Nieves Menéndez

Synthesis and characterization of CoFe2O4 ferrite nanoparticles obtained by an electrochemical method

Uniform size cobalt ferrite nanoparticles have been synthesized in one step using an electrochemical technique. Synthesis parameters such as the current density, temperature and stirring were optimized to produce pure cobalt ferrite. The nanoparticles have been investigated by means of magnetic measurements, Mössbauer spectroscopy, x-ray powder diffraction and transmission electron microscopy. The average size of the electrosynthesized samples was controlled by the synthesis parameters and this showed a rather narrow size distribution. The x-ray analysis shows that the CoFe(2)O(4) obtained presents a totally inverse spinel structure. The magnetic properties of the stoichiometric nanoparticles show ferromagnetic behavior at room temperature with a coercivity up to 6386 Oe and a saturation magnetization of 85 emu g(-1).

Heterobimetallic MoSn complexes with seven-coordinate molybdenum and five-coordinate tin

Abstract New heterobimetallic complexes [Mo(SnRCl2)(CH3CN)2(CO3)(Cl)] [(1)R  C6H5, (2) R  CH3] have been obt

One-pot electrochemical synthesis of polydopamine coated magnetite nanoparticles

Herein a facile and versatile one step synthesis of magnetite nanoparticles coated with polydopamine is described. Magnetite nanoparticles are synthesized electrochemically by electrooxidation of iron in an aqueous medium in the presence of dopamine. The oxidative conditions and alkaline pH involved in the synthesis favor the self-polymerization of dopamine that adheres at the surface of the magnetic nanoparticles in a simultaneous process. It is shown that the size of the magnetite nanoparticles as well as the polydopamine coating can be controlled by varying the synthetic approach that is, adding dopamine at the beginning of the electrosynthesis, in the middle or at the end of the process. The particle size of the core varies between a few nanometers and 25 nm while the shell can reach thicknesses of up to ∼5 nm. The obtained hybrid nanoparticles were characterized by thermogravimetric analysis (TGA), infrared spectroscopy (FTIR), X-ray diffraction (XRD) and transmission electron microscopy (TEM). In addition, the magnetic measurements of the different obtained materials were carried out showing a variety of magnetic behaviors depending on the synthetic procedure.

Structural and magnetotransport features in new electron-doped Sr2−xCexFeMoO6 double perovskites

A new series of double perovskites of stoichiometry Sr2−xCexFeMoO6 (0 ≤ x ≤ 1) has been prepared by a wet chemistry procedure yielding very reactive precursors, which were subsequently annealed under reducing conditions. The compounds have been studied by X-ray (XRD), neutron powder diffraction (NPD), Mossbauer spectroscopy, magnetic and magnetotransport measurements in polycrystalline samples, aiming to investigate the effect of electron doping in the parent perovskite Sr2FeMoO6. The evolution of the crystal structure has been analyzed from NPD data; a transition from tetragonal (I4/m) to monoclinic (P21/n) has been observed at 0.2 ≤ x ≤ 0.4 as a result of the decrease of the tolerance factor due to the electron injection on B sites and the reduction in ionic size at A positions of the ABO3 perovskite. A bond valence study shows that an important fraction of the injected electrons is localized at Mo sites, which promotes the occurrence of antisite disordering between Fe and Mo cations. The magnetic properties are characteristic of ferromagnetic materials, with an ordering temperature of around 400 K, showing a non-monotonic behavior along the series, which is understood as a compromise between the steric and the electron injection effects. The introduction of a magnetic rare-earth in the Sr sublattice improves the magnetoresistive response at low temperatures, as a result of the polarization of the Ce3+ moments: a magnetoresistance (MR) factor of 45% is observed at 5 K for the x = 0.6 sample. For the first time we show that the introduction of a magnetic element in the A sublattice of a double perovskite can be used as an effective way to enhance the magnetoresistive response in this attractive family of oxides.

Vibrational and 119Sn Mössbauer spectra of tin(IV) halide complexes with 1,3-dimethylurea and 1,3-dimethylthiourea

Tin(IV) halide complexes with 1,3-dimethylurea (dmu) and 1,3-dimethylthiourea (dmtu) have been prepared and characterised by IR, Raman and 119Sn Mossbauer spectroscopies. The previously reported isomer shift (IS) values of SnBr4(dmtu)2 and SnI4(dmtu)2 have been revised to 0.98 and 1.32 mm s−1, respectively. The IS increases with decreasing electronegativity of the halogen atoms, and on going from dmu to dmtu complexes. The variation of the IS of SnX4L2 complexes with the electronegativity of the halogen X and the donor atom of the ligand L is studied. The IS can be used to get information about the atoms bonded to tin. The shifts of v(CE) (EO, S) and vas(NCN) with respect to the free ligands, in the IR spectra of the complexes, indicate that the dmu and dmtu ligands coordinate through the O and S atoms, respectively. The structure of the complexes is discussed in terms of the Mossbauer quadrupole splitting and the tin-halogen stretching vibrations in the IR and Raman spectra.

Heterobimetallic MoSn complexes of the type [Mo(CO)2(phen){P(4-XC6H4)3}(Cl)(SnRCl2)]

Abstract New heterobimetallic complexes [Mo(CO) 2 (phen){P(4-XC 6 H 4 ) 3 }(Cl)(SnRCl 2 )] (X = Cl, F, me, MeO; R = Cl, Me, Ph) were prepared by reaction between [Mo(CO) 3 (phen){P(4-XC 6 H 4 ) 3 }] and SnRCl 3 . The reactions involve retention of the phosphine ligand and elimination of CO. A seven-coordination at the molybdenum is produced by two cis -CO, NN-bidentate and phosphine ligands, the other two positions being occupied by the chlorine and SnRCl 2 groups, as deduced by IR and NMR ( 1 H and 31 P) spectroscopy. Mossbauer spectral data agree with a five-coordination at tin(IV) due to the presence of a chlorine group bridging the MoSn bond.

Mössbauer, far-infrared and Raman spectra of tetrachloro[1,4-di(p-methoxyphenyl)-2,3-dimethyl-1,4-diazabutadiene] tin(IV)

SummaryThe dependence of the logarithm of the area under the tin-119 Mössbauer resonance (A at different temperatures are normalized toA77) vs. temperature is measured for tetrachloro[1,4-di(p-methoxyphenyl)-2,3-dimethyl-1,4-diazabutadiene] tin(IV). The slope of the plot of lnA vs.T is −2.46·10−2K−1, suggesting a monomeric structure. In the vibrational spectra the significant stretching vibrations agree with acis octahedral configuration of C2v symmetry.ZusammenfassungDie Abhängigkeit des Logarithmus der Fläche unter der Zinn-119-Mössbauer-Resonanz (A in verschiedenen Temperaturen werden zuA77 normalisiert) von der Temperatur wurde für Tetrachlor[1,4-di(p-methoxyphenyl)-2,3-dimethyl-1,4-diazabutadien]zinn(IV) gemessen. Der Anstieg lnA gegenT von −2.46·10−2K−1 spricht für eine monomere Struktur. In den Vibrations-Spektren stimmen die wichtigsten Dehnungsschwingungen mit einercis-oktaedrischen Konfiguration von C2v-Symmetrie überein.