Philippe Deniard

Electrochemical Cyclability of Orthorhombic LiMnO2 Characterization of Cycled Materials

The electrochemical removal of lithium from orthorhombic LiMnO 2 (o-LiMnO 2 ) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li 2 MnO 2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for 0.3 μm crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO 2 .

Potential of the INEL X-ray position-sensitive detector: a general study of the Debye–Scherrer setting

The INEL diffractometer, equipped with a CPS120 curved detector and set up in a Debye-Scherrer geometry, is a unique tool for carrying out powder diffraction studies on air-sensitive and/or small-volume samples. Although it has routinely been used in powder diffractometry because of its minute acquisition times, its accuracy in d-spacing and intensity measurements has not been clearly demonstrated before now. Concerning the d spacings, proper linearization of the CPS120 with a cubic Na2Ca3Al2F14 standard allowed a mean δ2θ difference of 0.006°. Intensity accuracy was measured with different highly and poorly absorbing samples. The accuracy is fairly good for the latter but poor for the former, except when special procedures such as the dilution of the sample with boron powder are used. A Rietveld calculation carried out on TI4V207 showed a very good agreement between the INEL Debye-Scherrer-geometry results and those obtained with a Philips diffractometer and Bragg-Brentano geometry.

Electrochemical cyclability of orthorhombic LiMnO{sub 2}: Characterization of cycled materials

The electrochemical removal of lithium from orthorhombic LiMnO 2 (o-LiMnO 2 ) leads to a phase transition with a first plateau at about 3.7 V. This corresponded to the formation of a spinel-like material; a possible transition to a rhombohedral Li 2 MnO 2 phase was ruled out through structural and crystal-site energy considerations. Several electrochemical cycles were necessary to achieve a complete phase transformation; the smaller the crystallites/crystals the fewer the number of cycles needed. The capacity difference between large and small crystallite/crystal compounds was ascribed to kinetic reasons as shown by ex situ x-ray diffraction analyses and quasi-equilibrium electrochemical studies. Capacities as high as 200 Ah/kg were found for 0.3 μm crystal size materials. Contrary to the spinel prepared at high temperature, the electrochemically obtained spinel-like phase cycled very well in the 2.5 to 4.3 V range, suggesting structural differences between the two materials. An extended x-ray absorption fine structure study at the manganese K edge confirmed this observation through a marked difference between the manganese second neighbors for two compounds. This can be related to the orthorhombic-to-cubic phase transition itself and/or to the memory effect of the stacking faults originally present in o-LiMnO 2 .

Tuning the size and color of the p-type wide band gap delafossite semiconductor CuGaO2 with ethylene glycol assisted hydrothermal synthesis

Delafossite CuGaO2 is a prototype p-type transparent semiconducting oxide. We report here a new ethylene glycol (EG) mediated hydrothermal synthesis route to this material from soluble hydrated metal nitrates. We found that EG acts as a reducing agent which eases the formation of CuGaO2 by stabilizing Cu(I) at low temperature in hydrothermal conditions. Moreover, we observed that the initial pH of the precursors solution is a key parameter to tune the particle sizes (from 2 µm to 300 nm) and subsequently the color (from dark brown to light gray) of the CuGaO2 powder. The optimal synthesis conditions to obtain the brightest product are described. In these conditions, the CuGaO2 powder exhibits a pale grayish color and contains 300 nm large nano-layered particles approximately 20 nm thick.

Determination of the crystallized fractions of a largely amorphous multiphase material by the Rietveld method

The Rietveld method has proved to be a very effective means to characterize and quantify the crystalline phases and the amorphous phase in glass ceramic materials using X-ray powder diffraction data. The technique was applied to a borosilicate glass of the type used for high-level nuclear-waste containment, in order to measure the proportions of the crystallized phases after heat treatment and, thus, to qualify the thermal stability of the glass. Six crystalline phases were analysed in this way in an almost entirely (>95 wt%) amorphous material after adding a known proportion of an internal standard (TiO2). The quantitative analyses were corrected to allow for microabsorption effects resulting from grain-size and absorption-contrast effects. In addition to the quantitative data, unit-cell parameters and site-occupancy refinements revealed solid-solution and substitution phenomena in the crystal.

Electrochemical behavior of orthorhombic LiMnO2: influence of the grain size and cationic disorder

Abstract Stoichiometric orthorhombic O-LiMnO 2 , prepared at high temperature from a mixture of Mn 2 O 3 and LiOH.H 2 O, is electrochemically active in a lithium deintercalation/intercalation process. The samples with the smallest grains ( O μ m), and corresponding to the phases prepared with some lithium hydroxide deficiency developed the higher weight capacities. These could reach up to 160 mAh/g under a C/15 regime following a forming that spanned over about thirty cycles. The cells capacity remained constant after forming, showing the very good cyclability of the system. It has not been clearly demonstrated whether the Li/Mn substitution in the O-LiMnO 2 samples played an important role in the electrochemical characteristics of the Li/Li x MnO 2 cells, although the group with small grains also showed, on average, a higher cationic disorder. A phase transition took place upon the first oxidation of O-LiMnO 2 . The new phase presented an electrochemical behavior resembling that of spinel LiMn 2 O 4 . A modeling of the discharge curves showed a progressive forming of the materials during a few cycles.

Photo- and electrochromic properties of covalently connected symmetrical and unsymmetrical spiropyran-polyoxometalate dyads.

Polyoxometalates covalently linked to one or two spiropyran entities have been isolated. These organic-inorganic hybrids exhibit multi-electrochromic and photochromic properties.

Smart Heterostructures for Tailoring the Optical Properties of Photochromic Hybrid Organic–Inorganic Polyoxometalates

A new concept of photoresponsive composites has been elaborated by intimately connecting a Photochromic Phase (PP), (H(2)DABCO)(2)(HDMA)(0.5)Na(0.75)(H(3)O)(0.75)[Mo(8)O(27)]·3H(2)O (1), with a second hybrid organic-inorganic molybdate material, (H(2)DABCO)(HDABCO)[Fe(OH)(6)Mo(6)O(18)]·4H(2)O (2) acting as an Oxidation Catalytic Phase (OCP) toward the former once photoexcited. The association of both the PP and the OCP in the composite drastically improves the bleaching process of the PP alone because of efficient electronic transfers through the OCP-PP interface without affecting significantly its photoinduced color change characteristic. Two OCP-PP composites with different PP weight percents have been obtained by associating 1 with 2. The optical properties of these composites before and after UV irradiation have been investigated by Diffuse Reflectance Spectroscopy, and the strong impact of the OCP on the fading kinetics of the PP has been clearly highlighted.

Properties of the transition metal dichalcogenides: The case of IrS2 and IrSe2

The syntheses and structure determination of IrSe{sub 2} and IrS{sub 2} were undertaken. Pure phases were obtained and powder data were measured on an X-ray diffractometer. Rietveld refinement was successfully carried out for both structures (IrSe{sub 2}, 269 observations R (weighted profile) 0.062; IrS{sub 2}, 205 observations R (weighted profile) 0.074). The Pnam space group (No. 62) was found to yield the best results, eliminating Pna2{sub 1} as the other possible space group considered in a previous study of IrSe{sub 2}. Chalcogen pairs are found in both structures and these may correspond, in a first approach, to the charge balance Ir{sup 3+}X{sup 2{minus}}(X{sub 2}){sup 2{minus}}{sub 1/2}. However, long bond lengths were found in the chalcogen pairs (S-S 2.299(11) {angstrom}, Se-Se 2.555(4) {angstrom}) along with an unexpectedly small effective cationic radius of 0.50 {angstrom} for Ir{sup 3+}. These features of IrS{sub 2} and IrSe{sub 2} are discussed.

Structural and spectroscopic characterization of a series of potassium- and/or sodium-substituted β-tricalcium phosphate

In this paper, we report X-ray diffraction investigations as well as Raman and solid-state (31)P and (23)Na magic angle spinning nuclear magnetic resonance (NMR) characterization of three series of calcium orthophosphates. The general formulae of the studied compounds are Ca(10.5-x/2)M(x)(PO(4))(7), where M=K or Na and x=0, 0.25, 0.50, 0.75, 1.0; and Ca(10)K(x)Na(1-x)(PO(4))(7), where x=0, 0.25, 0.5, 0.75, 1.0. These calcium orthophosphates are found to be isostructural with β-tricalcium phosphate (β-TCP, Ca(3)(PO(4))(2)) with the substitution of some calcium sites by potassium and/or sodium cations. The unit cell parameters vary continuously with the level of substitution, a characteristic of these solid solutions. The Raman spectra show the different vibrational bands of the phosphate groups PO(4), while the NMR chemical shifts are sensitive to the non-equivalent phosphorus and sodium ions present in these substituted samples. As both Raman and NMR spectroscopies are local probes, they offer tools to distinguish between these different phosphorus and phosphate groups, according to their structural site and local environment, especially the type of cation substituent. A convenient decomposition of the Raman and NMR spectra into Gaussian-Lorentzian components leads us to propose an assignment of the main observed bands of these substituted β-TCPs.