Lithium orthosilicate (Li4SiO4) was synthesized by three different techniques:  the solid-state reaction, precipitation, and sol−gel (using a microwave oven) methods. The better results were obtained by the two first methods. In the third case, pure Li4SiO4 could not be obtained, because the microwaves produced the lithium sublimation. The samples were characterized by X-ray diffraction, scanning electron microscopy, N2 adsorption, and thermogravimetric analysis under a flux of CO2. Different particles sizes were obtained as a function of the method of synthesis, and the CO2 sorption analyses gave different results. The particle size modified the stability of the Li4SiO4 during the CO2 sorption/desorption cycles, due to lithium sublimation, as Li2O. Conversely, the isothermal study allowed measuring the kinetic parameters for the chemisorption and diffusion processes, as a function of the particle size. As could be expected, the activation energies obtained, for the small particles, were smaller than those obtained for the large particles. These results were explained in terms of reactivity, for the chemisorption process, and in terms of geometry, for the diffusion process.

The effect of boron excess in the structure and superconducting properties of NbB 2 is reported. Rietveld refinements of the x-ray diffraction patterns indicate that boron excess induces significant changes in the Nb-B bond length, increasing the c-axis. In contrast, the B-B bond length remains essentially constant. Magnetization behaviour was studied in the temperature range from 2 to 15 K. We found that for (B/Nb) exp 2.20(2) of boron excess samples display superconductivity with a maximum T C of about 9.8 K at (B/Nb) exp = 2.34(1). High pressure measurements in samples with two different boron contents reveal that T C decreases at different ratios, dT C /d P. Superconducting parameters were determined, indicating that NbB 2+x is a type II superconductor. We correlated the change of T C with the evolution of the structural parameters and found that it coincides with theoretical predictions of band structure.

ABSTRACT. Polycrystalline samples of LaFeO3 and GdFeO3 were synthesized by the molten salt method. Some properties and the quality of the resulting compounds were investigated. The crystal structure and purity of the samples was determined through X-ray diffraction and Rietveld analysis. The vibrational properties were characterized by Raman and IR spectroscopy. Mössbauer spectroscopy was used to determine the ionic state of the Fe ions and the internal hyperfine magnetic fields Considerable reduction of the heat treatment (temperature and time) for the reaction to take place was achieved without detriment of the quality of the compounds.

Polycrystalline samples of FeSe 1-x Te x (x = 0.00, 0.25, 0.50, 0.75 and 1.00) were synthesized by solid-state reaction to study the effects of substituting Se with Te in the system. The magnetization properties of the resulting compounds were investigated and the crystallographic structures of the samples analyzed through X-ray diffraction.Mössbauer spectroscopy was used to determine the ionic state of the Fe ions and the hyperfine fields. The magnetic susceptibility curves of the samples with x = 0.25, 0.50 and 0.75 show superconducting behavior. The lattice parameters and the cell volume increase monotonically with increasing Te concentration and the Mössbauer spectra reveal the absence of internal magnetic hyperfine fields.

The effects of Ca substitutions on the structure, magnetism and electrical properties of YCrO 3 ceramics are investigated by X-ray diffraction, magnetic susceptibility and electrical conductivity measurements. The cell volume decrease occurs through the change from Cr(III) to Cr(IV) as a result of the charge compensation of the Ca doping. No changes are observed in the antiferromagnetic transition temperature while strong changes are observed in the transport measurements due to Ca content. The increase of the electrical conductivity as well as the decrease of the activation energy are caused by the formation of the small-polarons localized in the O-Cr-O lattice distortion. The origin of small-polarons in the undoped sample is different in nature from the calcium doped. "Local non-centrosymetry" is the source of the small-polaron formation in undoped sample, while the change from Cr(III) to Cr(IV) through the charge compensation of Ca(II) in the Y(III) site is the source of small-polarons formations. The decrease of the average bond length Cr-O as well as effective moments in the paramagnetic state and the increase of the electrical conductivity areclear evidence that the Ca doping induces localized polarons, which in turn, these quasiparticles move from site to site by a thermally activated process in the doped YCrO 3 compound. Here, we also discuss a possible mechanism of small-polaron injections in YCrO 3 matrix.

ZnO nanoparticles with Wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. Nanoparticles are in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization vs temperature, M(T), and isothermal measurements M(H) were determined. The coercive field clearly shows ferromagnetism above room temperature. An exchange bias was observed, and we related this behavior at a core shell structure presented in the samples. The chemical synthesis, structure, defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior.

Crystal structure, thermal and magnetic properties were systematically studied in the Y1-x Prx CrO3 with 0 ≤ x ≤ 0.3 compositions. Magnetic susceptibility and specific heat measurements show an increase of the antiferromagnetic transition temperature (TN ) as Pr is substituted in the Y sites and notable magnetic features are observed below TN . Strong coupling between magnetic and crystalline parameters is observed in a small range of Pr compositions. A small perturbation in the lattice parameters by Pr ion is sufficient to induce a spin reorientation transition followed by magnetization reversal, to finally induce exchange bias effect. The spin reorientation temperature (TSR) is increased from 35 K to 149 K for 0.025 ≤ x ≤ 0.1 compositions. It is found that the Cr spins sublattice rotates continuously from TSR to a new spin configuration a lower temperature. In addition, magnetization reversal is observed at T * ∼ 35 K for x= 0.05 up to T * ∼ 63 K for x = 0.20 composition. The M − H curves show negative exchange bias effect induced by Pr ions, which are observed below of 100 K and being more intense at 5 K. At 10 K, the magnetic contribution of the specific heat, as well as the ZFC magnetization, show the rise of a peak with increasing Pr content. The magnetic anomaly could be associated with the freezing of the Pr magnetic moment randomly distributed at the 4c crystallographic site. A clear correspondence between spin reorientation, magnetization reversal and exchange bias anisotropy with the tilting and octahedral distortion is also discussed.

Polycrystalline samples of NbB 2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by X-ray photoelectron spectroscopy (XPS). The spectra revealed Nb and B oxides on the surface of the samples, mainly B 2 O 3 and Nb 2 O 5 . After Ar ion etching the intensity of Nb and B oxides decreased. The Nb 3d 5/2 and B 1s core levels associated with the chemical states (B/Nb) were identified and they do not change with etching time. The Binding Energy of the Nb 3d 5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shift in the core levels. On the other hand, analysis of Valence Band spectra showed that the contribution of the Nb 4d states slightly decreased while the contribution of the B 2p π states increased as the boron content increased. As a consequence, the electronic and superconducting properties were substantially modified, in good agreement with band-structure calculations.