M6ssbauer Spectra Research Articles

Properties of Iron Atoms at Grain Boundaries in Fe and Fe72Al28

ABSTRACTLocation of diffused 57Co atoms in single crystals, bicrystals and polycrystals of pure iron and Fe72Al28alloy were investigated by means of emission Mössbauer spectroscopy. To interpret the results, first principles calculations of iron atom magnetic moments and hyper-fine field were carried out. From comparison of M6ssbauer spectra of single crystals with those of bicrystals and polycrystals, an information about grain boundary positions occupied by diffusing atoms is obtained. It is shown that about 5% of the diffusing atoms at the {112} grain boundary of iron are located at the positions either having impurity atoms in the nearest neighbourhood or characterized by larger atomic spacing in comparison with the bulk. In the Fe72Al28 a dominating portion of diffusing atoms have different surrounding than in grain volume. An enrichment of grain boundaries by aluminum could explain their hyperfine parameters.

57Fe Mössbauer Spectroscopy on Two Romanian Carpathian Pumpellyites

The authors obtained the Fe~~ M6ssbauer spectra of two pumpellyite samples obtained from two different mineral sources in the Romanian Carpathians. Using a specialized computation program of the quadrupole splitting spectral parameter, QSCOMP, the observed quadrupole doublets were attributed to the ferric and ferrous ions in octahedral sites. The refined analysis of spectral parameters permitted us to characterize both the oxidation state, the electronic configuration and the coordination symmetry with the distortions and occupancies of pumpellyite constituting iron ions. The differences of the spectral parameters giving the data of the iron abundance, the chemical iron-oxygen bonds, the deformation and occupancies of iron octahedra, observed at the two studied samples, are discussed and correlated by the different genesis conditions of two pumpellyite-bearing occurrences.

Spectroscopic studies of iron(III) complexes of D-saccharose and D-glucose in the solid state and in solution

Iron(Ill) complexes of D-saccharose and D-glucose were prepared. The compositions of the complexes were determined by standard analytical methods. The M6ssbauer spectra reflected the presence of high-spin iron(Ill) in the polynuclear species. EPR spectroscopy demonstrated 13 antiferromagnetically coupled iron(IlI) centers within the solid complexes. The C NMR spectra indicated the presence of a mixture of coordination isomers of iron(IIl) complexes containing the sugar ligand in differently bound forms.

Mineralogical analysis of a weathering mantle derived from tuffite

M6ssbauer spectra are presented at 15 and 298 K for magnetically-separated extracts of magnetic soils from a tuffite-derived weathering mantle from Minas Gerais, Brazil. The principal iron-bearing mineral is maghemite in which the degree of Ti or Mg substitution is found to vary with depth. The Mg-maghemite, with lattice parameter a0 = 0.838 nm, is a new natural mineral. It is associated with anatase in the magneticseparates.

Structural and valence state of iron ions in sodium-aluminum-phosphate glass

(SAP) glass by means of electron paramagnetic resonance (EPR), M6ssbauer spectroscopy (MS), and nuclear magnetic resonance (NMR). Our results enabled us to determine the characteristics of the occupancy and distribution of iron ions over tetra- and octahedral positions as a function of the ratio of the concentration of iron and aluminum oxides, to determine the concentration of Fe 3+ and Fe 2+ ions and the number of phosphorus and aluminum atoms in the nearest-neighbor environment of the impurity iron ions, as well as to estimate the effect of the concentration of aluminum oxide on the average distance between the phosphorus atoms in the SAP glass. Experimental Conditions. To synthesize the SAP glasses, we prepared a charge consisting of powders of the pure reagents NaPO3, AI203, and NaNO 3. The charge was held at 1100~ in an alundum crucible for 4 h in air to obtain frits of three-component glass, corresponding to the following oxide composition (mass %): 25 Na20-(10-21)AIxO 3-(5363)P205. The frit was mixed with Fe203 powder and held at 1050~ for 6 h, after which the melt was allowed to cool in air to room temperature within 3 h. The samples of SAP glass contained from 0.4 to 6 mass % Fe203. The EPR, M6ssbauer, and NMR spectra were recorded using glass powders with grain size not exceeding 120/zm; all measurements were performed at room temperature. The EPR spectra of the Fe 3 + ions were recorded on a RI~- 1301 radio spectrometer, the 57Fe M6ssbauer spectra were recorded on a YAGRS-4 spectrometer with a 57Co ,,/-ray source in chromium with the signals recorded in a NTA-1024 multichannel analyzer, followed by least-squares analysis of the two doublets on a IBM PC/AT computer. The 27AI and 31p spectra were recorded on a Brucker (Germany) SKHR-90 pulsed Fourier spectrometer, using a "solid-echo" sequence in the high-power regime with external stabilization of the magnetic field. The chemical shifts in the 3tp NMR spectra were read from the signal from an 85 % solution of H3PO 4 used as an external standard. EPR Spectra of Fe 3+ Ions. Figure 1 displays the EPR spectrum of Fe 3 + ions in SAP glass, containing 6 mass % Fe203. For [Fe203] < 4 mass% the EPR spectrum consists of two lines with g-factors of 4.3 and 2, which are determined by the isolated Fe 3+ ions occupying, respectively, tetra-(FeO4) and octahedral positions (FeO6) in the glass structure [1]. The third (wider) line in the region g = 2 (dashed line in Fig. 1) is manifested in the spectrum only with Fe203 content in the glass exceeding 4 mass%. This line was identified with clusters of iron ions, which are characterized by ferromagnetic properties. Figure 2 displays the intensity (Iv_) of the EPR spectrum as a function of the iron oxide concentration in SAP glass with the following composition (mass%): 25 Na20-21AI203-53P xOs-xFe203 (x = 0.4-6 mass%). In the region [FexO 3] <4% In is a linear function of [Fe203]. At high iron oxide concentration the linear relation breaks down because of the sharp increase in the intensity of the line corresponding to iron-ion clusters, which eliminate the direct proportionality between the iron concentration in the clusters and the intensity of the EPR spectrum.

A new preparative procedure for ultrafine spinel ferrites

Ferrites have been used as industrial catalysts of butene-1 to butadiene. Recently, great attention has been focused on ultrafine ferrites in our laboratory. It is well established that preparatory procedures have a major influence on the performance of catalysts. Thus, a series of ultrafine ferrites were prepared by different procedures. In an attempt to find suitable procedures, the explosion method of excessive urea was first developed by ourselves. The process was as follows. First, excessive urea was added to the solution of nitrates. On controlled heating, a white slurry was formed. The slurry became hardened at room temperature. Eventually, the slurry was placed in a muffle furnace at a calcination temperature, whereupon a severe explosion occurred and the corresponding ultrafine powder was obtained. If the slurry producing step was well controlled, the single spinel phase would be obtained. It was found that the samples obtained in this way, denoted as E-type samples, exhibited the best activities and selectivities in oxydehydrogenation of ethylbenzene [1]. The M6ssbauer spectra of ultrafine magnesium ferrites taken at 298 K are shown in Fig. 1. The solid lines through the data points are the result of computer simulations of the spectra. The parameters derived from the least-square fits are listed in Table I. The C-type magnesium ferrite shown in Fig. 1 was prepared by hydrolysis in citric acid in the conventional way. The two kinds of magnesium ferrites both have spinel structures as illustrated by X-ray powder diffractometry (XRD). Observations by transmission electron microscopy (TEM) suggested that the particle sizes of the two samples were all below 20 nm. However, the E-type magnesium ferrite exhibited completely superparamagnetic spectrum only with doublets, whereas the C-type magnesium ferrite mainly exhibited normal Zeeman-split sextets, one due to the Fe 3÷ ions at the tetrahedral (A) site and (a)

Magnetic Properties Of Iron/Silica Gel Nanocomposites

ABSTRACTHomogeneous gelled composites of iron and silica containing 5–30 wt. % Fe have been prepared by low temperature polymerization of aqueous solutions of ferric nitrate, tetraethoxysilane, and ethanol (with an HF catalyst). X-ray diffraction data, characterized by the presence of a diffuse scattering peak centered at 20≈24 degrees and the absence of any strong Bragg scattering from the iron-containing regions, indicates that these bulk materials are comprised of nanometer-sized regions of iron compounds embedded in a silica gel matrix. Scanning electron microscopy observations show that this matrix is characterized by the presence of many interconnected pores and that the size of these pores is related to the particle size of the Fe-containing regions. The paramagnetic nature of these materials at room temperature, as well as the small size of the iron-containing regions, is indicated by the appearance in many of the samples of only a high intensity central doublet in the 57Fe M6ssbauer spectra. The Mössbauer effect data demonstrates that the form of the iron can be changed by a subsequent treatment in an atmosphere of ammonia or hydrogen at elevated temperatures: for a 10 wt. % Fe sample treated with ammonia, only a central doublet was observed but with a much larger quadrupole splitting and isomer shift. Both of these subsequently treated materials became superparamagnetic at room temperature. In addition, magnetic susceptibility measurements indicate that the hydrogen treated material becomes a spin glass at low temperatures.

Interpretation of Mössbauer Spectra of Nontronite, Celadonite, and Glauconite

A new approach to the interpretation of M6ssbauer spectra of Fe3+-phyllosilicates having vacant trans-octahedra is based on (1) crystal structure simulation methods that allow for the size and the shape ofa Fe3+-octahedron as a function of the nearest surrounding cations; and (2) calculations of electric field gradients (EFG) on Fe 3+ in terms of the ionic point-charge model Calculations were performed by direct summation within the region of radius -<50/~. Coordinates for the anions in the coordination octahedra have been assigned to take into account the nearest cationic environment. Atomic coordinates for the rest of the summation volume are those for the average unit cell. EFG calculations for cation combinations responsible for the visible quadrupole splitting Avi~ in the spectra of nontronite, "red" muscovite, and celadonite have led to good agreement between Avis and A~ c. Computer fitting of the nontronite and celadonite spectra based on EFG calculations for the rest of the possible cation combinations suggests that the distribution of tetrahedral cations in nontronite obeys the Loewenstein rule, and in celadonite, the distribution of R 3+ and R 2+ over cis-octahedra is predominantly ordered, in agreement with electron diffraction and infrared spectroscopy data. The M6ssbauer spectrum of one of the glauconites suggested the presence of celadonite-like and muscovite-like domains in its 2:1 layers.

57Fe Mössbauer Spectroscopy of Montmorillonites: A New Interpretation

The 57Fe M6ssbauer spectra of several montmorillonites, measured at room temperature and 453 K, showed a considerably broadened Fe 3+ resonance which can be computer-fitted with a similarly broadened Fe 3+ doublet. In some spectra, particularly if all linewidths were constrained to be equal, this broadened Fe 3§ resonance was further resolved into overlapping inner and outer Fe 3+ doublets, also having broad linewidths. In accordance with recent electron diffraction evidence, the assignment by previous workers of the inner doublet to Fe 3+ in the octahedral sites having the cis-arrangement of OH groups and the outer doublet to the octahedral site having the trans-arrangement OH groups is incorrect. Instead, the Fe 3+ was found to be located largely in the trans-octahedral sites. Because of the relatively low iron content of the montmorillonite examined, the next and more distant neighboring-cation envi- ronment varied considerably about the octahedral Fe 3+ ions. This variation produced a broadened ex- perimental resonance, and the resulting two-doublet computer fits probably represent the mean extremes of a continuum of slightly different Fe 3+ resonances arising from the variable nature of the environment surrounding these such trans-sites, rather than distinct cis- and trans-sites. In addition, a small resonance indicating the substitution of Fe 3 + into the tetrahedral sites was observed. The interlayer species probably influenced the Mrssbauer resonance of Fe 3+ in the tetrahedral and octahedral sites.

Magnetic properties of sheet silicates; 2:1:1 layer minerals

Magnetization, susceptibility and M6ssbauer spectra are reported for representative chlorite samples with differing iron content. The anisotropy of the susceptibility and magnetization of a clinochlore crystal is explained using the trigonal effective crystal-field model developed earlier for 1:1 and 2:1 layer silicates, with a splitting of the Tzg triplet of 1,120 K. Predominant exchange interactions in the iron-rich samples are ferromagnetic with J= 1.2 K, as for other trioctahedral ferrous minerals. A peak in the sus- ceptibility of thuringite occurs at Tm = 5.5 K, and magnetic hyperfine splitting appears at lower temperatures in the M6ssbauer spectrum. However neutron diffraction reveals no long-range magnetic order in thuringite (or biotite, which behaves similarly). The only magnetic contribution to the diffraction pattern at 1.6 K is increased small angle scattering (q<0.4~ 1). A factor favouring this random ferromagnetic ground state over the planar antiferromag- netic state of greenalite and minnesotaite is the presence of pairs of ferric ions on adjacent sites, in conjunction with magnetic vacancies in the octahedral sheets. Monte Carlo simulations of the magnetic ground state of the sheets illus- trate how long range ferromagnetic order may be destroyed by vortices forming around the Fe 3 +--Fe 3 + pairs. tral in chlorites. Substitution of trivalent ions for Mg in the brucite layers is compensated by replacement of Si by A1 in the tetrahedral sheets of the talc layers (Bailey 1980). The general formula for a trioctahedral chlorite is