Search

International audience; The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure, and dynamics, 7Li and 23Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. 7Li MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. 23Na MAS NMR showed that some Na+ ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li0.42Na0.05]Na0.32CoO2 formula for the material. In addition, 7Li and 23Na MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for 7Li up to 450 K, the 23Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.

The OP4-(Li/Na)

International audience; NaMoO2 was synthesized as a layered oxide from the reaction between the layered oxide Na2/3MoO2 and metal sodium. Its structure was determined from high-resolution powder X-ray diffraction, and it can be described as an α-NaFeO2 distorted structure in which sodium ions and molybdenum atoms occupy octahedral interstitial sites. Chains of “diamond-like” clusters of molybdenum were evidenced in the [MoO2] layers resulting from the Peierls distortion expected in a two-dimensional triangular lattice formed by transition metal atoms with a d3 electronic configuration. Molybdenum–molybdenum distances as short as 2.58 Å were found in these clusters. The magnetic moment recorded at low temperatures and at room temperature showed that NaMoO2 presents a very low magnetic susceptibility compatible with the localization of the 4d electrons in the Mo–Mo bonds. This localization was confirmed by DFT calculation that showed the NaMoO2 was diamagnetic at 0 K. A sodium battery was built using NaMoO2 as the positive electrode material, and we found that sodium ions can be reversibly deintercalated and intercalated in NaMoO2, indicating that this compound is one of the many phases existing in the NaxMoO2 system.

NaMoO

International audience; The phase diagram of the layered NaxMoO2 system for low sodium contents (x ≤ 0.5) was explored using electrochemical (de)intercalation in a sodium ion battery from the Na2/3MoO2 composition which is the only one obtained by solid state reaction at high temperature. This study was realized using electrochemistry combined with in situ X-ray diffraction for sodium contents x below 0.5. For the first time, four single phase domains (x = 0.5, 0.4 ≤ x ≤ 0.42, x = 0.37, and x = 0.33) were evidenced during the cycling of the NaxMoO2 based battery in the 0.25 ≤ x ≤ 0.5 composition domain. All structural phase transitions are fully reversible. The existence of a superstructure was observed for the Na0.5MoO2 and Na0.33MoO2 compositions, attributed to the regular arrangements of both the sodium and molybdenum cations.

International audience; Sodium electrochemical deintercalation in a battery was used as a route to synthesize new vanadium oxides. Sodium layered oxides were used as positive electrode materials in batteries and sodium was electrochemically deintercalated up to a precise composition resulting in the discovery of a new phase Na1/2VO2. As for many vanadium oxides, this compound presents a magnetic transition, slightly above room temperature, around 325 K, as well as a structural transition involving displacements of vanadium ions. The discovery of this new vanadium oxide and the study of its unique electronic properties have only been possible because the battery permits the synthesis of this new phase at room temperature. This shows that many more new systems could be explored using electrochemical (de)intercalation in a battery and new materials that would be impossible to synthesize by classical solid state reaction could be obtained.

International audience; This paper gives an overview of the research carried out on lithium and sodium layered materials as positive electrodes of lithium (sodium)‐ion batteries. It focuses on the solid‐state chemistry contribution to discover new materials and to optimize the properties versus the requirements imposed by the applications. Among, all material structures, which are considered, the layered ones (lithium based), are the best candidates for high energy density batteries for mobile applications. Recently, the homologous Na materials, which have lower energy, are considered for stationary applications due to their low price. Starting for LiMO2 materials or NaxMO2 (0.5 < x < 1), many substituted phases, obtained by high‐temperature solid‐state chemistry, have allowed stabilizing the layered structure in large composition domains to increase the specific capacity, which is directly related to the number of exchanged electrons during the cycling process.

Sodium transition metal layered oxides are a class of materials which exhibits fascinating properties, such as high thermoelectric power. Whereas most of the work conducted so far focused on 3d transition metals, mainly cobalt, compounds with 4d metals could be excellent materials to obtain new strongly correlated electron systems. This work is focused on Na

International audience; Sodium transition metal layered oxides are a class of materials which exhibits fascinating properties, such as high thermoelectric power. Whereas most of the work conducted so far focused on 3d transition metals, mainly cobalt, compounds with 4d metals could be excellent materials to obtain new strongly correlated electron systems. This work is focused on NaxRhO2 compounds, with O3- and P2-type structures. The P2-type structure was obtained by ion exchange from the potassium phase P2-K0.62RhO2. This type of synthesis was conducted here for the first time on layered oxides with 4d transition metals. The phase diagram of both structures was explored by sodium electrochemical deintercalation/intercalation in a battery. The existence of single phases was shown with presumably different physical properties. As an example, the O′3-Na1/2RhO2 compound electrochemically obtained for the first time exhibits a metallic behavior, whereas the O3-NaRhO2 phase is a semiconductor. The synthesis of each single phase existing in both the O3- and P2-type systems should lead to new insights into the structure-properties relationships of this class of materials.

International audience; Phase diagrams and structural transformations in the complex NaxVO2 system have been studied using electrochemical (de)intercalation and in situ and operando high resolution synchrotron powder diffraction. Starting from O′3-Na1/2VO2 obtained by sodium electrochemical deintercalation of O3-NaVO2, the structural details of irreversible and reversible thermally driven transformations to P′3 and P3 type structures are presented. Subsequently, these P′3-NaxVO2 phases provide a platform for operando studies exploring the NaxVO2 phase diagram as a function of sodium electrochemical (de)intercalation. In this system, three single phase domains have been found: a line phase P′3-Na1/2VO2, one solid solution for 0.53 ≤ x ≤ 0.55 characterized by an incommensurate modulated structure, and a second solid solution for 0.63 ≤ x ≤ 0.65 with a defective structure resulting from a random stack of O′3 and P′3 layers. With further sodium intercalation (x > 0.65), the structure irreversibly transforms to the starting parent phase O3-NaVO2. This work reveals new details about the diverse structural polymorphs found in sodium layered oxides used as electrode battery materials and the transitional pathways between them as a function of temperature and composition.

International audience; The comparative structural, Mössbauer, and electrochemical studies of the NaxMn2/3Fe1/3O2 and NaxMn1/2Fe1/2O2 systems show that the change in the Mn/Fe ratio has a significant influence on the overlap between the Mn3+/4+ and Fe3+/4+ redox couples. The P2-type structure is maintained in the 0.3 < x < 0.8 domain. For the highest intercalation amount, structural distortions occur due to the Jahn–Teller effect of the Mn3+ ions. The macroscopic distortion results from a competition between the opposite effects of Mn3+ and Fe3+: the isotropic character of Fe3+ tends to prevent the macroscopic distortion. For the lower sodium amounts, the instability of the interstitial trigonal prismatic space leads to the formation, by slab gliding, of a very disordered structure. Even if this structural transition is reversible, a strong capacity fading is observed if the cell is charged above 4 V verus Na/Na+.

The development of very large scale renewable energy systems requires optimizing the lifetime, the price and the material availability. From these points of view, sodium based batteries have to be investigated. Our research group studied layered oxides as positive electrode for 30 years. Now a special focus is devoted on P2-type layered phases. One of the main interests of this structure is the existence of an ion conduction plane made of face sharing trigonal prims which exhibits a high ionic diffusivity thanks to the existence of a large bottleneck (oxygen rectangle) for sodium diffusion. This structure is able to accommodate a lot of transition metal cations, allowing the optimization of the properties by cationic substitution. Studies was performed on the P2- Nax(Mn,Co)O2, Nax(Mn,Fe)O2, Nax(Mn,Fe,Co)O2 and Nax(Mn,Fe,Ni)O2 systems with various amounts of transition metal ions. All materials crystallize in the hexagonal system (P63/mmc space group). The structure is made of MO2 slabs built of corner sharing MO6 octahedra. The sodium ions are in trigonal prismatic sites. Half of them share edges (Nae) with the MO6 octahedra while the other one share faces (Naf). As the two types of prisms share a common face, they cannot be occupied simultaneously at the atomic scale. The sodium distribution depends of the cationic charge distribution and of the sodium amount is order to minimize the Na+-Na+ and Na+-Mn+ electrostatic repulsions. This lead to a large number of ordered Na+ distribution as it was shown in the case of the NaxCoO2 systems. In materials with several transition metal cations, their statically distribution in the slabs prevent generally from Na+ ordering. In all systems, the P2 type packing is preserved in the 0.3 < x < 1 range. The fully intercalated phase is difficult to be obtained due to the very low ionic conductivity when the amount of vacancies is very small. For these compositions, only the Naf prisms are occupied. The specific capacity is in the 130-170 mAh.g-1 depending on the voltage range. For large deintercalation amounts (x < 0.3) a large polarization occurs in discharge. The GITT experiment shows that this “polarization” is not kinetics. The in-situ XRD during battery cycling from the Na0.7(Mn0.5Fe0.3Ni0.2)O2 phase shows the existence at high voltage of two phases with different interslab distances. The change in the shape of the diffraction line profiles is characteristic of the continuous formation of stacking faults. For manganese based materials, structural distortions (Jahn-Teller effect) occurs at the end of discharge. Even if these distortions are fully reversible, they have a very bad effect on the cycling behavior. In the intermediate composition range the reversibility is very good. Therefore, these materials can be interesting for practical applications in stationary batteries due to their low price and the availability of the raw materials.

International audience; Sodium intercalation in P2-Na2/3Co2/3Mn1/3O2 (obtained by a coprecipitation method) was investigated by ex situ and in situ X-ray absorption spectroscopy. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and the Mn edges, suggesting that the redox pairs, that is, Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction.

International audience; Layered sodium transition metal oxides represent a complex class of materials that exhibit a variety of properties, for example, superconductivity, and can feature in a range of applications, for example, batteries. Understanding the structure–function relationship is key to developing better materials. In this context, the phase diagram of the NaxMoO2 system has been studied using electrochemistry combined with in situ synchrotron X-ray diffraction experiments. The many steps observed in the electrochemical curve of Na2/3MoO2 during cycling in a sodium battery suggest numerous reversible structural transitions during sodium (de)intercalation between Na0.5MoO2 and Na∼1MoO2. In situ X-ray diffraction confirmed the complexity of the phase diagram within this domain, 13 single phase domains with minute changes in sodium contents. Almost all display superstructure or modulation peaks in their X-ray diffraction patterns suggesting the existence of many NaxMoO2 specific phases that are believed to be characterized by sodium/vacancy ordering as well as Mo–Mo bonds and subsequent Mo–O distances patterning in the structures. Moreover, a room temperature triclinic distortion was evidenced in the composition range 0.58 ≤ x < 0.75, for the first time in a sodium layered oxide system. Monoclinic and triclinic subcell parameters were refined for every NaxMoO2 phase identified. Reversible [MoO2] slab glidings occur during the sodium (de)intercalation. This level of structural detail provides unprecedented insight on the phases present and their evolution, which may allow each phase to be isolated and examined in more detail.

International audience; Using auto-combustion synthesis route followed by a 1000°C heat-treatment, we succeeded to prepare four Nax(Mn,Fe)O2 phases: P2-Na0.67Mn2/3Fe1/3O2, P2-Na0.71Mn1/2Fe1/2O2, and new Na-deficient O3-Na0.82Mn1/3Fe2/3O2 and O3-Na∼0.8Mn1/2Fe1/2O2 phases. We studied their structures by X-ray Diffraction and their electrochemical properties as positive electrode in Na-cells that were reversibly charged and discharged in the 1.5 and 3.8 V range vs. Na+/Na, leading to discharge capacities between 135 and 155 mAh.g−1. The shapes of the cycling curves are discussed together with the stacking of the phases and the redox processes involved in the Na intercalation/deintercalation reaction. In some cases the Fe(IV) state is clearly reached.

The stress-optic response of glass refers to the birefringence induced by application of anisotropic stress. We showed in an earlier publication [M. Guignard, L. Albrecht, J.W. Zwanziger, Chem. Mater. 19 (2007) 286] that this effect may be predicted based on the combination of bond metallicity and coordination number in the glass, and from the combination of these effects a variety of zero stress-optic glasses can be generated. In the present work we study in detail the relation of the structure and the bond metallicity to the stress-optic response in three glasses, two of which show zero and negative stress-optic response while the other shows only positive response. In particular we show by solid-state nuclear magnetic resonance spectroscopy that barium and lead phosphate glasses are much more similar in structure than tin phosphate glass, but that the lead and tin glasses have similar stress-optic response. The reason is the similar bond metallicity, which we explore by first-principles calculations, and the low metal ion coordination numbers that the lead and tin glasses exhibit.

By comparing data on a variety of examples, an empirical correlation for the photoelastic response of simple metal oxides is discovered and used to predict new families of zero-stress optic glasses. The birefringence induced by uniaxial stress on glass is found to correlate well with the ratio of the metal oxygen bond metallicity to the metal coordination number; the metallicity itself is quantified through the metal oxygen bond length. This correlation was obtained by consideration of the stress optic response of a number of oxide crystals, obtained both from the literature when possible and also from first principles calculations. The correlation obtained provides a simple rule for choosing the composition of oxide glass so as to minimize the stress optic response; this rule is shown to agree with known data on lead oxide glasses and to predict the existence of previously unknown lead-free, zero-stress optic glasses. These glasses were then synthesized, tested, and shown to give the predicted response.

This work presents a laser desorption study of Ga 5 Ge 20 Sb 10 S 65 chalcogenide glasses. The desorption products have been analyzed by UV multi-photon ionization and time-of-flight mass spectrometry. Desorption mechanisms and plume dynamics have been investigated by studying the desorption products velocity distribution profiles in the frame of a Knudsen layer model. A peculiar behavior (broadening of the mass spectral lines) of the Ga component was found at high desorption–ionization delays and a tentative explanation has been proposed, based on the very different thermal properties of the sample constituents.

We investigate the linear and nonlinear coefficient of chalcogenide glasses containing different amount of tellurium (Ge 10 As 10 Se 80− x Te x , where x = 0, 10, 15, 20). We give, for the first time to our knowledge, the values of the linear indices as well as the nonlinear refraction coefficient near the band gap of these materials. We give also, in the transparency region, the Cauchy coefficients for all the compositions (near infrared domain). An estimation of the linear absorption is also made using a ‘cut-back’ method. Nonlinear characterization of these materials is performed using the Z -scan technique at 1064 nm and 15 ps pulsewidth. A good agreement is found when we compare some of the results with those obtained using other techniques (at the same incident intensity). We show that the nonlinear refraction coefficients measured here are among the largest values reported on chalcogenide glasses. Furthermore, we found that the addition of tellurium does not enhance significantly the nonlinear refraction coefficient but exalts more the nonlinear absorption.

We show that highly nonlinear two-photon absorption coefficients can be obtained in infrared glasses (tellurium based chalcogenide glasses) at 1.06 lm and in the picosecond range (twice that of As2Se3). We observe an intensitydependant two-photon absorption coefficient. This dependence is attributed to higher order effects occurring in nonlinear absorption process in our materials. The corresponding nonlinear coefficients are deduced from experimental data using a closed-form relation relating the effective two-photon absorption coefficient to the incident intensity. These coefficients become larger as the photon energy increases relative to the band gap. � 2003 Elsevier B.V. All rights reserved.

A simple experimental method is used to obtain the evolution of both the refractive index and the linear absorption coefficient as a function of the optical wavelength in the near infrared range (from 900 up to 1700 nm with 10 nm resolution). Several chalcogenide glasses (As2S3, As2Se3, GeSe4) are tested and the corresponding Cauchy coefficients are determined. Comparison of our results shows a good agreement with values available in the literature at some wavelength. Application of this method is used to estimate Cauchy coefficients of Ge10As10Se80 for the first time to our best knowledge.

International audience; We have investigated the microscopic magnetic nature of a novel Na battery material, P2-Na0.5VO2, in which V ions form a two-dimensional triangular lattice, by means of muon-spin rotation and relaxation (μ+SR) measurements down to 50 mK. Although magnetization measurements indicated the presence of an antiferromagnetic transition at 13 K, the internal magnetic field due to the formation of magnetic order appears not at 13 K but at 2 K. Furthermore, the magnetic order is found to have a wide field distribution even at 50 mK. Such wide field distribution is reasonably explained by a combination of multiple muon sites and the formation of a long-range magnetic order, while the reliable spin structure is still unknown.

International audience; The electrochemical properties of the O3-type NaxMn1/3Fe2/3O2 (x = 0.77) phase used as positive electrode material in Na batteries were investigated in the 1.5–3.8 V, 1.5–4.0 V and 1.5–4.3 V ranges. We show that cycling the Na cells in a wider voltage range do not induce a significant gain on long term cycling as the discharge capacities reached for the three experiments are identical after the 14th cycle. The structural changes the material undergoes from 1.5 V (fully intercalated state) to 4.3 V were investigated by operando in situ X-ray powder diffraction (XRPD) and were further characterized by ex situ synchrotron XRPD. We show that the low amount of Mn3+ ions (≈33% of total Mn+ ions) is enough to induce a cooperative Jahn–Teller effect for all MO6 octahedra in the fully intercalated state. Upon deintercalation the material exhibits several structural transitions: O′3 → O3 → P3. Furthermore, several residual phases are observed during the experiment. In particular, a small part of the O3 type is not transformed to P3 but is always involved in the electrochemical process. To explain this behaviour the hypothesis of an inhomogeneity, which is not detected by XRD, is suggested. All phases converge into a poorly crystallized phase for x ≈ 0.15. The short interslab distance of the resulting phase strongly suggests an octahedral environment for the Na+ ions. X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy were used to confirm the activity of the Mn4+/Mn3+ and Fe4+/Fe3+ redox couples in the low and high voltage regions, respectively. 57Fe Mössbauer spectroscopy also showed an increase of the disorder into the material upon deintercalation.

Zero stress-optic glasses are achieved traditionally through a high content of lead oxide or other closely related p-block metal oxide. We have found that the underlying cause for this behavior is the combination of high metallicity and low coordination numbers adopted by compounds such as lead oxide. Here we test this idea by showing that barium tellurite glasses also show zero stress-optic and negative stress-optic response, at very low barium content. This response results from the fact that barium oxide bonds have very high metallicity, and at the same time barium modifies tellurite by lowering the Te coordination number. The two effects together are sufficient to produce zero stress optic response.

International audience; Sodium layered oxide, Na0.70Mn0.60Ni0.30Co0.10O2, with a P2-type structure was synthesized by a co-precipitation method and characterized as positive electrode of sodium batteries. The P2-NaxMn0.60Ni0.30Co0.10O2 electrode delivered a high specific capacity of 185 mAh/g in the 1.5–4.3 V range at 1st cycle at C/20 rate; however, rapid capacity decay is observed. In the 1.7–4.0 V range the capacity is equal to 125 mAh/g with a fading considerably reduced. The rate capability is reasonably good. According to the electrochemical profile, a potential plateau around 4.2 V vs. Na+/Na, meaning a bi-phasic domain, and several voltage drops that could be related to single phase domains in a narrow sodium content range are observed. Even if the capacity decreases when the battery is cycled above 4.0 V, there is no irreversible structural change.

International audience; The electrochemical properties of the P2-type NaxMn1/2Fe1/2O2 (x = 0.62) phase used as a positive electrode in Na batteries were tested in various voltage ranges at C/20. We show that, even if the highest capacity is obtained for the first cycles between 1.5 and 4.3 V, the best capacity after 50 cycles is obtained while cycling between 1.5 and 4.0 V (120 mAh g(-1)). The structural changes occurring in the material during the (de)intercalation were studied by operando in situ X-ray powder diffraction (XRPD) and ex situ synchrotron XRPD. We show that a phase with an orthorhombic P'2-type structure is formed for x ≈ 1, due to the cooperative Jahn-Teller effect of the Mn(3+) ions. P2 structure type stacking is observed for 0.35 < x < 0.82, while above 4.0 V, a new phase appears. A full indexation of the XRPD pattern of this latter phase was not possible because of the broadening of the diffraction peaks. However, a much shorter interslab distance was found that may imply a gliding of the MO2 slab occurring at high voltage. Raman spectroscopy was used as a local probe and showed that in this new phase the MO2 layers are maintained, but the phase exhibits a strong degree of disorder.

International audience; The new layered phase P2-Na1/2VO2 has been synthesized by sodium electrochemical deintercalation. Its structure has been studied by high resolution powder diffraction, pair distribution function analysis, and nuclear magnetic resonance spectroscopy between 300 and 350 K. An increase of 2 orders of magnitude in its electronic conductivity has been observed at approximately 322 K, and a structural transition has been found to occur simultaneously. The arrangement of sodium ordering in P2-Na1/2VO2, which maximizes sodium-sodium distances to lower electrostatic repulsions between alkali ions, is found to be unchanged across this transition. At room temperature, high resolution powder diffraction and pair distribution function analysis reveal the triangular lattice formed by vanadium ions to be distorted by the formation of pseudotrimers clusters with vanadium-vanadium distances as short as 2.581 Å. Above the transition, the pseudotrimers disappear and the triangular vanadium lattice becomes more regular with a mean vanadium-vanadium distance of 2.88 Å. At 350 K, the increase in P2-Na1/2VO2 electronic conductivity is due to enhanced charge transport resulting from the declustering of vanadium ions. These results highlight how sodium ordering between the MO2 layers and the electronic transport within the MO2 layers are intimately correlated in NaxMO2-type sodium-layered oxides.

International audience; Layered oxides are the subject of intense studies either for their properties as electrode materials for high-energy batteries or for their original physical properties due to the strong electronic correlations resulting from their unique structure. Here we present the detailed phase diagram of the layered P2-NaxVO2 system determined from electrochemical intercalation/deintercalation in sodium batteries and in situ X-ray diffraction experiments. It shows that four main single-phase domains exist within the 0.5≤x≤0.9 range. During the sodium deintercalation (intercalation), they differ from one another in the sodium/vacancy ordering between the VO2 slabs, which leads to commensurable or incommensurable superstructures. The electrochemical curve reveals that three peculiar compositions exhibit special structures for x = 1/2, 5/8 and 2/3. The detailed structural characterization of the P2-Na1/2VO2 phase shows that the Na+ ions are perfectly ordered to minimize Na+/Na+ electrostatic repulsions. Within the VO2 layers, the vanadium ions form pseudo-trimers with very short V-V distances (two at 2.581 Å and one at 2.687 Å). This original distribution leads to a peculiar magnetic behaviour with a low magnetic susceptibility and an unexpected low Curie constant. This phase also presents a first-order structural transition above room temperature accompanied by magnetic and electronic transitions. This work opens up a new research domain in the field of strongly electron-correlated materials. From the electrochemical point of view this system may be at the origin of an entire material family optimized by cationic substitutions.

International audience; The electrochemical cycling in a sodium battery of the lamellar oxide NaVO(2) is reversible in the Na(x)VO(2) composition range 1/2 ≤ x ≤ 1. The complex electrochemical curve reveals the presence of several transitions taking place during deintercalation. With the help of in situ X-ray diffraction, we observed the structural transitions taking place between Na(2/3)VO(2) and Na(1/2)VO(2). The diffractograms show the presence of several monophasic domains separated by biphasic domains. All phases present a monoclinic distortion of the α-NaFeO(2) structure in the composition range 1/2 ≤ x ≤ 2/3. Moreover the presence of a superstructure is evidenced for Na(1/2)VO(2). It is the first time that an ordered structure is reported at the Na(1/2)MO(2) composition with an O'3 oxygen stacking. A thorough investigation of electrochemically obtained O'3-Na(1/2)VO(2) was performed. The structure refinement reveals the existence of a sodium/vacancy ordering, with a peculiar arrangement of the V-V distances hinting at a pairing of vanadium atoms. Our first measurements of the physical properties of O'3-Na(1/2)VO(2) show a semiconductor behavior and a complex thermal dependence of the magnetic susceptibility related to the pairing of the vanadium atoms.

International audience; In this contribution, new data on the reversible Li+ intercalation in iron substituted sodium manganates are provided. Novel Na2/3Mn1−yFeyO2 (y = 0, 1/3 and 2/3) compounds with a P′3-type structure are prepared from freeze-dried citrate precursors at 500 °C. A new structural element is the development of three-layer oxygen stacking contrary to the well-known P2-type Na2/3MnO2 with a two-layer sequence. The effect of Fe additives on the structure of Na2/3MnO2 was examined by XRD powder diffraction and TEM analysis. The oxidation state and the distribution of transition metal ions in Na2/3Mn1−yFeyO2 were analysed using electron paramagnetic resonance spectroscopy. The lithium intercalation in Na2/3Mn1−yFeyO2 was investigated in two-electrode lithium cells of the type Li|LiPF6 (EC:DMC)|Na2/3Mn1−yFeyO2. The stability of the layered phases during lithium intercalation was studied by ex situ Raman spectroscopy. It was found that the intermediate Na2/3Mn2/3Fe1/3O2 composition is able to intercalate Li+ reversibly in high amounts. Details of the structure and its stability during the Li+ intercalation are discussed.

International audience; Manganese substituted sodium cobaltate, Na(2/3)Co(2/3)Mn(1/3)O(2), with a layered hexagonal structure (P2-type) was obtained by a co-precipitation method followed by a heat treatment at 950 °C. Powder X-ray diffraction analysis revealed that the phase is pure in the absence of long-range ordering of Co and Mn ions in the slab or Na(+) and vacancy in the interslab space. The oxidation states of the transition metal ions were studied by magnetic susceptibility measurements, electron paramagnetic resonance (ESR) and (23)Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The charge compensation is achieved by the stabilization of low-spin Co(3+) and Mn(4+) ions. The capability of Na(2/3)Co(2/3)Mn(1/3)O(2) to intercalate and deintercalate Na(+) reversibly was tested in electrochemical sodium cells. It appears that the P2 structure is maintained during cycling, the cell parameter evolution versus the sodium amount is given. From the features of the cycling curve the formation of an ordered phase for the Na(0.5)Co(2/3)Mn(1/3)O(2) composition is expected.

International audience; We report a X-ray absorption study at Ti K- and L2,3-edges to determine the role of TiO2 content as a nucleating agent in glass ceramics. It is found that the local Ti environment is not modified with TiO2 addition, indicating that medium range organization is responsible for its ability to promote internal versus bulk nucleation. We have identified a Ti coordination change between the nucleation front and the crystallized part of the glass ceramic. These changes correspond to conversion from 5-fold coordinated to 4-fold coordinated Ti in the remaining glassy part, resulting from compositional changes. This reveals that active sites for nucleation could be experimentally detected and that reorganization of the glass matrix during nucleation has a major influence on the ongoing nucleation processes.

International audience; This paper describes the preparation of NaxVO2 phases by means of electrochemical deintercalation. The curves V = f(x) show that the process is reversible up to x = 0.5 and they indicate the presence of multiple single phase domains in the range 1 < x < 0.5. Two single phases have been successfully isolated for Na1/2VO2 and Na2/3VO2 compositions; we report here the first X-ray powder diffraction measurements of these phases. They present a O'3-type monoclinic distortion of the O3 rhombohedral phase observed for Na1VO2. The evolution of cell parameters with sodium deintercalation is comparable with other AxMO2 layered oxides.

This work presents recent studies, concerning non linear optical properties of chalcogenide glasses. Second and third order properties are presented, on bulk glasses and in fibers. Chalcogenide glasses are based on sulfur, selenium, tellurium and the addition of other elements such as arsenic germanium, antimony gallium, etc. The interest of chalcogenide glasses is to associate high non linear properties with their infrared transmission from 0.5 - 1 /spl mu/m to 12 - 18 /spl mu/m depending on the composition.

International audience; Ordering of disordered materials occurs during the activated process of nucleation that requires the formation of critical clusters that have to surmount a thermodynamic barrier. The characterization of these clusters is experimentally challenging but mandatory to improve nucleation theory. In this paper, the nucleation of a magnesium alumino-silicate glass containing the nucleating oxide TiO 2 is investigated using neutron scattering with Ti isotopic substitution and 27 Al NMR. We identified the structural changes induced by the formation of crystals around Ti atoms and show important structural reorganization of the glassy matrix.

article i nfo Nucleation is the initiating event for the formation of any crystal from undercooled melt. Although the classical nucleation theory describes qualitatively many aspects of the crystallization process, the mechanisms favoring the first steps of nucleation are still unknown and required the full identification of the catalytic sites. In this work, we report a structural investigation of the system 2MgO-2Al2O3-5SiO2+ TiO2, a major glass-ceramic for scientific and industrial interest, in order to understand the structural influence of Ti as a nucleating agent. X-ray scattering and 27 Al Nuclear Magnetic Resonance studies were carried out on 2MgO-2Al2O3-5SiO2+xTiO2 glasses, with 0 mol% ≤x ≤15.5 mol%. No evidence of phase separation can be detected in the initial glasses by Transmission Electron Microscopy at nanoscale and Ti atoms appear to be homogeneously distributed within the glassy structure. We explain that the presence of TiO2 favors nucleation by the formation in the initial glass of high-coordinated Al species and the presence of structural fluctuations that mimic the initial crystalline phase precipitating in the glass. The understanding of the parent glass structure appears as a critical constraint to understand the pathways promoting nucleation.

International audience; High second-order susceptibility has been created in a chalcogenide glass from Ge-Sb-S system. A thermal poling process was used to produce this non-linear effect and a second harmonic generation experiment allowed characterizing the phenomenon. A maximum χ(2) value of 8.0±0.5 pm/V was measured for the first time to our best knowledge in sulfide glasses.

Second harmonic generation (SHG) has been obtained in a sample of Ga5Ge20Sb10S65 glass submitted to a thermal poling treatment. An original characterization method is used for the determination of the induced second-order nonlinear profile. A reproducible chi(2) susceptibility of 4.4 +/- 0.4 pm/Volt was achieved for specific poling conditions.

International audience; The structure of the glass 2MgO–2Al2 O3 –5SiO2 –TiO2 was investigated using neutron diffraction with isotopic substitution. Reverse Monte Carlo (RMC) modeling was used to reproduce experimental structure factors derived from diffraction experiments. The local environment of titanium atoms was determined and it corresponds to an average of 5.4 ± 0.2 oxygen atoms at a mean distance of 1.86 ± 0.02 ̊A. This coordination number agrees with the predominance of fivefold coordination, with the coexistence of four- and sixfold coordination in similar amounts. 27 Al nuclear magnetic resonance (NMR) results revealed that the proportion of highly coordinated aluminum atoms in this titanium-bearing glass was higher than in the titanium-free sample. RMC modeling was used to interpret the structural role of these [5] Al species and we show a trend for preferential bonding between [5] Al and Ti atoms. This favored linkage is important to understand the role of titanium dioxide as a nucleating agent in inorganic glass-ceramics fabrication.

International audience; New class of chalcohalide glasses has been prepared in the GeS2-In2S3-CsI system with regard to their potential non-linear properties. The study of glass-forming region was undertaken to select glassy compositions, which present high non-linear (NL) optical properties with a low two-photon absorption. Thermal analyses, structural examination by Raman spectroscopy, non-linear optical measurements were investigated as a function of CsI contents. Introduction of CsI has shifted the band-gap edge towards the blue region of the absorption optical spectrum and therefore has limited the two-photon absorption. Their NL refractive index n2 are 60 times higher than silica glasses without any NL absorption. Moreover, second harmonic signal was observed in thermally poled samples similar to silica glass. However, this second order non-linearity is not temporally stable.

International audience; The structure of six glass compositions in theMgO–Al2O3–SiO2 systemwas investigated using neutron and X-ray diffraction. Reverse Monte Carlo modeling was used to reproduce experimental structure factors derived from diffraction experiments. The environment of Mg was determined and corresponds to an average of 5.1±0.1 oxygens at a mean distance of 2.00±0.02 Å with little change as the compositions vary. Tetrahedral, pentahedral and octahedral polyhedra are found with a majority of MgO5. The Al environment reveals the presence of highcoordinated Al in agreement with 27Al Nuclear Magnetic Resonance results. The Reverse Monte Carlo models were used to interpret the structural role of Mg and Al. The [5]Al species showa trend to be localized in the same regions, which have important implications to quantify the Si/Al ordering in aluminosilicate glasses and melts

International audience; High second-order susceptibilities are created by thermal poling in bulk germanium disulfide based chalcogenide glasses. Experimental conditions of the poling treatment (temperature, voltage, time) were optimized for each glass composition. The second-order nonlinear signals were recorded by using the Maker fringes experiment and a second-order coefficient χ(2) up to 8 pm V-1 was measured in the Ge25Sb10S65 glass. This value is obtained using a simulation based on accurate knowledge of the thickness of the nonlinear layer. Two mechanisms are proposed to explain the creation of a nonlinear layer under the anode: the formation and the migration of charged defects towards the anode may mainly occur in Ge20Ga5Sb10S65 and Ge25Ga5S70 glasses, whereas the migration of Na+ ions towards the cathode may be responsible for the accumulation of negative charges under the anode in Ge33S67 and Ge25Sb10S65 glasses. Different electronic conductivity behaviors seem to be at the origin of the phenomenon. In parallel, the potential effect of the poling treatment on the structural and electronic properties is studied using Raman spectroscopy and secondary ion mass spectroscopy measurements.

International audience; The non-linear optical response of sulphide glass-containing β-GeS2 microcrystallites, has been studied at a fundamental wavelength of λ=5.5 μm under additional irradiation by IR-pulses of λ=3.7 μm. A substantial increase of the non-linear susceptibility with IR-power and size of the microcrystallites has been found. Molecular dynamics calculations linked to quantum chemical calculations show that this effect could be explained by substantial contributions of the microcrystallite interfaces and due to the phonon sub-system. The role of anharmonic electron-phonon effects is experimentally confirmed by the observation of an optimal delay of 8 ps between the pump laser pulses at 3.7 μm and the probing laser pulses at 5.5 μm, which generate the observed second harmonic signal.

International audience; Permanent second-harmonic generation (SHG) has been demonstrated in chalcogenide based glass-ceramics containing non-linear micro-crystals with a size of a few micrometers. A glass composition from the Ge-Sb-S system was chosen as the reference glass for its stability against crystallization and atmospheric corrosion. Metallic cadmium was introduced in this matrix to promote crystallite formation resulting in infrared transparent glass-ceramics. A volume crystallization of β-GeS2 phase was obtained within the glass media by heating the glass samples at 370 °C for different durations. The glass-ceramics were then investigated by Raman spectroscopy, X-ray diffraction and NanoSIMS. The second-order non-linear signals were recorded by using Maker fringes experiment and were studied as a function of the crystallized volume fraction. The results indicated a non-linearity in chalcogenide glass-ceramics about one hundred times lower than α-quartz for a 1 mm thick sample heat treated 144 h.

International audience; The aim of this work is to investigate and qualify vitreous regions in new chalcogenide systems containing highly polarizable elements like S, Se, As, Sb, Bi, Pb and I. The sulfur based system, As2S3-Sb2S3-Bi2S3-PbI2, as well as the selenium containing glasses As2Se3-Sb2Se3-PbI2 have been studied. Large vitreous regions have been defined in the both systems: sulfur and selenium. Indeed, several glass compositions can accept in the vitreous network a molar concentration near 50% of PbI2. Physical properties such as glass transition temperature, crystallization temperature, optical transmission, band-gap wavelength have been measured versus glassy compositions.

Second harmonic generation was obtained with an interesting efficiency in thermally poled sulfide glass. The best results obtained to date for chalcogenide glasses were on a Ge-Sb-S system thanks to an adapted treatment of thermal poling. The poling parameters like temperature (100-310 °C). applied voltage (2.5-4 kV) and duration (5-60min) were explored. A large NL second-order susceptibility X (2) of about 10 ± 0.5 pm/V was measured. The nonlinear susceptibility profile as a function of the depth under the anode for Ge 25 Sb 10 S 65 poled glass was determined using the analyze of remained second harmonic signal during the NaOH etching treatment. In parallel. a study of the concentration variation of elements being able to be involved in the formation of a charge space was achieved by using the secondary ion mass spectroscopy.

Charging-discharging phenomena of P2-Na2/3Co2/3Mn1/3O2 prepared by a co-precipitation method were investigated by ex situ and in situ X-ray absorption spectroscopy [1, 2]. The electronic transitions at the O K-edge and the charge compensation mechanism, during the sodium intercalation process, were elucidated by combining Density Function Theory (DFT) calculations and X-ray absorption spectroscopy (XAS) data. The pre-edge of the oxygen K-edge moves to higher energy while the integrated intensity dramatically decreases, indicating that the population of holes in O 2p states is reduced with increasing numbers of sodium ions. From the K-edge and L-edge observations, the oxidation states of pristine Co and Mn were determined to be +III and +IV, respectively. The absorption energy shifts to lower positions during the discharging process for both the Co and Mn edges, suggesting that the redox pairs, i.e. Co3+/Co2+ and Mn4+/Mn3+, are both involved in the reaction. [1] J.-H. Cheng, C.-J. Pan, J.-F. Lee, J.-M. Chen, M. Guignard, C. Delmas, D. Carlier, and B.-J. Hwang, Chem. Mater., 26 (2014 ) 1219–1225. [2] D. Carlier, J. H. Cheng, R. Berthelot, M. Guignard, M. Yoncheva, R. Stoyanova, B. J. Hwang and C. Delmas, Dalton Trans.,40 ( 2011), 9306.

Introduction Sodium–ion batteries are considered to be one of the favourite for the future hybrid and electric vehicles because of their low price and sodium abundant resources. Sodium layered oxides (NaxMO2) are the expectable candidates as cathode material in respect of potential and sodium–ion diffusion. Especially, O3 type is mainly focused, however, the O3 type structure can be transformed into the spinel type structure that leads to capacity decay in several cycles. From the structural stability point of view, the P2 type structure is able to avoid converting to the spinel type structure due to different oxygen stacking. We focus on synthesizing the new P2 material including three transition metals, such as Mn, Ni, and Co, in order to achieve higher capacity and cyclability. Experimental We adopted a co−precipitation method in order to make the aimed compound. A solution of transition metal nitrates, and another solution of sodium carbonate were dripped into a beaker with distilled water simultaneously. The washed precipitate was dried at 80 ̊C for two days. The obtained powder was mixed with sodium carbonate with an excess amount, and annealed at 900 ̊C in air atmosphere, and finally quenched to room temperature. The crystal structure was studied by XRD. The electrochemical performances were evaluated in two electrodes cell configuration. The positive electrode was a mixture containing 88 wt. % of the active material, 10 wt. % of graphite as the conductor and 2 wt. % of polytetrafluoroethylene (PTFE). The electrolyte was 1 M NaPF6 in propylene carbonate (PC) with fluoroethylene carbonate (FEC) at 2 wt. %. Galvanostatic performances were carried out between 4.0 V and 1.5 V vs. Na+/Na at the current rate of 0.05 C and 25 ̊C. Cycle performance was evaluated between 3.8 V and 1.5 V at same condition. Results Among all composition studied, a pure phase with the P2 structure was obtained for the Na0.7Mn0.6Ni0.3Co0.1O2 composition. Fig. 1 shows the XRD pattern of Na0.7Mn0.6Ni0.3Co0.1O2 phase. It indicates that the sample crystallizes in the hexagonal system (S.G. , P63/mmc) and there were no impurities in the material. The cell parameters are ahex. = 2.8847 Å, chex. = 11.0833 Å. The material has excellent electrochemical performance as shown in Fig. 2. At first, it was discharged until 1.5 V in order to insert sodium ions into the remaining vacancies. The amount of sodium at the end of the first discharge (x = 0.96) shows that almost all sites are occupied between MO2 slabs. In the curves, there were two large potential drops around x = 0.7 and x = 0.5 (close to x = 1/2) identified with the single phase domain. The three sloping curves parts indicate solid solution behaviour. The specific capacity calculated from the sodium content was 153 mAh/g, which was similar to that of the O3 phase [1]. A good cycle retention was observed during 10 cycle test as shown in Fig. 3. In the presentation, we will discuss the crystal structural changes occurring during the sodium extraction deintercalation / intercalation. [1] M. Sathiya, et al., Chem. Mater. 2012, 24, 1846−1853.