SYNTHESIS AND RIETVELD WHOLE-PATTERN ANALYSIS OF Sr-DOPED PEROVSKITE BISMUTH FERRITE

Doping of perovskite bismuth ferrite (BiFeO 3 ) affect greatly its microstructure characteristics. X-ray diffraction analysis using Reitveld method of the whole-pattern fitting is a very useful approach to study the structure variation in relation to doping level. Phase identification, crystallite size, residual microstrain, lattice parameters and bond lengths have been investigated. The conventional solid state reaction method at 650 and 800 °C for 1 h with intermediate grinding was used for the preparation of a series of Bi 1-x Sr x FeO 3 (x = 0.1 -0.5). FULLPROF program was used and modified THOMPSON Cox-HASTING PESUDO-VOIGT function was applied and the background was modeled with fourth order. The sample of x = 0.1 consists of two polymorphic phases perovskite cubic and rhombohedral, the cubic one as the major phase and the rhombohedral one as the minor one. For samples of x ═ 0.2 -0.45, only single phase of cubic perovskite structure was identified while for x 0.5 the sample consists of cubic perovskite and traces of unidentified phase. A continuous decrease in the lattice parameter as well as in the octahedral and dodechaderal bond lengths was observed with increasing Sr 2+ content and this may be attributed to the increasing number of oxygen vacancies with increasing doping level. The micostrain was found to increase with increasing the Sr 2+ content, which was attributed also to the formation of point defects (oxygen vacancies) as well as to ionic size mismatch between host and substituent cations that is led to local structural disorder. The crystallite size increases with increasing the doping level which seems to be due to the increase in the surface free energy resulting in the enhancement of the size growth by the presence of oxygen vacancies

On the Wavy Behaviour of Grain Growth in Sprayed Thin Films

Tin oxide films were prepared by spray pyrolysis technique with different deposition times (60 - 240 s) while the substrate temperature (773 K), the deposition rate (0.22 ml/s) and the concentration (0.2 M) of tin chloride (hydrous) solution were kept constant. Variation of grain size with film thickness (deposition time) is found useful to model a mechanism for film growth. Depending on the mode of layer growth followed by 3D crystallite growth (Stranski and Krastaniv, SK) as well as on the variation of the standard deviation of pole densities, a proposed model of growth mechanism of thin film was suggested. Consider the fact that what you see depends on how you look , a wavy nature was suggested for the variation of the mean grain size with the film thickness using XRD analysis and SEM/AFM images. The discrepancy between the present result and others was understood on the basis of either the definition of the grain size measured by the different techniques (XRD or SEM) or the proposed wavy behaviour of the grain growth with the film thickness. Interpretation of the results of the grain size variation that extracted from the combined use of XRD and SEM/AFM can provide useful information about the growth mechanism

Thermal and spectroscopic characterization of N-methylformamide/Ca-, Mg-, and Na-exchanged montmorillonite intercalates

The progressive adsorption of N-methylformamide (NMF) onto Ca-, Mg-, and Na-exchanged Wyoming bentonite (Mn+ - SWy-2) together with its subsequent thermal desorption has been studied using a variety of complementary techniques. The derivative thermograms (DTG) for the desorption of NMF from Mn+-SWy-2 exhibit three maxima at temperatures which depend on the exchange cation. In Mg-SWy-2 these maxima occur at 130, 200, and 400 degrees C, whereas in Na-SWy-2 they occur at 100, 150, and 190 degrees C. Each of these maxima has been assigned to different sites and/or environments for sorbed NMF using variable temperature-X-ray diffraction (VT-XRD) and variable temperature-diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS). Each fully loaded Mn+-SWy-2/NMF complex has two layers of NMF in the gallery, which decreases to a single layer prior to the complete removal of NMF from the complex. The temperatures at which major weight losses occur coincide with decreases in the interlayer spacing. VT-DRIFTS has shown that at low temperatures NMF was removed from NMF clusters, similar to those in liquid NMF, while at high temperatures the NMF molecules are firmly bound and directly coordinated to the exchangeable cations. Unusual shifts in the C-H and N-H absorption bands were observed, indicating a unique orientation of these groups, which probably reflects their keying into the hexagonal cavities of the tetrahedral sheet of the aluminosilicate layer