L.C.J. Pereira

Single-ion magnet behaviour in [U(Tp(Me2))(2)I].

[U(Tp(Me2))(2)I] exhibits at low temperatures single molecule magnet (SMM) behaviour comparable to its bipyridine derivative and related single ion U(III) complexes recently reported as SMMs. The trend of variation of the energy barrier for the magnetic relaxation in these compounds is well reproduced by quantum chemistry calculations.

Behavior of REE and other trace and major elements during weathering of granitic rocks, Évora, Portugal☆

The distribution of elements within a weathering profile depends on several factors, including the relative stability of the mineral phases of the parent rock, redox and leaching conditions. Three main processes are involved in weathering: ( 1 ) the breakdown of the parent minerals; (2) the removal in solution of some of the released constituents; and (3) the formation of secondary phases with components from the atmosphere (water, oxygen and carbon dioxide), from the parent rock and from shallower levels, and transported by the percolating solutions (Fritz and Ragland, 1980; Nesbitt, 1980; Chesworth et al., 1981 ). The purpose of this work is to investigate how major and trace elements behave during the weathering of granitic rocks from l~vora, Portugal. The prevailing climatic characteristics of this region are: 700 mm annual rainfall; dry summers; and a mean annual temperature of 16°C.

Effects of surfactants on the magnetic properties of iron oxide colloids.

Iron oxide nanoparticles are having been extensively investigated for several biomedical applications such as hyperthermia and magnetic resonance imaging. However, one of the biggest problems of these nanoparticles is their aggregation. Taking this into account, in this study the influence of three different surfactants (oleic acid, sodium citrate and Triton X-100) each one with various concentrations in the colloidal solutions stability was analyzed by using a rapid and facile method, the variation in the optical absorbance along time. The synthesized nanoparticles through chemical precipitation showed an average size of 9 nm and a narrow size distribution. X-ray diffraction pattern and Fourier Transform Infrared analysis confirmed the presence of pure magnetite. SQUID measurements showed superparamagnetic properties with a blocking temperature around 155 K. In addition it was observed that neither sodium citrate nor Triton X-100 influences the magnetic properties of the nanoparticles. On the other hand, oleic acid in a concentration of 64 mM decreases the saturation magnetization from 67 to 45 emu/g. Oleic acid exhibits a good performance as stabilizer of the iron oxide nanoparticles in an aqueous solution for 24h, for concentrations that lead to the formation of the double layer.

[Fe(nsal2trien)]SCN, a New Two-Step Iron(III) Spin Crossover Compound, with Symmetry Breaking Spin-State Transition and an Intermediate Ordered State

We report the synthesis of the iron(III) complex of the hexadentate Schiff base ligand nsal2trien obtained from the condensation of triethylenetetramine and 2 equiv. of 2-hydroxy-1-naphthaldehyde. The study of the salt [Fe(nsal2trien)]SCN (1) by magnetic susceptibility measurements and Mössbauer spectroscopy reveals a rather unique behavior that displays thermally induced spin crossover (SCO) with two well-separated steps at 250 (gradual transition) and 142 K (steep transition). Single crystal X-ray structures were obtained at 294, 150, and 50 K, for the high spin (HS), intermediate (Int), and low spin (LS) phases. The HS and LS phases are isostructural, and based on a single Fe(III) site (either HS or LS) an unusual symmetry break occurs in the transition to the Int ordered phase, where the unit cell includes two distinct Fe(III) sites and is based on a repetition of the [HS-LS] motif. The two-step SCO behavior of 1 must result from the existence of structural constraints preventing the full conversion HS ↔ LS in a single step.

Magnetic properties of Co-doped TiO2 anatase nanopowders

This letter reports on the magnetic properties of Ti1−xCoxO2 anatase phase nanopowders with different Co contents. It is shown that oxygen vacancies play an important role in promoting long-range ferromagnetic order in the material studied in addition to the transition-metal doping. Furthermore, the results allow ruling out the premise of a strict connection between Co clustering and the ferromagnetism observed in the Co:TiO2 anatase system.

Clay mineral assemblages in weathered basalt profiles from central and southern Portugal: climatic significance

Abstract Mineralogical and chemical changes during weathering of basalts from central and southern Portugal were studied. Both regions have a temperate Mediterranean type climate, the Algarve being more arid. The areas concerned are the Lisbon (72 Ma) and the Runa Volcanic Complexes (100–70 Ma), and Rio Maior (160–130 Ma) in central Portugal, and Budens (170–180 Ma) in the south. The basalts are generally rich in olivine phenocrysts. Clay minerals are in general well crystallised and comprise smectites, associated with 1:1 minerals (principally halloysite), illite and palygorskite. Kaolin minerals prevail in topographic highs, whilst smectites predominate in lower areas. Smectites were analysed from the

Thermal and magnetic properties of chitosan-iron oxide nanoparticles

Chitosan is a biopolymer widely used for biomedical applications such as drug delivery systems, wound healing, and tissue engineering. Chitosan can be used as coating for other types of materials such as iron oxide nanoparticles, improving its biocompatibility while extending its range of applications. In this work iron oxide nanoparticles (Fe3O4 NPs) produced by chemical precipitation and thermal decomposition and coated with chitosan with different molecular weights were studied. Basic characterization on bare and chitosan-Fe3O4 NPs was performed demonstrating that chitosan does not affect the crystallinity, chemical composition, and superparamagnetic properties of the Fe3O4 NPs, and also the incorporation of Fe3O4 NPs into chitosan nanoparticles increases the later hydrodynamic diameter without compromising its physical and chemical properties. The nano-composite was tested for magnetic hyperthermia by applying an alternating current magnetic field to the samples demonstrating that the heating ability of the Fe3O4 NPs was not significantly affected by chitosan.

A new hybrid material exhibiting room temperature spin-crossover and ferromagnetic cluster-glass behavior

A new compound, [Fe(5–Cl-qsal)2][Ni(α-tpdt)2].CH3CN, where α-tpdt = 2,3-thiophenedithiolate and H5–Cl-qsal = N-(8-quinolyl)-5-chlorosalicylaldimine, was prepared and structurally and magnetically characterized. The crystal structure is based on an arrangement of alternate layers of [Fe(5–Cl-qsal)2]+ cations and [Ni(α-tpdt)2]− anions. The magnetic measurements and Mossbauer spectroscopy revealed hybrid behavior in this compound, where a ferromagnetic cluster-glass behavior, ascribed to the anions network, and a spin crossover (SCO) of the [Fe(5–Cl-qsal)2]+ cations were observed. The glassy behavior, with a blocking temperature of ca. 7.5 K, results from the disorder in the anionic layer and the competition between ferro- and antiferromagnetic interactions in the anionic layers. The SCO process with T1/2 = 298 K (high spin fraction = γHS = 0.5) is rather sluggish in the limits of the conversion (γHS ∼ 0 and γHS ∼ 1), which is attributed to the effect of the anions that seem to restrict somehow the structural distortions of the cations associated with the SCO process.

Magnetic Properties of the Layered Lanthanide Hydroxide Series YxDy8-x(OH)20Cl4·6H2O: From Single Ion Magnets to 2D and 3D Interaction Effects

The magnetic properties of layered dysprosium hydroxides, both diluted in the diamagnetic yttrium analogous matrix (LYH:0.04Dy), and intercalated with 2,6-naphthalene dicarboxylate anions (LDyH-2,6-NDC), were studied and compared with the recently reported undiluted compound (LDyH = Dy8(OH)20Cl4·6H2O). The Y diluted compound reveals a single-molecule magnet (SMM) behavior of single Dy ions, with two distinct slow relaxation processes of the magnetization at low temperatures associated with the two main types of Dy sites, 8- and 9-fold coordinated. Only one relaxation process is observed in both undiluted LDyH and intercalated compounds as a consequence of dominant ferromagnetic Dy-Dy interactions, both intra- and interlayer. Semiempirical calculations using a radial effect charge (REC) model for the crystal field splitting of the Dy levels are used to explain data in terms of contributions from the different Dy sites. The dominant ferromagnetic interactions are explained in terms of orientations of easy magnetization axes obtained by REC calculations together with the sign of the superexchange expected from the Dy-O-Dy angles.

Crystal structure diversity in the bis[hydrotris(3,5-dimethylpyrazolyl)borate]iodouranium(III) complex: from neutral to cationic forms

The iodouranium(III) complex with two hydrotris(3,5-dimethylpyrazolyl)borate ligands is shown to adopt three closely related forms in the solid state. In addition to the previously reported structure for [U(Tp(Me2))2I], in which one of the pyrazolyl rings coordinates side-on to the U atom, another structure incorporating solvent molecules presents undistorted pyrazol rings, and a third one is the ionic compound [U(Tp(Me2))2]I. The implications of this structural diversity for the recently reported single ion magnet behaviour in this complex are discussed, namely on the basis of quantum chemistry calculations. The main effect of the bonding of the iodine atom to uranium is the increase of the size of the first coordination sphere and lowering of the symmetry of the molecule, resulting in a smaller crystal field splitting.