Search

Studies indicate that the degree of polymorphism of niobium pentoxide (Nb2O5) is related to the origin of its extraction and that adequate heat treatments can improve its quality concerning the number of structural phases. In this study, the niobium pentoxide extracted from the region of the Triângulo Mineiro in the state of Minas Gerais in Brazil was heat treated and its structural characterization reveals to obtaining the single monoclinic phase (H-Nb2O5). The thermal treatments were carried out from 900 to 1150 °C and the desired single-crystal phase sample was obtained with the treatment at 1100 °C. The treated material was characterized by Dynamic Light Scattering (DLS), and its morphology by scanning electron microscopy (SEM). X-ray diffraction (XRD) and Rietveld refinement show the presence of the crystalline phase and a good agreement between the experimental and the calculated results were obtained. Functional groups, characteristic bands, and peaks were analyzed by infrared spectroscopy using Fourier Transform with Photoacoustic Detection (FTIR-PAS) and Raman spectroscopy. Thermal analysis was also made using thermogravimetric analysis (TGA), differential thermal analysis (DTA), and dilatometry techniques.

The (La 0.8 Ca 0.2 ) 0.9 Bi 0.1 FeO 3 (LCBFO) compound has been synthesized by the sol-gel method and characterized by X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM), Raman spectroscopy and electrical impedance spectrometry . XRD results revealed that (La 0.8 Ca 0.2 ) 0.9 Bi 0.1 FeO 3 crystals are orthorhombic, belonging to the Pnma space group. The SEM measurements showed that the sample presents a large distribution of nano-grains connected to each other. The relaxation process and the electrical conductivity are awarded to the same type of charge carriers characterized by similar values of the activation energy determined from loss factor tangent t g δ , the imaginary part of the permittivity and from the Modulus spectrum. The ac -conductivity was analysed to examine the conduction mechanism, using the Jonscher’s universal power-law given by: σ ac ω = σ dc + A ω s . Based on the parameter s behavior, the conductivity was studied according to the NSPT model (non-overlapping small polaron tunneling).

Producción Científica, Lithium ferrites are well known materials due to their numerous technological applications especially in microwave devices. Mn-doped lithium ferrite nanoparticles were prepared by sol-gel technique by means of Pechini method, and then annealed at different temperatures in 250–1000 °C range. XRD confirms spinel formation with particle size in the 15–200 nm range, with increased size with annealing temperature. Microwave magnetoabsorption data of annealed lithium ferrite nanoparticles, obtained with a broadband system based on a network analyzer operating up to 8.5 GHz are presented. At fields up to 200 m T we can observe a broad absorption peak that shifts to higher frequencies with magnetic field according to ferromagnetic resonance theory. The amplitude of absorption, up to 85%, together with the frequency width of about 4.5 GHz makes this material suitable as wave absorber. Samples annealed at higher temperatures show a behaviour similar to polycrystalline samples, thus suggesting their multidomain character., Ministerio de Economía, Industria y Competitividad, Agencia Estatal de Investigación with FEDER (project MAT2016-80784-P)

In this work we explore the formation of enhanced multiferroic interfaces in bismuth ferrite crystalline fibers grown by laser floating zone technique. An underlying mechanism of self-segregation during the fibers growth process enables to establish a textured microstructure of a dominant BiFeO3 phase bordered by the presence of Bi25FeO40 secondary phase. The crystallites c axis of the BiFeO3 phase shows a preferential orientation along the longitudinal axis of the fibers, together with grain boundaries that also present a significant alignment with the same direction. These features induce a systematic disturbance of the antiferromagnetic structure of the BiFeO3 phase at the interfaces with the Bi25FeO40 diamagnetic phase. The structural anisotropy confirmed by High Resolution X-ray diffraction and scanning electron microscopy images is also manifested in the magnetic properties of the fibers, which reveal an enhanced susceptibility response in comparison to the conventional BiFeO3 phase diagram. Keywords: Multiferroic, Interfaces, BiFeO3, Composite, Laser floating zone

Producción Científica, Magnetic after-effects in ferrite nanoparticles have been measured with magnetic disaccommodation technique. For higher annealing temperature multidomain formation is detected with low temperature relaxation process similar to single crystal and polycrystalline NiFe2O4. As expected, no room temperature processes are observed due to low vacancy content. On the other hand, logarithmic relaxations are observed at lower annealing temperatures, revealing monodomain character., Ministerio de Economía, Industria y Competitividad (project MAT2016-80784-P)

a b s t r a c t TiO2 fibres grown by the laser floating zone technique show the rutile crystalline phase as identified by Xray diffraction and Raman spectroscopy. Scanning electron microscopy evidences a uniform surface, free of inclusions and without low-angle grain boundaries and bubbles. At low temperatures, the fibres luminescence is dominated by a richly structured optical centre with two well-defined zero-phonon lines at 1.579 eV and 1.574 eV followed by an intense vibronic sideband with a maximum close to 1.47 eV. Electron paramagnetic resonance revealed that substitutional chromium and iron ions are present as contaminants in the grown fibres, alongside with Ti3+ interstitials and other structural defects. The chemical nature of the defect responsible for the structured near-infrared luminescence is discussed.

We report on the structural, morphological and optical characterization of glass and glass–ceramic samples, with nominal composition of 91SiO2:4Li2O:4Nb2O5:1Dy2O3 (% mole). The samples were heated treated in air in the temperature range between 500 °C and 800 °C. The glass–ceramics samples start to crystallize at temperatures above 600 °C and besides the SiO2 and LiNbO3 crystalline phases, sub micron DyNbO4 precipitates were identified by X-ray diffraction and scanning electron microscopy for temperatures above 750 °C. Using Raman spectroscopy was possible to identify the vibration modes assigned to the crystalline phases detected by the XRD analysis. Also, the performed Photoluminescence Spectroscopy analysis suggest that the optical activation of Dy3+ ions in the glass and glass–ceramics hosts was found to be dependent of the heat treatment temperature. Intraconfigurational transitions between the 4F9/2 and 6HJ(=15/2, 13/2) multiplets of Dy3+ ions were only observed for samples without the presence of the DyNbO4 crystalline phase. Moreover, for samples heat treated between 500 °C and 700 °C the intensity ratio of the 4F9/2 6H13/2 and 4F9/2 6H15/2 transitions of the lanthanide ion changes, suggesting different ion environments which are dependent on the thermal annealing temperature. The dielectric measurements revealed that e′ value depends mainly in the number of network modifier ions and the relaxation phenomenon observed at frequencies above 10 kHz, in all samples, can be assigned to the increase of the volume ratio between the LiNbO3 particles and the glass network.