Mössbauer spectroscopy,intracluster atomic mobility and thermodynamics of ultrafine oxide clusters

Summary Dynamic properties of ultrafine clusters of γ-Fe₂O₃ (ferric oxide, FO) were studied by M?ssbauer spectroscopy and by thermodynamic analysis. The data obtained for FO clusters allowed the conclusion that dynamic properties of clusters as well as the decrease of melting point and the appearance of a gap between freezing and melting points depend on intracluster atomic mobility. Intracluster atomic mobility in FO clusters was shown to increase by the action of surfactants which decrease intercluster interactions. The increase in intracluster atomic mobility was suggested to proceed via formation of a solid-liquid state. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Paramagnetic relaxation in high-spin ferrous compounds

Measurements of three high-spin Fe^IIS₄ complexes at various temperatures and applied magnetic fields are reported and the results are compared with those of reduced rubredoxin. Simulations of the measured spectra by means of the fully developed stochastic theory of lineshape prove that intermediate relaxation behavior is present and is determined essentially by the phonon energy distribution and by one single parameter W₀, the scaling factor, which contains the strength of spin-lattice coupling.     

Limitation of semi-empirical mo-calculations in deriving charge densities ρ(0) in iron-oxygen compounds

For fifteen iron-oxygen compounds we carry out semi-empirical MO cluster calculations. The derived electronic structure is used to calculate electron charge densities ρ(0) and electric field gradients V_pq at the iron nucleus. The ρ(0)-values correlate with experimental isomer shifts δ, however, extreme cases like the Fe(VI)-compound BaFeO₄ are beyond the scope of accuracy of our method. We discuss variations of our calculational method, with one of them leading to ρ(0)-values for all fifteen compounds (including BaFeO₄) which satisfy the relation Δδ = αΔρ(0). The isomer shift calibration constant is in the range -0.195 mms^?1a₀³ to -0.25 mms^?1a₀³. Calculated quadrupole splittings are comparable with experimental data.     

Archimedean Synthesis and Magic Numbers: "Sizing" Giant Molybdenum-Oxide-Based Molecular Spheres of the Keplerate Type

Pythagorean harmony can be found in the spherical polyoxometalate clusters described here (see illustration for an example of a structure), since there are interesting relationships between the so-called magic numbers (12, 32, 42, 72, 132) relevant for spherical viruses and the number of the building blocks in the cluster. The size of these Keplerate clusters can be tailored by varying the type of connections between the pentagons by means of different spacers.     

Calculated electron densities and experimental isomer shifts of Fe57 in the deoxy- and CO-compounds of myoglobin and hemoglobin

Relativistic calculations of various electronic configurations of the iron atom were used in conjunction with Hückel-type self consistent field molecular orbital calculations for CO-myoglobin and CO-hemoglobin (mbCO, hbCO) and with limited configuration interaction calculations for deoxymyoglobin and deoxy-hemoglobin (mb, hb) to determine electron densities at the iron nucleus, ?(0). The calculations included all effects of overlap of iron core and next nearest neighbour (ligand) orbitals, and the effect of potential distortion of iron core orbitals due to molecular configurations 3d ^m4s ⁿ. From the calculated electron densities we found that the change in the experimental Mössbauer isomer shift, Δδ =δ_{mbCO, mbCO} ?δ_mb,hb, was mainly due to changes in the so-called overlap distortion of iron cores orbitals. The considerably higher electron density ?(0) in mbCO, hbCO than in mb, hb corresponds to the stronger interaction between iron and ligands in mbCO, hbCO compared to mb, hb. From the calculated values for ?(0) and the experimental isomer shifts we derived an isomer shift calibration constant, α = Δδ/Δ?(0), of the value ?0.242 ± 0.039a ₀ ³ mm sec^?1, which agrees reasonably well with the work of other investigators.     

Approximations in the theoretical interpretation of Mössbauer spectra

An account is given how semiempirical molecular orbital calculations and accurate relativistic atomic calculations can be utilized to understand Mössbauer spectra of the heavy main-group elements Sn, Sb, Te and I. The various approximations are described and it is shown in which way experimental and semiempirical theoretical information can be combined consistently to gain insight into the electronic structure of the respective compounds. A set of qualitative correlations between calculated valence orbital occupations, effective charges, coordination numbers and measured isomer shifts is derived and the limited applicability of phenomonological models is elucidated.     

Mössbauer, magnetic, X‐ray fluorescence and transmission electron microscopy study of natural magnetic materials from speleothems: haematite and the Morin transition

Detrital magnetic materials within cave stalagmitic formations, e.g., haematite or magnetite, carry remanence whose vector is of value in dating. Magnetometry measurements on a particular haematite‐bearing sample reveal that remanence was substantially restored and/or conserved on rewarming after cooling below the Morin transition temperature. Mössbauer measurements indicate the presence of two types of haematite, distinguished primarily by particle size. The majority is small in size, partially exhibiting superparamagnetism, and does not undergo a Morin transition above liquid nitrogen temperature. Superparamagnetic goethite is the second major component. Mine haematite samples of surface location with different color and mineralogical composition have also been studied. Possible relations between the mineralogical composition of the mine samples and detrital stalagmitic magnetic material, the modifications and the origin of this mineralization are discussed. Special attention is paid to the “irreversible” Morin transition in large enough (>20 nm) haematite particles and the possible loss of natural remanent magnetization and hence of palaeomagnetic records.     

SYNFOS

An expression for the amplitude of a pulse of synchrotron radiation coherently scattered in the forward direction by a Mössbauer absorber consisting of randomly oriented paramagnetic ironcontaining molecules (for example, a frozen solution of a ⁵⁷Fe protein) in an applied magnetic field is derived from the theory of optics. It is assumed that the hyperfine splittings present in the Mössbauer nuclei can be described in the framework of the spinHamiltonian formalism. In the general case of a thick Mössbauer sample of this kind the response on an incident monochromatic and fully polarized beam cannot be given analytically because of the integrations involved. How nuclear forwardscattering for this general case is evaluated in the program package called SYNFOS is outlined.