Madjid Arab

Carbonatation and Decarbonatation Kinetics in the La 2 O 3 -La 2 O 2 CO 3 System under CO 2 Gas Flows

The carbonatation of La2O3 oxide and the decarbonatation of lanthanum carbonate phase La2O2CO3 are investigated using thermal and thermogravimetry analyses under CO2 gas flow. The initial phase La2O3 is first elaborated from pyrolysis of a LaOHCO3 precursor. Then, thermal and thermogravimetry analyses are carried out under CO2 flow, as temperature increases then decreases. The carbonatation kinetics of La2O3 is determined at three fixed temperatures. Electrical impedance spectroscopy is performed to determine the electrical responses associated with ionic mobilities and phase changes, in the temperature range 25 to 900∘C. The electrical conduction during heating under CO2 gas flow should be linked to two regimes of ionic conduction of the carbonate ions. From these electrical measurements, the ionic mobility of carbonate ions CO32− is found to be close to 0.003 ·10−4 cm2 s−1 V−1 at 750∘C for the monoclinic La2O2CO3 phase.

Hierarchical design and control of NaCe(WO4)(2) crystals: structural and optical properties

Double rare earth tungstates NaCe(WO4)2 have been successfully synthesized hydrothermally for the first time with cerium as the trivalent cation in the tetragonal single phase. The resulting hierarchical superstructures were structurally characterized by X-ray diffraction in conjunction with Rietveld refinements and Raman spectroscopy. Their shape and size were observed by scanning and transmission electron microscopy coupled to energy dispersive spectroscopy. It was found that morphology modulation could be realized by controlling the amount of EDTA ligand (0.3–0.35 g) and the pH values (6–8) of the system solution. Hierarchical microstructures resulted firstly from the coalescence of nanosheets and secondly from the self-assembly of nanoplatelets, nanorods and nanosheets, giving rise to sphere, flower and novel spindle morphologies, respectively. Structural determinations showed that the crystal structure belongs to the scheelite family ABWO4, with Na and Ce atoms occupying the same sites. Refinement calculations revealed that the microstructures showed different distortions of the polyhedral structure according to the W–O and Ce(Na)–O bonds and angles splitting. The formation mechanisms of the resulting hierarchical architectures are put forward based on a series of time-dependent experiments. Furthermore, the optical properties of the microstructures were strongly related to their morphologies, crystallite size and polyhedral distortion.

Morphological and structural investigation of SrWO4 microcrystals in relationship with the electrical impedance properties

In this paper, strontium tungstate (SrWO4) microcrystals have been synthesized at room temperature using a wet chemical method, without any surfactant or template, by adjustment of the precursor concentration and pH solution. The developed mineralization process allows obtaining homogeneous structures with controlled morphologies, including spindles and spheres. Scanning electron microscopy, X-ray diffraction combined with Rietveld analyses and Raman spectroscopy, are used to characterize the detailed scheelite-type tetragonal structures and morphologies and to highlight the size and microstrain effects. In addition to the Raman bands characteristic of the scheelite phase, it was found a strong low frequency modes assigned to high distortion of the SrO8 polyhedra. The mechanisms underlying growth and shape evolution are proposed taking into account the effects of precursor concentration and thermal treatment on the stability of the different morphologies which have been investigated in detail. Electrical conduction analyses have been carried out as a function of temperature in both AC and DC current modes. The results are interpreted in terms of a Nyquist representation, such that the spindles show a single semicircle at high frequencies, while a second semicircle appears at lower frequencies for the spheres. The measured conductivities have similar electrical transitions. Below 500 °C, the conductivity is mainly due to electron charge carriers; however above this temperature, the conductivity becomes mixt due to the order–disorder transition. Moreover, the associated activation energies of the spheres are higher than those of the spindles.

A facile one step route to synthesize WO3 nanoplatelets for CO oxidation and photodegradation of RhB: microstructural, optical and electrical studies

Two-dimensional nanoplatelets of tungsten trioxide (NP-WO3) were synthesized at room temperature, using a wet chemical method, without any surfactants or templates; aqueous mineralization was obtained by simply adjusting the pH. The resulting nanostructures were characterized using X-ray diffraction combined with Rietveld refinements, Raman and UV-Vis spectroscopies. Their morphologies and sizes were analyzed by scanning and electron microscopies. The electrical, optical, catalytic and photocatalytic properties of the NP-WO3 nanoplatelets were then determined and compared to the ones of pseudospherical (PS-WO3) standard samples. Nanoplatelets as well as pseudospherical particles crystallized in the single orthorhombic WO3 phase. The Rietveld refinement calculations and HRTEM analyses revealed a strong distortion of the WO6 octahedra, according to the W–O splitting. The electrical conductivity of WO3 compact pellets showed that both samples were semi-conducting with a weak difference in activation energies. Using a homemade photocatalytic device, the NP-WO3 particles used as photocatalyst in an aqueous medium, exhibited a significant efficiency to decompose rhodamine B over their large exposed surface (010), compared to PS-WO3 particles. These NP-WO3 particles were also used as the catalytic material for oxidation in an air–CO gas flow. They exhibited catalytic activity higher than the one in the PS form.

Synthesis, characterization and luminescent properties ofSr 1-x Pb x WO 4 solid solution (x=0, 0.5 and 1)

In this work, a study of the role of chemical substitution in the properties of the solid solution Sr(1-x)PbxWO(4) (x = 0, 0.5 and 1) is presented. Polycrystalline samples were synthesized by solid state reaction at 1100 degrees C. Using Rietveld method, the structural parameters of all samples were refined. All structures are of scheelite type Scanning electron microscopy showed that a high level of crystallization characterized the samples, with modifications in sizes and shapes depending on composition x. The Infrared and Raman spectroscopies were performed to characterize the evolution of vibrational modes with composition x. Finally, a systematic study of luminescence under X-ray and UV excitations was performed. The energies and intensities of luminescence depended on composition x and on the nature of excitations.

Influence of chemical substitution on the photoluminescence of Sr{sub (1−x)}Pb{sub x}WO{sub 4} solid solution

The solid solution Sr{sub 1−x}Pb{sub x}WO{sub 4} based on luminescent tungstates SrWO{sub 4} and PbWO{sub 4} has been synthesized by solid-state reaction for all compositions 0≤x≤1. Using Rietveld method, the structural data of all polycrystalline samples have been refined and crystal cell parameters exhibited a linear behavior as a function of x. All substituted structures are of scheelite type. Scanning electron microscopy showed that a high level of crystallization characterized the samples, with modifications in sizes and shapes depending on composition x. Infrared and Raman spectroscopy have been performed to characterize the evolution of vibrational modes with substitution rate. Finally, a systematic study of luminescence under X-ray excitation has been performed: in the composition range x=0.2 to 0.4, intensities of emission exhibited increased values. The luminescence profiles have been interpreted in terms of four Gaussian components, two of them depending on substitution rate. - Graphical abstract: Photoluminescence under X-ray excitation of Sr{sub 1−x}Pb{sub x}WO{sub 4} solid solution: (left) decomposition of emission large band into four components for composition x=0.3; (right) variation of total emission intensity with composition x. - Highlights: • Structural study of solid solution Sr{sub 1−x}Pb{sub x} WO{sub 4} using Rietveld analyses. • Variation of Debye–Waller factor withmorexa0» composition x, disorder parameter. • Existence of four components of luminescence under X-ray excitation. • Effect of substitution of Sr by Pb on two components, due to Pb–O–W interactions. • Enhancement of luminescence intensity in a specific composition range.«xa0less