ABSTRACT
Single domain superparamagnetic ferrite nanoparticles with the composition MFe2O4 (M = Fe, Co, Zn) have been prepared by thermal decomposition of metal acetylacetonates in diphenyl ether or dibenzyl ether, using oleic acid in the presence of oleylamine as a stabilizing agent. The Fe, Co and Zn ferrite nanoparticles are monodisperse with diameters of 4.9, 4.4 and 4.7 nm, respectively. The TG and IR results indicate that four or six carboxylate groups per nm2 are bonded at the surface of the particles acting as chelating and/or bridging bidentate ligands depending on the composition. The oleate groups minimize the interparticle interactions in Fe and Zn ferrite samples, while in the Co ferrite sample dipolar interactions produce broad maxima in the ZFC and energy barriers distribution curves. The inversion degree has been estimated from the Raman spectra and the obtained x values have been used to calculate the saturation magnetization and compare them with the experimental MS values. Compared to bulk materials, the magnetization value is higher for the Zn ferrite sample due to its mixed spinel cation distribution. For the Co ferrite sample, and probably for the Fe one, the low value of saturation magnetization seems to be due to the surface disordered layer of canted spins. Compared to non-coated nanoparticles with the same composition and similar size, the oleate groups, covalently bonded to the superficial cations, increase the anisotropy field and decrease the magnetization.
ABSTRACT
CoFe2O4 particles of 16 nm and 17 nm embedded in a silica matrix have been prepared through the hydrothermal method and the sol-gel method, respectively. From neutron powder diffraction a cation distribution of (Fe(0.72)Co(0.28))[Fe(1.28)Co(0.72)]O4 has been determined for Co-ferrite particles of 17 nm, which is in agreement with its particle size taking into account the reported x values for other nanometric Co-ferrite particles. Magnetic measurements were performed up to 700 K as the prepared ferrite samples present blocking temperatures above room temperature. The temperature dependence of the superparamagnetic moment has been analyzed and presents for both samples an abrupt drop in the magnitude once the blocking temperature is overcome. The temperature dependence of the calculated magnetic field needed to reach the magnetic saturation of the samples allows us to determine the temperature range for which the nanoparticles show superparamagnetic behaviour. The ordering temperature is in both cases lower than the tabulated one for bulk Co-ferrite (793 K) which has been ascribed mainly to two factors: a different cation distribution and the nanometric particle size, both contributing to lowering of the strength of the superexchange interactions.
ABSTRACT
Two new cation-ordered polymorphs of Mn2ScSbO6 have been synthesised at high-pressure. At 5.5 GPa and 1523 K Mn2ScSbO6 crystallizes in the Ni3TeO6-type structure with the polar R3 space group and cell parameters a = 5.3419 (5) Å and c = 14.0603 (2) Å. Below TC = 42.0 K it exhibits ferrimagnetic order with a net magnetization of 0.6µB arising from unusual site-selective Mn/Sc disorder and is thus a potential multiferroic material. A double perovskite phase obtained at 12 GPa and 1473 K crystallizes in the non-polar P21/n monoclinic space group with cell parameters a = 5.2909 (3) Å, b = 5.4698 (3) Å, c = 7.7349 (5) Å and ß = 90.165 (6) °. Magnetization and neutron diffraction experiments reveal antiferromagnetic order below TN = 22.3 K with the spins lying in the ac plane.
ABSTRACT
We describe the synthesis, crystal structure and lithium deinsertion-insertion electrochemistry of two new lithium-rich layered oxides, Li3MRuO5 (M = Mn, Fe), related to rock salt based Li2MnO3 and LiCoO2. The Li3MnRuO5 oxide adopts a structure related to Li2MnO3 (C2/m) where Li and (Li0.2Mn0.4Ru0.4) layers alternate along the c-axis, while the Li3FeRuO5 oxide adopts a near-perfect LiCoO2 (R3[combining macron]m) structure where Li and (Li0.2Fe0.4Ru0.4) layers are stacked alternately. Magnetic measurements indicate for Li3MnRuO5 the presence of Mn(3+) and low spin configuration for Ru(4+) where the itinerant electrons occupy a π*-band. The onset of a net maximum in the χ vs. T plot at 9.5 K and the negative value of the Weiss constant (θ) of -31.4 K indicate the presence of antiferromagnetic superexchange interactions according to different pathways. Lithium electrochemistry shows a similar behaviour for both oxides and related to the typical behaviour of Li-rich layered oxides where participation of oxide ions in the electrochemical processes is usually found. A long first charge process with capacities of 240 mA h g(-1) (2.3 Li per f.u.) and 144 mA h g(-1) (1.38 Li per f.u.) is observed for Li3MnRuO5 and Li3FeRuO5, respectively. An initial sloping region (OCV to ca. 4.1 V) is followed by a long plateau (ca. 4.3 V). Further discharge-charge cycling points to partial reversibility (ca. 160 mA h g(-1) and 45 mA h g(-1) for Mn and Fe, respectively). Nevertheless, just after a few cycles, cell failure is observed. X-ray photoelectron spectroscopy (XPS) characterisation of both pristine and electrochemically oxidized Li3MRuO5 reveals that in the Li3MnRuO5 oxide, Mn(3+) and Ru(4+) are partially oxidized to Mn(4+) and Ru(5+) in the sloping region at low voltage, while in the long plateau, O(2-) is also oxidized. Oxygen release likely occurs which may be the cause for failure of cells upon cycling. Interestingly, some other Li-rich layered oxides have been reported to cycle acceptably even with the participation of the O(2-) ligand in the reversible redox processes. In the Li3FeRuO5 oxide, the oxidation process appears to affect only Ru (4+ to 5+ in the sloping region) and O(2-) (plateau) while Fe seems to retain its 3+ state.
Subject(s)
Iron/chemistry , Lithium/chemistry , Manganese/chemistry , Oxygen/chemistry , Ruthenium Compounds/chemistry , Electrochemical Techniques , Ions/chemistry , Magnetics , Oxidation-Reduction , Oxides/chemistry , Photoelectron SpectroscopyABSTRACT
The perovskite form of the Mn2FeSbO6 oxide has been obtained at 5.5 GPa and 1523 K. X-ray and neutron diffraction data reveal that this compound crystallizes in the monoclinic P21/n space group with a = 0.5234 nm, b = 0.5389 nm, c = 0.7642 nm and ß = 90.372° lattice parameters. AC and DC magnetic susceptibility measurements suggest the existence of a complex magnetic behavior below 200 K. The magnetic structure has been determined and can be described on the basis of an elliptical spiral with an incommensurate propagation vector, k = [0,0.426,0], where the magnetic moments of 2Mn(2+) and Fe(3+) cations are confined to the ac-plane.
ABSTRACT
Tetrachlorocobaltates(II) of diprotonated alpha,omega-diaminoalkanes with the formula [NH(3)(CH(2))(n)NH(3)]CoCl(4), where n = 5 (cadaverine; 1,5-pentanediammonium tetrachlorocobaltate), 8 (1,8-octanediammonium tetrachlorocobaltate) and 10 (1,10-decanediammonium tetrachlorocobaltate), were prepared. The compounds were studied by mass spectrometry, FT-IR and visible spectroscopy, magnetic susceptibility techniques and thermal analysis. The compounds contain the tetrahedral tetrachlorocobaltate(II) ion and the corresponding diprotonated diamine (cadaverine, 1,8-octamethylenediamine and 1,10-decamethylenediamine). The compound corresponding to cadaverine crystallizes in the monoclinic space group P2(1)/c, with lattice parameters a = 7.1633 (7), b = 15.940 (3), c = 11.137 (2) Å, beta = 98.44 (1) degrees and Z = 4. Its crystal structure contains slightly distorted tetrahedral CoCl(4)(2-) ions: the largest difference in Co-Cl bond lengths is 0.029 Å and the largest difference in Cl-Co-Cl angles is 7.76 degrees. The compound also contains diprotonated cadaverine ions. An extensive hydrogen-bonding network connects these ions. The slightly positive deviations of the magnetic susceptibility from the Curie-Weiss law are in agreement with the (4)A(2) ground state for the tetrachlorocobaltate anion. The compounds with eight and ten C atoms show phase transitions in the solid state and a greater complexity is observed in their differential scanning calorimetry curves. Unfortunately, no suitable single crystals of these could be obtained.
ABSTRACT
Different physicochemical studies were undertaken with polycrystalline samples of the complex [Cu2(carnosine)2(H2O)2].2H2O. The infrared spectrum was discussed in comparison with that of free carnosine and on the basis of the known structural data. Magnetic susceptibility measurements were performed between 4.2 and 300 K, showing an effective magnetic moment of 1.79 BM. Both the electronic (reflectance) and ESR spectra were compatible with the existence of a dx2-y2 ground state. The axial reversed ESR spectrum could be explained on the basis of a very weak interdimeric coupling mechanism. The electrochemical behavior, investigated by cyclic voltammetry, shows that the complex possesses a very high redox stability. The possible SOD-like activity was tested using the NBT/superoxide reduction assay. The results show a negligible SOD activity.
