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Nano-octahedra of cobalt ferrite Co x Fe3 − xO4 (1 ≤ x

Nanocubes of cobalt ferrite with stoichiometric composition, 20 nm in size, were obtained by a solvothermal route, with acetylacetonates precursors and benzyl alcohol as a solvent, in presence of oleylamine and a mixture oleic acid/oleylamine. The cobalt ferrites were characterized by X-ray diffraction and transmission electron microscopy coupled with energy dispersive spectroscopy. Synthesis parameters such as temperature, duration of the thermal treatment, concentration of precursors, were optimized.

1st International Conference on Advances in Functional Materials (AFM), Stony Brook Univ, Stony Brook, NY, JUN 29-JUL 03, 2015; International audience; Tungsten trioxide WO3 powder have been synthesized at room temperature with aqueous mineralization processes, with two distinct morphologies: nanoplates and pseudospheres. The aim of this work is tohighlight the influence of the nanoparticles morphology on the carbon monoxide CO catalytic gaseous transformation and photocatalytic decomposition of Rhodamine in aqueous medium. The samples were characterized by X-ray diffraction, scanning and transmission electron. The nanoplates of WO3 show significantly higher photocatalytic activity than the pseudospheres, due to the large surface area. The CO oxidation was performed under air and the conversion reaction is not complete leaving residual CO and without shape effect. (C) 2016 Elsevier Ltd. All rights reserved.

International audience; 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.

SrBi 2 Ta 2 O 5 films have been deposited by a spin-coating method on an oxide electrode, Sr 2 RuO 4 , chosen to minimise the fatigue phenomenon and for the possibilities of epitaxy between the substrate and the film. X-Ray Diffraction, Scanning Electron Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy, with both medium and high resolution, were used to study the morphology, structure and defects of the obtained films. The films are polycrystalline with grains having an average size of 50 nm. Images and diffraction patterns of SrBi 2 Ta 2 O 9 grains on Sr 2 RuO 4 show chiefly four orientations, namely (110), (100), (001) and (105), which illustrate possibilities of epitaxy between the film and the substrate.

Sr-doped PZT ceramic, with a composition in the morphotropic phase boundary, was synthesised by solid state reaction. In a first step, from room temperature X-ray powder diffraction data, the structures of the tetragonal (space group P4mm, Z=1, at=4.040(1) A and ct=4.123(1) A) and rhombohedral (space group R3m, Z=3, ahex=5.755(4) A and chex=7.089(1) A) phases have been refined using a Rietveld-type method. Chemical occupancies, cation displacements and spontaneous polarisation were derived for the two coexisting phases. Moreover, to investigate the ferroelectric domains reorientation under a static electrical field, the ceramic was poled and then analysed by X-ray powder diffraction. The effect of the electrical field on the macroscopic polarisation of the ceramic was studied using the ratios of selected Bragg reflections intensities. An electrical field of 1 kV mm−1 necessary to trigger the domains reorientation in the two phases was determined and the percentage of domains switching has been estimated from simple models. On the other hand, to appreciate the degree of orientation, diffraction patterns were simulated using a preferred orientation function. Finally, the depoling process of the ceramic has been studied from successive thermal treatments.

Temperature-dependent neutron powder diffraction experiments (diffractometer 3T2-LLB, Saclay, France, λ = 1.227 Å) have been performed on the perovskite-like lead hafnate titanate PbHf0.4Ti0.6O3. This compound belongs to the solid solution denoted PHT, which derives from the well known ferroelectric PZT series. It exhibits a ferroelectric-to-paraelectric phase transition around 620 K, between the low-temperature tetragonal phase and the high-temperature cubic phase. The tetragonal structure of the ferroelectric phase has been refined at 10 and 300 K using a Rietveld-type method: space group P4mm with Z = 1; at = 3.999 (1), ct = 4.120 (1) Å, c/a = 1.030, V = 65.89 Å3 at 10 K; at = 4.012 (1) and ct = 4.100 (1) Å, c/a = 1.022, V = 65.99 Å3 at 300 K. The cubic structure of the paraelectric phase has also been refined at 720 K: space group Pm3¯m, Z = 1, ac = 4.046 (1) Å, V = 66.23 Å3. Cation displacements and oxygen-octahedra elongations have been observed as a function of temperature. Evidence for peculiar behaviour associated with the relative shifts of the Hf and Ti atoms (thought until now to be on the same crystallographic site) was found through an anomaly of the mean-square atomic displacements of the Hf/Ti pseudo-nucleus. The PDF Nos for PbHf0.4Ti0.6O3 are 48-49-9 and 48-49-10.

This paper reviews some possibilities for predicting the possible structures and solid state transitions for some oxides. It is well known that some systems (metal, oxygen) exist in many different stable phases while some others present only a few phases and, finally, others have only one possible oxide. The vanadium–oxygen system is a very good example of the first possibility (more than 15 phases V x O y are to be found in the literature) while the iron–oxygen system belongs to the second category; finally, only one stable oxide, NiO, is to be found in the nickel–oxygen system. Predicting the stable (or metastable) structures, may help us to understand the transitions between different polymorphs of one composition or, also, the relations between the different oxides as reduction occurs. We will start by a review of some recent results on two polymorphs of vanadium dioxide VO 2 where the experimental evidence is well explained, using the Static Concentration Waves (SCW) theory. We will then, as an other example, employ this theory to deduce the known structures of iron oxides (FeO, Fe 3 O 4 and Fe 2 O 3 polymorphs α, β, γ).

International audience; The electric and magnetic properties as well as the thermal stability of CoxFe3-xO4 nanopowders, (0.6

Thermodynamical calculations on the tetramethylsilane (TMS) pyrolysis were performed under two assumptions: (1) gaseous equilibria; (2) gaseous and SiC solid-vapour equilibria. From the results obtained by these calculations, the evolution of the main species under different experimental conditions was shown. For every component, a comparison of the curves, obtained under the two assumptions, permits us to give possible chemical reactions producing the SiC layers.

Thermodynamical calculations on tetramethylsilane (TMS) pyrolysis were performed in order to study the evolution of ethylene and acetylene partial pressures as a function of the experimental conditions. The different preferential orientations of the deposited SiC films are correlated with the variation of the partial pressure of these byproducts.

Sic polycrystalline layers were grown by LPCVD in a hot wall reactor using tetramethylsilane (TMS) diluted in hydrogen as precursor. The morphology and the structure of the films were analyzed in terms of deposition temperature, total pressure in the reactor and TMS flow rate. The layers have been characterized using various techniques : SEM, X-ray diffraction and TEM (HREM and EELS).

Ab initio studies have theoretically predicted room-temperature ferromagnetism in crystalline ${\text{SnO}}_{2}$, ${\text{ZrO}}_{2}$, and ${\text{TiO}}_{2}$ doped with nonmagnetic element from the 1A column as K and Na. Our purpose is to address experimentally the possibility of magnetism in both ${\text{Sn}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{O}}_{2}$ and ${\text{Sn}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{O}}_{2}$ compounds. The samples have been prepared using equilibrium methods of standard solid-state route. Our study has shown that both ${\text{Sn}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{O}}_{2}$ and ${\text{Sn}}_{1\ensuremath{-}x}{\text{K}}_{x}{\text{O}}_{2}$ structure is thermodynamically unstable and leads to a phase separation, as shown by x-ray diffraction and detailed microstructural analyses with high-resolution transmission electron microscopy. In particular, the crystalline ${\text{SnO}}_{2}$ grains are surrounded by K-based phase. In contrast to Ca:${\text{SnO}}_{2}$ samples, we have obtained a magnetic phase in K:${\text{SnO}}_{2}$ ones but no long-range ferromagnetic order. The K:${\text{SnO}}_{2}$ samples exhibit a moment on the order of $0.2\text{ }{\ensuremath{\mu}}_{\text{B}}/\text{K}$ /ion, in contrast to ab initio calculations which predict $3\text{ }{\ensuremath{\mu}}_{\text{B}}$, where K atoms are on the Sn crystallographic site. The apparent contradictions between our experiments and first-principles studies are discussed.

It has been reported that using oxide bottom electrodes reduces the fatigue phenomenon in ferroelectric random access memories (FeRAM). For that aim, strontium ruthenate SrRuO3 was already used as bottom oxide electrode for ferroelectric thin films [1], but compounds with less ruthenium are more interesting from the economical and environmental point of view. We elaborate Sr4Ru2O9 thin films and bilayers Bi3.25La0.75Ti3O12/Sr4Ru2O9 by pulsed laser deposition. Sr4Ru2O9 compound differs from the other strontium ruthenates. It has a hexagonal perovskite structure, contains pentavalent ruthenium atoms, with octahedral sharing faces, thus building dimmers Ru2O9 [2]. All the other strontium ruthenates belong to the Ruddlesden Poppers homologous series, Srn+1RunO3n+1 (Sr2RuO4 corresponds to n=1 and SrRuO3 to n=∞). The common features of these compounds are an orthorhombic structure, tetravalent ruthenium atoms, and an octahedral surrounding by oxygen of ruthenium atoms.

Nanomaterials based on spinel ferrites have already numerous applications, mainly based on their magnetic properties. Recently, catalytic properties of nickel, copper and cobalt ferrites in the conversion of CO and CH4 were evidenced, opening a new field of applications for these materials [1]. Magnetic properties of nanoparticles, as well as catalytic properties, will depend on their size, but also on their shape and size distribution. These parameters are linked to the elaboration method. The properties of transition metal spinel oxides depend also on the nature of the transition metal and on his site occupation in the structure. The location of the cations in the spinel structure is related to their octahedral or tetrahedral sites preference, but also to the synthesis method. For magnetite Fe3O4, which adopts the inverse spinel structure, the tetrahedral sites are fully occupied by Fe3+ ions, whereas octahedral sites are occupied by Fe3+ and Fe2+. For Co3O4, the octahedral and tetrahedral sites are respectively occupied by Co3+and Co2+ (normal spinel structure). Intermediate situations occur for CoxFe3−xO4,. Thus, three different compositions were prepared CoFe2O4, Co0.6 Fe2.4O4, Co1.4 Fe1.6O4.

Sr 4 Ru 2 O 9 thin films have been grown by pulsed laser deposition (PLD) on Si [1 0 0] substrates, using a Sr 2 RuO 4 target and high oxygen pressures. The growth conditions for a single-phased film were a substrate temperature of 700 °C, and an oxygen pressure of 300 mTorr. An oxygen pressure of 50 mTorr and a substrate temperature of 700 °C lead to the occurrence of nanograins of SrRuO 3 at the surface of the Sr 4 Ru 2 O 9 grains. The Sr 4 Ru 2 O 9 growth is columnar, with a strong tendency to (0 0 l) texture. Planar defects parallel to the ( h 0 0) and (0 k 0) planes are presented. The occurrence of these defects can be explained by an ordering of ruthenium atoms in the Sr 4 Ru 2 O 9 structure.

Si3N4-SiC whisker composites were fabricated using several routes (i.e., pressure filtration or CIP) followed by HP or HIP. The fracture strength ranges from 650 MPa to 750 MPa on account of the whiskers orientation. Compared to the Si3N4 matrix, the toughness is increased. A strong R-curve effect can be obtained, suggesting that, to be efficient, the whisker diameter must exceed a critical size. 13 refs.

The difficult scaling of ferroelectric random access memories with the complementary metal-oxide semiconductor technology roadmap requires integration of three-dimensional (3D) ferroelectric capacitors (FeCAP’s). In this work the unusual electrical behavior of 3D FeCAP sidewalls was studied by comparing the electrical properties of two-dimensional and 3D integrated FeCAP structures. We evidenced composition variations of the SrBi2Ta2O9 (SBT) film in the sidewalls with marked bismuth segregation during metal-organic chemical-vapor deposition (MOCVD) of the SBT film. The segregation was reduced after decreasing the deposition temperature from 440°C, whereby the Bi-rich phase in the sidewalls does not contribute to polarization, down to 405°C, whereby sidewall SBT contributes to polarization. After further optimization of the MOCVD conditions at 405°C, the segregation is minimized and the ferroelectric contribution of the sidewall SBT is almost the same as the contribution of the planar SBT. As a result, 3D ...

As compared to usual planar capacitors, three-dimensional (3D) ferroelectric capacitors display a large polarization increase due to the additional electrical contribution of the capacitor sidewalls. However, in 3D capacitors the polarization increase with respect to the planar geometry is lower than expected. To uncover the origin of this discrepancy, the microstructure of 3D Sr0.8Bi2.2Ta2O9-based (SBT) capacitors was studied. First, high-resolution synchrotron x-ray diffraction has found a unit cell expansion of the SBT phase in the capacitor sidewalls. From local chemical analyses, this lattice distortion has been correlated to composition variations in this region. These microstructural changes are due to the peculiar topography of the etched bottom electrode and to the variety of layers in contact with the SBT film, which can explain the nonoptimized polarization in 3D capacitors.

International audience; Sr2RuO4 is a copper-free layered perovskite superconductor with the tetragonal K2NiF4-type structure. The precise nature of the pairing in the superconducting state of this material is still under debate. In this paper, we report about crystal growth and characterization of this compound. The crystals were grown by a floating zone technique using a light furnace equipped with double elliptical mirrors starting from off-stoichiometric Sr2RuO4. The crystals have been checked by X-ray diffraction and microanalysis. The superconducting properties were measured by AC-susceptibility, magnetization and specific heat. High crystalline quality centimetre-sized crystals have been grown and best crystals exhibit superconducting transitions at View the MathML source (typical dimensions : 4.5 mm diameter and 70 mm length with the (0 0 1) axis perpendicular to growth direction). The unconventional superconductors are characterized by a drastic impurity effect on the superconducting properties. We try to establish a correlation between the defect concentration and the physical properties of Sr2RuO4.

The up-conversion of infrared radiation into green, red, and 805-nm fluorescence has been studied for ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions in both ${\mathrm{Yb}}^{3+}$-doped and non-${\mathrm{Yb}}^{3+}$-doped ${\mathrm{BaF}}_{2}$-${\mathrm{ThF}}_{4}$ fluoride glass over a wide temperature range and several dopant concentrations. It has been found that the addition of ${\mathrm{Tm}}^{3+}$ preferentially quenches the up-conversion efficiency of the green emission corresponding to a transition from the $^{4}\mathrm{S}_{3/2}$ level of ${\mathrm{Er}}^{3+}$. The decrease in emission amounts to a factor of about 50 for a concentration of 0.5 mol % ${\mathrm{TmF}}_{3}$ at 300 K. The quenching effect is discussed in terms of energy transfer between ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions. A rate equation model and a Judd-Ofelt analysis used in conjunction with the appropriate optical data lead to an understanding of the up-conversion process. The absolute values of the energy transfer rates and the electron populations in the lower excited states of the excited ${\mathrm{Er}}^{3+}$ and ${\mathrm{Tm}}^{3+}$ ions can be determined. This information should enable researchers to tailor materials to improve the efficiency of the up-conversion and laser devices.