A. M. Gismelseed

Coordination versatility of tridentate pyridyl aroylhydrazones towards iron: tracking down the elusive aroylhydrazono-based ferric spin-crossover molecular materials

The two potentially tridentate and monoprotic Schiff bases acetylpyridine benzoylhydrazone (HL(1)) and acetylpyridine 4-tert-butylbenzoylhydrazone (HL(2)) demonstrate remarkable coordination versatility towards iron on account of their propensity to undergo tautomeric transformations as imposed by the metal centre. Each of the pyridyl aroylhydrazone ligands complexes with the ferrous or ferric ion under strictly controlled reaction conditions to afford three six-coordinate mononuclear compounds [Fe(II)(HL)(2)](ClO(4))(2), [Fe(II)L(2)] and [Fe(III)L(2)]ClO(4) (HL = HL(1) or HL(2)) displaying distinct colours congruent with their intense CT visible absorptions. The synthetic manoeuvres rely crucially on the stoichiometry of the reactants, the basicities of the reaction mixtures and the choice of solvent. Electrochemically, each of these iron compounds exhibits a reversible metal-centred redox process. By all appearances, [Fe(III)(L(1))(2)]ClO(4) is one of only two examples of a crystallographically elucidated iron(III) bis-chelate compound of a pyridyl aroylhydrazone. Several pertinent physical measurements have established that each of the Schiff bases stabilises multiple spin states of iron; the enolate form of these ligands exhibits greater field strength than does the corresponding neutral keto tautomer. To the best of our knowledge, [Fe(III)(L(1))(2)]ClO(4) and [Fe(III)(L(2))(2)]ClO(4) are the first examples of ferric spin crossovers of aroylhydrazones. Whereas in the former the spin crossover (SCO) is an intricate gradual process, in the latter the (6)A(1)↔(2)T(2) transition curve is sigmoidal with T(½)∼280 K and the SCO is virtually complete. As regards [Fe(III)(L(1))(2)]ClO(4), Mössbauer and EPR spectroscopic techniques have revealed remarkable dependence of the spin transition on sample type and extent of solvation. In frozen MeOH solution at liquid nitrogen temperature, both iron(III) compounds exist wholly in the doublet ground state.

Studies on Al Kidirate and Kapoeta meteorites

Mössbauer spectroscopy (20–300 K), magnetic susceptibility measurements (77–350 K), scanning electron microscopy and X-ray diffraction experiments have been performed on two meteorite samples: one from an old fall (Kapoeta) and another from a very recent fall (Al Kidirate). The two specimens differ in their mineralogy. Chondrules appear to be absent in Kapoeta and it is probably a pyroxene-plagioclase achondrite with ferrohypersthene as the most abundant mineral. On the other hand, the Al Kidirate meteorite is an ordinary chondrite and the specimen consists of olivine, orthopyroxene, troilite and kamacite. The Mössbauer measurements confirm the above characterization, showing a paramagnetic doublet for the Kapoeta sample and at least two paramagnetic doublets and magnetic sextets for the Al Kidirate specimens. The former were assigned to Fe in pyroxene sites, while the latter was assigned to Fe in pyroxene, olivine, Fe-S and Fe-Ni alloys. The difference in the mineralogy of the two meteorites has also been reflected in the temperature-dependent magnetic susceptibility. The magnetization and the hyperfine interaction parameters will be discussed in relation to the mineralogy.

Identification of Corrosion Products Due to Seawater and Fresh Water

Mossbauer and X-ray diffraction (XRD) measurements were performed on corrosion products extracted from the inner surface of two different metal tubes used in a desalination plant in Oman. One of the tubes corroded due to the seawater while the second was corroded due to fresh water. The corrosion products thus resulted due to seawater were scrapped off in to two layers, the easily removable rust from the top is termed outer surface corrosion product and the strongly adhered rust as internal corrosion product. The Mossbauer spectra together with the XRD pattern of the outer surface showed the presence of magnetite (Fe3O4), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), goethite (α-FeOOH) and hematite (Fe2O3). The inner surface however showed the presence of akaganite, goethite, and magnetite. On the other hand, the corrosion products due to the fresh water showed only the presence of goethite and magnetite. The mechanism of the corrosion process will be discussed based on the significant differences between the formation of the iron components of the corrosion products due to seawater and the fresh water.

Fe2P2O7 and Fe2P4O12 studied between 5–800 K

Mössbauer spectra of triclinic Fe2P2O7 indicate the existence of two crystallographic metal positions in the structure. In the paramagnetic region the two Mössbauer doublets are closely overlapping. The magnetic transition takes place at ≈ 21 K and the saturated fields are around 12 tesla for the two positions. In monoclinic Fe2P4O12 the two octahedrally coordinated metal positions give quite different quadrupole splittings (1.5 and 3 mm/s at room temperature) and hyperfine field values (42 and 12.5 Tesla at 5 K). The transition temperature is at ≈ 18.5 K.

Structural and magnetic analysis of the transformation of Sn-doped magnetite to Sn-doped hematite by mechanical milling

Spinel-related Sn-doped Fe3O4 has been ball milled for different times up to 35h. Milling was found to transform the material to corundum-related Sn-doped α-Fe2O3. The influence of the milling time, the crystallite size, and the cationic distribution on transformation process is being analyzed with x-ray diffraction, Mossbauer spectroscopy, and magnetic measurements. The relatively fast spinel-to-corundum structural transformation observed is associated with more Fe3+ ions being reduced to Fe2+ due to doping with Sn4+ ions.

Mössbauer studies on the metallic phases of Al Kidirate and New Haifa meteorites

We have studied the metallic phases in two equilibrated ordinary chondrites Al Kidirate (H6) and New Haifa (L4) using Mossbauer spectroscopy and synchrotron X-ray diffraction. Mossbauer spectra on taenite-enriched samples from the two meteorites identify the following Fe-Ni phases: the ferromagnetic ordered taenite (tetrataenite) with ∼ 50 % Ni, the ferromagnetic disordered taenite with ∼ 50 % Ni, the low-Ni (∼ 25 %) paramagnetic taenite, and small amounts of the α phase kamacite/martensite. The tetragonal unit cell parameters, in the space group P4/mmmm, of tetrataenite from Alkidirate meteorite are: a = 3.5680 ± 0.0018 A; c = 3.5901 ± 0.0015 A; c/a = 1.0062.

The electronic and magnetic structure of bcc iron-nickel alloy

The first principles discrete variational method and the local density approximation are used to calculate the electronic and magnetic properties of clusters of iron and nickel atoms representing the bcc iron-nickel alloy with nickel concentration up to about 30%. It is found that the presence of nickel atoms at sites neighbouring iron increases the iron local magnetic moment but decreases the magnitude of the magnetic hyperfine field. The magnetic hyperfine field at the iron site increases with increasing number of nickel atoms at the next-nearest-neighbour sites. Consequently, the experimentally observed maximum in both magnetization and hyperfine field versus nickel concentration is attributed to different iron sites. The 3d local densities of states at iron and nickel sites are calculated. The average density of states is found to remain unaltered for the majority sub-band, whereas it exhibits large deformation for the minority sub-band. An energy diagram is deduced from the density of states and is used to explain the formation of magnetic moment at iron and nickel sites. We deduce the following from these calculations. The observed increase in the local magnetic moment in the iron site is attributed to the reduction in the weight of the bonding d states and the accompanying constancy of the weight of the antibonding states, which ar driven by sd interaction.

Lattice parameters and magnetic properties of F-Sb-Te alloys with nickel arsenide structure

Abstract Single phase Fe-Sb-Te alloys with NiAs-type structure have been prepared in the composition range from Fe 1.173 Sb to Fe 1.105 Te and characterized by X-ray diffraction. Magnetic susceptibility measurements have been taken on these alloys in the temperature range from 77 up to 500 K. The lattice parameters show a linear variation with the alloy composition in the Sb-rich side but an anomalous change for the binary Fe 1.105 Te. The effective magnetic moments extracted from the susceptibility variations with temperature show a similar anomaly at approximately the same composition. These are discussed in terms of a model of additive magnetic moments of different atomic species.

Investigating Exchange Bias and Coercivity in Fe3O4–γ-Fe2O3 Core–Shell Nanoparticles of Fixed Core Diameter and Variable Shell Thicknesses

We have carried out extensive measurements on novel Fe3O4–γ-Fe2O3 core–shell nanoparticles of nearly similar core diameter (8 nm) and of various shell thicknesses of 1 nm (sample S1), 3 nm (sample S2), and 5 nm (sample S3). The structure and morphology of the samples were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The direct current (DC) magnetic measurements were carried out using a superconducting quantum interference device (SQUID). Exchange bias and coercivity were investigated at several temperatures where the applied field was varied between 3 and −3 T. Several key results are obtained, such as: (a) the complete absence of exchange bias effect in sample S3; (b) the occurrence of nonconventional exchange bias effect in samples S2 and S1; (c) the sign-change of exchange bias field in sample S2; (d) the monotonic increase of coercivity with temperature above 100 K in all samples; (e) the existence of a critical temperature (100 K) at which the coercivity is minimum; (f) the surprising suppression of coercivity upon field-cooling; and (g) the observation of coercivity at all temperatures, even at 300 K. The results are discussed and attributed to the existence of spin glass clusters at the core–shell interface.

Magnetic properties of nanocrystalline FexCu1-x alloys prepared by ball milling

X-ray diffraction, Mossbauer and magnetization measurements were used to study FexCu1−x alloys prepared by ball-milling. The X-ray data show the formation of a nanocrystalline Fe—Cu solid solution. The samples with x ≥ 0.8 and x ≤ 0.5 exhibit bcc or fcc phase, respectively. Both the bcc and fcc phases are principally ferromagnetic for x ≥ 0.2, but the sample with x = 0.1 remains paramagnetic down to 78 K. The influence of the local environment on the hyperfine parameters and the local magnetic moment are discussed using calculations based on the discrete-variational method in the local density approximation.