixture. The⁵⁷Fe Mössbauer spectra and EXAFS show that iron and iridium in the titania-supported iron-iridium catalysts are reduced in hydrogen at even lower temperatures and, after treatment at 600°C, are predominantly present as the iron-iridium alloy. The treatment of these reduced catalysts in carbon monoxide and hydrogen is shown by Mössbauer spectroscopy and EXAFS to induce the segregation of iron from the iron-iridium alloy and its conversion to iron oxide.     

The state of 57Fe in pure and oxidized tin chalcogenides by Mössbauer spectroscopy

A Mössbauer spectroscopy study has been made of the stabilization states of ⁵⁷Fe in pure tin chalcogenides. The values of the isomer shifts and the quadrupole splittings lead to the conclusion that iron presents the electronic configuration 3d⁵4s^x at high spin in the interstitial position. This result is confirmed varying the ⁵⁷Fe neighbours by partial substitution of tellurium with oxygen. In this case, too, the values of Mössbauer parameters confirm the same results. The quadrupole moment present in pure SnTe doped ⁵⁷Co, and the shift of isomer shift towards less positive values for the oxidizated samples have been qualitatively justified as well.     

Magnetic and Mössbauer spectral studies of nano crystalline cobalt substituted magnesium ferrites (MgxCo1 − xFe2O4)

The electronic structure of the heme unit of deoxyhemoglobin including the proximal imidazole has been studied using the first-principles Hartree-Fock procedure. Our results for the ^57mFe isomer shift and asymmetry parameter are in very good agreement with the values obtained from Mössbauer spectroscopy measurements. The ^57mFe nuclear quadrupole coupling constant is smaller than the experimental result and possible ways to improve the agreement in the future are discussed. Improved analysis of the Mössbauer data, removing some approximations made for deriving the magnetic hyperfine tensor for the ^57mFe nucleus, is suggested to allow quantitative comparison with our results in the future.     

Variations in quadrupole splitting of the 57 Fe in the M1 and M2 sites of meteoritic olivines with different origin

A comparative study of meteoritic olivine in bulk samples of Farmington L5 and Tsarev L5 ordinary chondrites and extracted from Omolon and Seymchan the main-group pallasites was performed using Mössbauer spectroscopy with a high velocity resolution. Mössbauer spectra for each specimen were measured at 295 and 90 K. Mössbauer spectral components related to the ⁵⁷Fe in crystallographically non-equivalent sites M1 and M2 in olivines were determined and their Mössbauer hyperfine parameters were evaluated. It was found small variations of quadrupole splitting for the ⁵⁷Fe in both the M1 and M2 sites of olivines in bulk ordinary chondrites and olivines extracted from pallasites.     

In situ characterisation of supported iron-iridium catalysts by iron-57 and iridium-193 Mössbauer spectroscopy

Some silica-supported iron-iridium catalysts with different iron to iridium ratios and formed by the incipient wetness technique have been examined in situ by⁵⁷Fe and¹⁹³Ir Mössbauer spectroscopy following pretreatment in hydrogen. The results show that the reduction of the iron component is enhanced by the presence of iridium metal. The pretreated catalysts were evaluated for the hydrogenation of carbon monoxide at 270°C and 50 atmospheres pressure. The presence of iridium, which was shown by Mössbauer spectroscopy to result in the formation of reduced iron, iridium, and iron iridium alloy in the pretreated catalyst, was found to increase the catalytic activity and also influence selectivity. However, the systematic variation of the iridium content which was shown by Mössbauer spectroscopy to determine the exact phase composition of the pretreated catalysts, appeared to have little effect on catalytic performance. The⁵⁷Fe Mössbauer spectra recorded from all the used catalysts showed the formation of large and small particle ε′-Fe_2.2C under the high pressure of the catalytic reaction. The results suggest that the formation of iron carbides on reduced iron, perhaps at the surface, and the adsorption of hydrogen on reduced iridium are important features of this catalytic system.     

Grazing‐incidence synchrotron‐radiation 57Fe‐Mössbauer spectroscopy using a nuclear Bragg monochromator and its application to the study of magnetic thin films

Energy‐domain grazing‐incidence ⁵⁷Fe‐Mössbauer spectroscopy (E‐GIMS) with synchrotron radiation (SR) has been developed to study surface and interface structures of thin films. Highly brilliant ⁵⁷Fe‐Mössbauer radiation, filtered from SR by a ⁵⁷FeBO₃ single‐crystal nuclear Bragg monochromator, allows conventional Mössbauer spectroscopy to be performed for dilute ⁵⁷Fe in a mirror‐like film in any bunch‐mode operation of SR. A theoretical and experimental study of the specular reflections from isotope‐enriched (⁵⁷Fe: 95%) and natural‐abundance (⁵⁷Fe: ～2%) iron thin films has been carried out to clarify the basic features of the coherent interference between electronic and nuclear resonant scattering of ⁵⁷Fe‐Mössbauer radiation in thin films. Moreover, a new surface‐ and interface‐sensitive method has been developed by the combination of SR‐based E‐GIMS and the ⁵⁷Fe‐probe layer technique, which enables us to probe interfacial complex magnetic structures in thin films with atomic‐scale depth resolution.     

Clays and clay minerals: The firing process

⁵⁷Fe Mössbauer spectroscopy reveals changes in iron valence and iron site geometry when clays and clay minerals are heated, and allows a distinction to be made between paramagnetic and magnetically ordered phases. Mössbauer spectra can thus reveal the extent of iron retention in silicate structures upon heating, the identity of iron oxides initially present or formed during the heating process and their transformations, and the character of the atmosphere under which heating was carried out. This makes Mössbauer spectroscopy the most effective tool for the characterization of changes induced by heating phyllosilicates and iron oxides.     

Perspectives in physical metallurgy

Physical metallurgy experienced a tremendous, impact with the advent of the Mössbauer effect. Some of these developments will be reviewed, in particular the role of⁵⁷Fe as the star performer in Mössbauer spectroscopy. Also, one must realize that in most casen metals are involved, either in the source, the absorber, or both. Mössbauer spectroscopy in its different variations (conversion electron-, conversion x-ray-, γ-transmission Mössbauer spectroacopy) is well suited for the analysis of iron based alloys. An instrument has been developed which allows simultancous triple radiation Mössbauer spectroscopy (STRMS). The backscalttering geometry of CEMS and CXMS in conjunction with the transmission mode permits non-destructive testing of surfaces and depth analysis.     

Mössbauer spectroscopic studies of defect structure and alloying effects in nanostructured materials

Mössbauer spectroscopy provides spectral information from both ordered and structurally disordered regions of a solid and is therefore well suited for the atomic-scale characterisation of materials with very high defect concentrations. This applies especially to nanocrystalline materials where 5–50% of the atoms may be located at planar defects such as grain boundaries. In this paper, the range of Mössbauer spectroscopy in exploring the structure of nanostructured materials will be discussed in the form of case studies dealing with (i) nanometer-sized antiphase domains in the intermetallic compound Fe_3?x Si_1+x , (ii) ball-milling induced structural changes and alloying effects in dilute Al(⁵⁷Fe) and Y(⁵⁷Fe) alloys, and (iii) the Mössbauer signature of grain boundaries in nanocrystalline W(⁵⁷Fe).     

Using the 57m Fe3+ Mössbauer probe to determine the EFG tensor parameters at CuO cation sites

Mössbauer emission spectroscopy of the ⁵⁷Co(^57m Fe) isotope has shown that the impurity iron atoms appearing at the CuO-lattice cation sites after the decay of ⁵⁷Co²⁺ are donors and can become stabilized in two charge states, ^57m Fe³⁺ and ^57m Fe²⁺, and that the population ratio of these states depends on the Fermi level, whose position is governed by native CuO-lattice defects. A satisfactory agreement between the calculated and experimental values of the quadrupole splitting in Mössbauer spectra has been obtained for the ^57m Fe³⁺ centers. This permits one to consider the results obtained in the ⁵⁷Co(^57m Fe) Mössbauer emission spectroscopy study of cuprates as reliable experimental data on the crystal-field EFG tensor parameters at copper sites.