Kisla Prislen Félix Siqueira

Effect of the processing parameters on the crystalline structure of lanthanide orthotantalates

The influence of the synthesis parameters on the crystalline structures of orthotantalate ceramics has been investigated. Powder materials were prepared by the solid-state reaction route. X-ray diffraction and Raman scattering measurements were employed to investigate the crystal structure of the produced materials. In this work, we analyzed three different examples in which the temperature and time were decisive on the final crystal structure of LnTaO 4 compounds besides the lanthanide ionic size. Firstly, the thermal evolution for NdTaO 4 samples showed that mixed crystal phases are formed up to 1100 °C, while well-crystallized M-NdTaO 4 (I2/a) materials are obtained in temperatures higher than 1200 °C. Also, the influence of the synthesis time was investigated for the LaTaO 4 ceramics: it was necessary 14 h to obtain samples in the P2 1 /c structure. Finally, two polymorphs could be obtained for the DyTaO 4 ceramics: P2/a and I2/a space groups were obtained at 1300 °C and 1500 °C, respectively. This study indicated that the temperature, time and lanthanide size are directly correlated with the crystalline arrangement of the orthotantalate materials.

Optical phonon modes and infrared dielectric properties of monoclinic CoWO4 microcrystals

The phonon characteristics of CoWO4 microcrystals with monoclinic Wolframite structure were investigated by far-infrared (IR) and Raman spectroscopies. Near-normal spectra were taken for IR light polarization along the principal b-axis (A u modes) and along several angles within the ac-plane (B u modes). The IR spectra were analyzed with a generalized Drude–Lorentz model, and all predicted polar phonon modes were fully determined, including their symmetries, the dielectric Lorentz parameters and the non-orthogonal phonon polarizations for the B u modes. Anomalous dispersion and negative values for the real and imaginary parts of the off-diagonal components of the dielectric tensor functions were identified and discussed under the light of the varying phonon polarization directions (spread out in the ac-plane). The obtained static and background dielectric tensors gave an average permittivity of 16.1 (at microwave region), refractive indices along the principal dielectric axes of 2.22, 2.33 and 2.44 (at 1 μm), the optical axes, and an estimated value for the biaxial angle of the crystal. Polarized Raman spectra on appropriate scattering configurations revealed the 18 non-polar gerade phonons of CoWO4 crystals, with their correct symmetries attributed.

Electrocatalytic performance of different cobalt molybdate structures for water oxidation in alkaline media

Cobalt molybdates with different crystalline structures, i.e., α, β, and hydrated (H)-CoMoO4, were synthesized, and their electrocatalytic activities were thoroughly examined for catalyzing the oxygen evolution reaction (OER) in alkaline media. The material characteristics were associated with the electrocatalytic properties by evaluating the CoMoO4 crystal structures (XRD and Raman), morphologies (TEM), and electrochemical features (electrochemically active surface area, roughness factor, electrochemical impedance, Tafel analysis, and controlled-current electrolysis). These combined findings revealed that the electrocatalytic performance is greatly influenced by the crystalline structures of CoMoO4, following the order α-CoMoO4 > H-CoMoO4 > β-CoMoO4. The H-CoMoO4 catalysts crystallized in the triclinic space group, P (#2), with Z = 4. On the other hand, the α- and β-CoMoO4 catalysts exhibited a monoclinic structure, C2/m (#12), with Z = 8. In the OER experiments, α-CoMoO4 showed an overpotential of 0.43 ± 0.05 V compared to the 0.51 ± 0.05 V and 0.56 ± 0.04 V exhibited by the H-CoMoO4 and β-CoMoO4 catalysts, respectively, to achieve 10 mA cm−2. All CoMoO4 structures displayed stability for at least 6 h at a controlled current density of 10 mA cm−2. Finally, computational simulations indicate that the coexistence of Co and Mo ions in edge-shared octahedral sites of α-CoMoO4 may favor the interaction between the O atom of the water molecule and the metal adsorption sites due to its surface being electronically less dense than β- and H-CoMoO4 surfaces, thus resulting in its higher performance for OER.

Microwave-Hydrothermal Synthesis of Transition Metal Tungstates with Nanosized Particles

Microwave-hydrothermal processing was employed to synthesize Mn and Co tungstates under environmentally friendly conditions. X-ray diffraction showed that crystalline, single-phase materials were produced. Incipient crystallized compounds were also obtained in some conditions, as verified by TEM. For these samples, nanostructured regions could be observed inside the amorphous matrix. Sintered solid-solutions of Mn1-xCoxWO4 compounds were produced to investigate the morphological and structural changes in the resulted ceramic. Raman spectroscopy was used to obtain a reliable set of spectra containing all the Raman-active bands predicted by group-theory calculations.