Simulation des diagrammes de diffraction par la méthode<br />combinée : application aux systèmes CaCO3

The tremendous development of powder diffraction techniques achieved over the lastten years even operational on strongly textured samples within good resolutions, together withthe fascinating development of individual calculating capabilities, make it possible to accessprecise information in reasonably short times. In this work, we apply the combined analysisbased on the Rietveld method to study samples of natural calcium carbonates from two livingspecies of molluscan shells and of synthetic CaCO3 in order to understand the influence of thedifferent parameters in the growth and shape of the crystallites of the different polymorphs ofCaCO3. In natural species we clearly put into evidence the existence of distortions ofcrystalline cells due to the presence of the intra- and extra-crystalline macromolecules, inopposition with similar studies operated on powderised samples. In synthetic samples westudied, using a simple synthesis method and taking as a model the polyacrylic acid, the roleof the soluble macromolecules in the growth of the CaCO3 polymorphs. The simulation of theanisotropic diffraction diagrams on both sample types allowed us to reach pertaininginformation within good precision (anisotropic sizes of the crystallites, structure, texture,volume fraction of the polymorphs).Le développement considérable réalisé ces dix dernières années dans les techniques dediffraction sur des échantillons sous forme de poudre même très texturés (tempsd'enregistrement raisonnable et très bonne résolution) et les progrès fascinants des moyens decalcul sur des machines individuelles rendent possible l'accès à des informations précises etavec des temps de calcul raisonnablement courts. Dans ce travail, nous avons appliquél'analyse combinée basée sur la méthode de Rietveld pour étudier des échantillons decarbonates de calcium naturels de coquilles de certaines espèces de mollusques (structure,texture, tailles anisotropes...etc.) et synthétiques par biomimétisme dans le but de comprendrel'influence des différents facteurs intervenants dans la croissance et la forme des grains desdifférents polymorphes de CaCO3. Dans les échantillons naturels, nous avons clairement misen évidence l'existence de déformations de mailles cristallines dues à la présence desmacromolécules intra-cristallines et extra-cristallines, en complément d'études du mêmegenre réalisées sur des échantillons broyés. Dans les échantillons synthétiques obtenus par uneméthode de synthèse simple, nous avons analysé l'effet de l'ajout d'acide polyacrylique, entermes de modulateur de croissance des polymorphes de CaCO3. La simulation desdigrammes de diffraction anisotropes réalisée sur ces deux types d'échantillons nous a permisd'accéder aux informations pertinentes de façon précise (tailles anisotropes des cristallites,structure, texture, proportion volumique des polymorphes)

Simulation des diagrammes de diffraction par la méthode<br />combinée : application aux systèmes CaCO3

The tremendous development of powder diffraction techniques achieved over the lastten years even operational on strongly textured samples within good resolutions, together withthe fascinating development of individual calculating capabilities, make it possible to accessprecise information in reasonably short times. In this work, we apply the combined analysisbased on the Rietveld method to study samples of natural calcium carbonates from two livingspecies of molluscan shells and of synthetic CaCO3 in order to understand the influence of thedifferent parameters in the growth and shape of the crystallites of the different polymorphs ofCaCO3. In natural species we clearly put into evidence the existence of distortions ofcrystalline cells due to the presence of the intra- and extra-crystalline macromolecules, inopposition with similar studies operated on powderised samples. In synthetic samples westudied, using a simple synthesis method and taking as a model the polyacrylic acid, the roleof the soluble macromolecules in the growth of the CaCO3 polymorphs. The simulation of theanisotropic diffraction diagrams on both sample types allowed us to reach pertaininginformation within good precision (anisotropic sizes of the crystallites, structure, texture,volume fraction of the polymorphs).Le développement considérable réalisé ces dix dernières années dans les techniques dediffraction sur des échantillons sous forme de poudre même très texturés (tempsd'enregistrement raisonnable et très bonne résolution) et les progrès fascinants des moyens decalcul sur des machines individuelles rendent possible l'accès à des informations précises etavec des temps de calcul raisonnablement courts. Dans ce travail, nous avons appliquél'analyse combinée basée sur la méthode de Rietveld pour étudier des échantillons decarbonates de calcium naturels de coquilles de certaines espèces de mollusques (structure,texture, tailles anisotropes...etc.) et synthétiques par biomimétisme dans le but de comprendrel'influence des différents facteurs intervenants dans la croissance et la forme des grains desdifférents polymorphes de CaCO3. Dans les échantillons naturels, nous avons clairement misen évidence l'existence de déformations de mailles cristallines dues à la présence desmacromolécules intra-cristallines et extra-cristallines, en complément d'études du mêmegenre réalisées sur des échantillons broyés. Dans les échantillons synthétiques obtenus par uneméthode de synthèse simple, nous avons analysé l'effet de l'ajout d'acide polyacrylique, entermes de modulateur de croissance des polymorphes de CaCO3. La simulation desdigrammes de diffraction anisotropes réalisée sur ces deux types d'échantillons nous a permisd'accéder aux informations pertinentes de façon précise (tailles anisotropes des cristallites,structure, texture, proportion volumique des polymorphes)

The effect of ergocalciferol on the precipitation of calcium carbonate

We have conducted the first study on the effect of ergocalciferol (vitamin D-2) on the polymorphism of calcium carbonate. CaCO3 particles are synthesised from aqueous solutions of sodium carbonate and calcium chloride, in the presence of vitamin D-2 with different concentrations, at temperatures ranging from 25 to 80 degrees C. The effect of the additive on the polymorphism and the morphology of the CaCO3 crystallites are characterised using X-ray quantitative phase analysis, scanning electron microcopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Our results show that the presence of vitamin D-2 has an impact on the morphology and composition of the CaCO3 crystallites. Vitamin D-2 was demonstrated to promote the formation of aragonite a 50 degrees C with a volume fraction that can reach 83%. Such effect can be attributed to the formation of D-2-Ca+2 ion solution complexes and their adsorption on the surfaces of the CaCO3 crystallites

The effect of ergocalciferol on the precipitation of calcium carbonate

International audienceWe have conducted the first study on the effect of ergocalciferol (vitamin D-2) on the polymorphism of calcium carbonate. CaCO3 particles are synthesised from aqueous solutions of sodium carbonate and calcium chloride, in the presence of vitamin D-2 with different concentrations, at temperatures ranging from 25 to 80 degrees C. The effect of the additive on the polymorphism and the morphology of the CaCO3 crystallites are characterised using X-ray quantitative phase analysis, scanning electron microcopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). Our results show that the presence of vitamin D-2 has an impact on the morphology and composition of the CaCO3 crystallites. Vitamin D-2 was demonstrated to promote the formation of aragonite a 50 degrees C with a volume fraction that can reach 83%. Such effect can be attributed to the formation of D-2-Ca+2 ion solution complexes and their adsorption on the surfaces of the CaCO3 crystallites

Stacking sequence variations in vaterite resolved by precession electron diffraction tomography using a unified superspace model

International audienceAs a metastable phase, vaterite is involved in the first step of crystallization of several carbonate-forming systems including the two stable polymorphs calcite and aragonite. Its complete structural determination would consequently shed important light to understand scaling formation and biomineralization processes. While vaterite’s hexagonal substructure (a0 ~ 4.1 Å and c0 ~ 8.5 Å) and the organization of the carbonate groups within a single layer is known, conflicting interpretations regarding the stacking sequence remain and preclude the complete understanding of the structure. To resolve the ambiguities, we performed precession electron diffraction tomography (PEDT) to collect single crystal data from 100 K to the ambient temperature. The structure was solved ab initio and described over all the temperature range using a unified modulated structure model in the superspace group C12/c1(α0γ)00 with a = a0 = 4.086(3) Å, b = 3−−√a0 = 7.089(9) Å, c = c0 = 8.439(9) Å, α = β = γ = 90° and q = 23a* + γc*. At 100 K the model presents a pure 4-layer stacking sequence with γ = 12 whereas at the ambient temperature, ordered stacking faults are introduced leading to γ < 12. The model was refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement

Stacking sequence variations in vaterite resolved by precession electron diffraction tomography using a unified superspace model

International audienceAs a metastable phase, vaterite is involved in the first step of crystallization of several carbonate-forming systems including the two stable polymorphs calcite and aragonite. Its complete structural determination would consequently shed important light to understand scaling formation and biomineralization processes. While vaterite’s hexagonal substructure (a0 ~ 4.1 Å and c0 ~ 8.5 Å) and the organization of the carbonate groups within a single layer is known, conflicting interpretations regarding the stacking sequence remain and preclude the complete understanding of the structure. To resolve the ambiguities, we performed precession electron diffraction tomography (PEDT) to collect single crystal data from 100 K to the ambient temperature. The structure was solved ab initio and described over all the temperature range using a unified modulated structure model in the superspace group C12/c1(α0γ)00 with a = a0 = 4.086(3) Å, b = 3−−√a0 = 7.089(9) Å, c = c0 = 8.439(9) Å, α = β = γ = 90° and q = 23a* + γc*. At 100 K the model presents a pure 4-layer stacking sequence with γ = 12 whereas at the ambient temperature, ordered stacking faults are introduced leading to γ < 12. The model was refined against PEDT data using the dynamical refinement procedure including modulation and twinning as well as against x-ray powder data by the Rietveld refinement

Structure and microstructure study of Charonia lampas lampas shell

International audienc

Radiation-Induced Synthesis and Superparamagnetic Properties of Ferrite Fe₃O₄ Nanoparticles

Ultra-small magnetic Fe3O4 nanoparticles are successfully synthesized in basic solutions by using the radiolytic method of the partial reduction in FeIII in the presence of poly-acrylate (PA), or by using the coprecipitation method of FeIII and FeII salts in the presence of PA. The optical, structural, and magnetic properties of the nanoparticles were examined using UV–Vis absorption spectroscopy, high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and SQUID magnetization measurements. The HRTEM and XRD analysis confirmed the formation of ultra-small magnetite nanoparticles in a spinel structure, with a smaller size for radiation-induced particles coated by PA (5.2 nm) than for coprecipitated PA-coated nanoparticles (11 nm). From magnetization measurements, it is shown that the nanoparticles are superparamagnetic at room temperature. The magnetization saturation value Ms = 50.1 A m2 kg−1 of radiation-induced nanoparticles at 60 kGy is higher than Ms = 18.2 A m2 kg−1 for coprecipitated nanoparticles. Both values are compared with nanoparticles coated with other stabilizers in the literature

Effects of sol concentration on structural, morphological and optical waveguiding properties of sol-gel ZnO nanostructured thin films

Nanostructured ZnO thin films with different precursor concentrations (0.5–0.8 M) have been deposited on glass substrates by sol-gel dip coating technique. X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), UV-visible spectrophotometer, and m-lines spectroscopy have been employed to investigate the effect of solution concentration on structural, morphological, optical and waveguiding properties of the ZnO thin films. XRD spectra have shown that all the films are polycrystalline and exhibit the wurtzite hexagonal structure. SEM micrographs and AFM images have revealed that morphology and surface roughness of the thin films depend on sol concentration. The UV-visible transmittance results show a high transparency in the visible range and a shift of the maximum transmittance to the higher wavelength with increasing sol concentration. Waveguiding properties such as refractive index, number of propagating modes and attenuation coefficient measured at 632.8 nm wavelength by m-lines spectroscopy indicate that our ZnO slab waveguides are single mode and demonstrate optical losses estimated around 1.5 decibel per cm (dB/cm) for the thin film prepared with a sol concentration of 0.7 M