A. Van Alboom

Evaluation of ferrous and ferric Mössbauer fractions

The Mössbauer fractions f for various ferrous- and/or ferric-containing oxides and oxyhydroxides, silicates and carbonates were evaluated from the experimental temperature dependence of their center shifts, using the Debye approximation for the second-order Doppler shift. It is concluded that ferrous ions exhibit a lower fraction as compared to ferric ions. Using standard mixtures of α-Fe2O3 with selected Fe2+ or Fe3+ compounds, it is found that the calculated Fe3+f values are somewhat overestimated with respect to those of Fe2+. Possible explanations for this shortcoming are discussed and it is suggested that a different temperature dependence of the intrinsic isomer shift is the most likely reason. This suggestion is corroborated by analyses of hematite and hedenbergite data which are available for temperatures up to 900 K and 800 K respectively.

Study of the temperature dependence of the hyperfine parameters in two orthopyroxenes by 57Fe Mössbauer spectroscopy

Abstract57Fe Mössbauer measurements were performed on two natural orthopyroxenes in the temperature range between 17 and 490 K. The temperature variations of the center shifts and of the quadrupole splittings have been interpreted using the Debye model for the lattice vibrations and the crystal-field model respectively. Two approaches have been applied to evaluate the crystal field. The first one, which is commonly used by Mössbauer spectroscopists, emanates from the approximative and simplified symmetry of the ferrous sites, whereas the second one takes into account the real C1 symmetry of the ferrous sites, thus leading to a pointcharge calculation. For comparison, analogous calculations have been carried out on literature data for an iron-rich orthopyroxene (specimen XYZ).

Temperature dependence of the 57Fe Mössbauer parameters in riebeckite

Mössbauer spectra for two riebeckite minerals were collected at temperatures in the range 4.2 to 500 K. The magnetic-ordering temperatures were found to be 33±1 and 31±1 K respectively. Fitting the paramagnetic spectra with a discrete number of doublets (three or four) did not lead to consistent results. Instead, a superposition of an Fe3+ (M2) doublet and one distributed ferrous component was found to produce adequate fits with reasonable parameter values. For both samples, a minor fraction of ferrous ions was observed to be present at the M4 sites and for one of the samples at the M2 sites as well. The temperature variations of the center shifts were well reproduced using the Debye model of the lattice vibrational spectrum to evaluate the second-order Doppler shift. The characteristic Mössbauer temperatures were calculated to be in the range 340–390 K for Fe2+, and 520 K for Fe3+. The temperature dependences of the various ferrous quadrupole splittings could not be explained in terms of the point-charge model and assuming a temperature-independent energy-level scheme for the 5D term. It is suggested that a gradual change with temperature of the orbital-level splittings takes place. All calculations yielded a positive sign for the principal component of the electric field gradient (EFG). The spectrum recorded at 4.2 K for one of the riebeckites was fitted with a superposition of an Fe3+ and a Fe2+ hyperfine-field distribution, the latter one primarily characterizing the Fe2+ (M1) cations. The following relevant hyperfine data were calculated: Hhf=161 kOe, ΔEQ=3.11 mm/s, and Vzz<0, all referring to the maximum-propability values. For the second riebeckite at 4.2 K, an additional distributed ferrous component could independently be resolved. The two maximum-probability hyperfine fields were found to be 189 and 98 kOe and the corresponding ΔEQ values 3.10 and 2.67 mm/s. Both components exhibit a negative Vzz. The subspectra were attributed to M1 and M3 sites respectively. The Fe3+ hyperfine fields are 548+-2 kOe for both riebeckites. The different values found for the Fe3+ quadrupole shift 2ɛQ for the two samples is explained by a different angle between the hyperfine field and the EFGs principal axis. The magnetic spectra recorded at 15 K and higher, could not be reproduced adequately with reasonable parameter values.

TEMPERATURE-DEPENDENCE OF THE MOSSBAUER PARAMETERS OF THE FE-NI PHASES IN THE SANTA CATHARINA METEORITE.

The temperature variation in the range 8–760K of the hyperfine parameters of the Fe−Ni phases in the Santa Catharina meteorite has been determined. It is suggested that the disordered 50–50 Fe−Ni phase actually consists of two distinct fractions, i.e. a completely disordered phase and one with intermediate long-range ordering parameter. The single-line subspectrum of the 28%-Ni phase was found to display magnetic ordering below approximately 25K.

Mössbauer spectroscopic study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O

A Mössbauer study of synthetic leucophosphite, KFe2(PO4)2(OH)·2H2O is reported. The sample was prepared by the reaction of synthetic goethite (α-FeOOH) with a K-phosphate solution of pH 2 at 373 K for 20 days. The obtained cell parameters are a = 9.771(1), b = 9.675(2), c = 9.747(2) Å, β = 102.45(1)°. The Mössbauer spectra from 295 K down to 40 K show the existence of two Fe3+ doublets with ∆EQ1 ≈ 0.62 mm/s and ∆EQ2 ≈ 0.91 mm/s (at 80 K), respectively, whereas for lower temperatures the spectra are composed of two sextets with Hhf1 ≈ 48.3 T and Hhf2 ≈ 48.8 T at 4.2 K. The magnetic transition temperature was determined to be 36 K from a Mössbauer thermoscan experiment. The magnetic ordering is presumed to be antiferromagnetic. The temperature variations of the hyperfine fields cannot be explained by the simple molecular field approximation. Using a model that takes into account the exchange magnetostriction associated to a non-second-order transition, an excellent reproduction of the Hhf(T) curves was obtained.

Crystal-field interpretation of the temperature dependence of the57Fe Mössbauer quadrupole splitting in two orthopyroxenes

The temperature dependences of the M2 and the M1 ferrous quadrupole splittings in two orthopyroxenes have been studied. The valence contributions to the EFG were calculated from the Boltzmann averages of the expectation values of the EFG components over the corresponding5D levels, while the lattice contributions were obtained from lattice summations. The proper5D level schemes were each determined by diagonalization of the full crystal-field Hamiltonian, for which the operator coefficients were calculated so as to take into account the real C1 symmetry of the FeO610− clusters. The calculations were based on the lattice and the charge data related to ferrosilite.

Mossbauer study of the hematite phase formed during synthesis of ferri-diopsides

Abstract The hematite phase, which is formed in the process of synthesizing ferri-diopsides with iron contents exceeding the solubility limit, has been studied by 57 Fe Mossbauer spectroscopy at variable temperatures between 80 and 900 K, in both ascending and descending sequences. The temperature variations of the hyperfine parameters were derived and interpreted. The Morin-transition temperature was found to be unusually high, i.e., ≈350 K, and possible explanations for this feature are discussed.

Deficiency of water molecules in the crystallographic structure of vauxite

A vauxite mineral sample from Huanuni, Bolivia, was studied by XRD, TGA and Mössbauer spectroscopy. The XRD revealed the sample as having the typical triclinic structure of vauxite. The chemical formula was determined as (Fe0.88Mn0.01)Al1.99(PO4)2(OH)1.75(H2O)5.31, implying some Fe2+, OH− and H2O deficiencies. The TGA curve showed ca. 27% loss of weight over a temperature range from 80 to 400 °C, supposedly due to the loss of water and hydroxyl groups. For the first time, Mössbauer spectra for vauxite were collected over a wide temperature range between 9 and 310 K. No magnetic ordering was detected. The spectra could be successfully and consistently analyzed by a superposition of four doublet subspectra. On the basis of the relation between the center shift and the mean Fe-ligand distance on the one hand and the center shift values for the various doublets on the other hand, one doublet was assigned to Fe(2). For the other doublets, it is proposed that, as a result of the H2O deficiency in the structure of the present vauxite sample, vacancies are present in the second coordination spheres of some Fe(1) and that these vacancies affect the quadrupole splitting of the corresponding Fe(1) cations, thus causing three Fe(1) doublet components in the Mössbauer spectra. The temperature variations of center shift and quadrupole splitting of the various doublet contributions are presented and discussed.

Crystallization behaviour of a ferri-silicate α-waste glass concluded from Mössbauer spectroscopy

A reference, ferri-silicate α-waste glass containing 10 mole% Fe2O3, was thermally annealed atTa=620, 700 and 800°C, resulting in the formation of crystalline substances. Standard characterization techniques have revealed that the crystal growth is either three-dimensional or rather two-dimensional depending onTa. The Mössbauer spectra recorded at 80 and ≈=295 K and fitted with a sum of two ferrous and two ferric doublets, do not show any effect doublet areas are found to be consistent with earlier suggestions that in silicate glasses nearly all of the Fe2+ ions have an octahedral coordination. The chemical environment of the Fe3+ ions in the parent glass seems to remain essentially unchanged upon crystallization, except for theTa=800°C sample for which the high degree of crystallinity is reflected in a drastic change of the hyperfine parameters. The nature of the crystalline phase, suggested to be diopside-like from diffraction experiments, is discussed. For that purpose. Mössbauer spectra were collected for a synthetic ferri-diopside sample.