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Fluorine-containing polymers occupy a peculiar niche among conventional polymers due to the unique combination of physicochemical properties. Direct surface fluorination of the polymeric materials is one of the approaches for the introduction of fluorine into the chemical structure that allows one to implement advantages of fluorinated polymers in a thin layer. Current review considers the influence of the surface interaction of the polymeric materials and membranes with elemental fluorine on gas, vapor and liquid transport as well as swelling and related phenomena. The increase in direct fluorination duration and concentration of fluorine in the fluorination mixture is shown to result mostly in a reduction of all penetrants permeability to a different extent, whereas selectivity of the selected gas pairs (He-H2, H2-CH4, He-CH4, CO2-CH4, O2-N2, etc.) increases. Separation parameters for the treated polymeric films approach Robeson’s upper bounds or overcome them. The most promising results were obtained for highly permeable polymer, polytrimethylsilylpropyne (PTMSP). The surface fluorination of rubbers in printing equipment leads to an improved chemical resistance of the materials towards organic solvents, moisturizing solutions and reduce diffusion of plasticizers, photosensitizers and other components of the polymeric blends. The direct fluorination technique can be also considered one of the approaches of fabrication of fuel cell membranes from non-fluorinated polymeric precursors that improves their methanol permeability, proton conductivity and oxidative stability.

International audience; Silicon-mediated fluoride abstraction is demonstrated as a means of generating the first fluorido-cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans-[M IV F 4 (CN) 2 ] 2À (M = Re, Os), obtained from their homoleptic [M IV F 6 ] 2À parents. As shown by combined high-field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans-[ReF 4 (CN) 2 ] 2À as compared to [ReF 6 ] 2À , reflecting the severe departure from an ideal octahedral (O h point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building-blocks for the design of high-performance molecule-based magnetic materials.

International audience; After a review on the wide variety of inorganic fluorinated components in modern technologies, in particular for energy conversion/storage systems, the use of fluorinated carbons as electrodes for primary lithium batteries will be highlighted; in particular conventional graphite fluorides will be compared to recently investigated fluorinated carbon nanoparticles (F-CNPs) prepared from electrochemical reduction of molten carbonates.

International audience; Fluorine–graphite intercalation compounds were prepared from highly oriented pyrolytic graphite (HOPG) and natural graphite under high pressure of fluorine (16 MPa) at room temperature and were analyzed by means of X-ray diffraction. The rate of intercalation under high pressure of fluorine was higher than that under ordinary pressure. The compositions of the compounds obtained from HOPG and natural graphite were estimated by the weight change before and after fluorination to be C3.41F and C5.62F, respectively. They were blue–black compounds composed of two different phases. One phase is a conventional stage II CxF with the repeat distance of ca. 0.93–0.96 nm which was the same compound as that obtained under ordinary pressure of fluorine, and the other was also a stage II with a different repeat distance of ca. 1.03–1.18 nm. The fluorine species intercalated between the graphene layers can easily move between both phases and the phase transition occurs periodically. The decomposition rate of fluorine-intercalated HOPG was extremely low and it shows a long-term stability in air for 26 years.

International audience; Heat capacity, Mössbauer and Raman spectra as well as magnetic properties of fluoride CsFe2F6 with defect pyrochlore structure were studied. In addition to recently found above room temperature three successive structural transformations Pnma-Imma-I41amd-Fd-3m, phase transition of antiferromagnetic nature with the 13.7 K Neel temperature and a broad heat capacity anomaly with a maximum at about 30 K were observed. The room temperature symmetry Pnma is unchanged at least down to 7 K. Simple model of indirect bond used to estimate the exchange interactions and to propose a magnetic structure model.

International audience; The physicochemical properties of amorphous silica prepared by the fluoride technology are studied; the surface of silica is modified by direct fluorination with F2 gas or cC4F8 radio frequency plasma. The modification of the surface physical chemical properties after fluorination are compared with the initial silica and analyzed by X-ray photoelectron spectroscopy, IR spectroscopy, and transmission electron microscopy.

International audience; The microwave-assisted route allows the synthesis of a new Ti hydroxyfluoride adopting a derived form of a ReO3-type network. Combined techniques such as (i) powder X-ray and neutron diffraction coupled with electron microscopy; (ii) chemical analysis and density measurements; and (iii) FTIR and TGA analyses support the occurrence of Ti vacancies and the stabilization of hydroxyl groups in the vicinity of Ti4+ cations. A superstructure of the ReO3 network has been proposed with two Ti sites and two anionic (OH/F) positions. This original compound exhibits UV-shielding properties with an optical band gap around 3.2 eV and could be considered for potential applications as protective UV absorbers.The O(2p)−Ti(3d) charge transfer band, responsible for this absorption, implies the OH groups and the nonbonding character of the 2p valence band. By changing the synthesis conditions which become reducing, Ti3+ can be stabilized in this network in an elongated octahedral site as revealed by ESR experiments and UV−visible spectroscopy. This is a second example following the recently prepared Al-based fluoride hydrate with Al vacancies, in which a polarizing cation such as Ti4+ can accommodate cationic vacancies.

Microwave-assisted synthesis has been applied for the preparation of aluminum hydroxyfluorides derived from β-AlF3. In the course of obtaining high-surface-area and homogeneous nanocrystallites, several key parameters have been adjusted. A relationship between the OH/F molar ratio and the stabilized allotropic forms has been pointed out. The surface area has been monitored depending on the water/isopropanol volume ratio. The microstructural properties of the as-prepared material have been thoroughly investigated highlighting X-ray line broadening arising from strains and size effects. It has been concluded from X-ray diffraction data refinements and TEM results that the prepared crystallites possess a platelet shape with an average particle size of 15 nm associated with a high surface area of 82 m2 g−1. The occurrence of hydroxyl groups inside the network has been detected by FTIR spectroscopy and considered angles and bond distances are probably located in peculiar crystallographic sites, i.e., 8f (F1) a...

A neutron diffraction study of temperature induced phase transition in Rb2KFeF6 elpasolite has been performed. At room temperature the structure is cubic with space group Fm3m. At T = 170 K a structural phase transition takes place. Space group and atom coordinates in this phase has been defined. The crystal structure is good refined by orthorhombic space group Pmnn. Possible models are discussed.

International audience; Cs2NH4GaF6 prepared by solid-state reaction exhibits at room temperature a cubic FmView the MathML sourcem elpasolite structure with a0=9.167 Å. Calorimetric measurements have shown the presence of a first order phase transition at T0=161 K. Pressure dependence of the phase transition temperature has been studied by DTA under pressure. The data are analyzed in the framework of orientational disordering of [NH4]+ and [GaF6]3− ions.

Biointegration is the ideal outcome which is expected for an artificial implant. That means that the phenomena which seats at the interface between the implant and the host tissues does not induce neither any deleterious effect, such as chronic inflammatory response, nor the formation of unusual tissues. Thus it is of paramount importance to design biomaterials, used for the fabrication of implants, with the best appropriate surface properties. At the same time these biomaterials must feature bulk properties which meet other requirements, especially mechanical properties, deriving from the intended function of the implant in which they are involved. As it is quite impossible to design biomaterials which fulfil at the same time both types of requirements, it is commonly agreed that the solution to this issue goes through the selection or the design of biomaterials with adequate bulk properties, and a further treatment of the surface which would improve the properties of the latter. In this respect ionizing radiations and plasma based treatments, offer a wide panel of possibilities; as an example we describe here how the surface of expanded poly(tetrafluoroethylene) samples can be activated using cold plasma, in order to open a way to chemical modifications of such a surface. Subsequently, Radio Frequency Glow Discharge (RFGD) containing oligopeptides, known for their role in mediating the adhesion of cells to the extracellular matrix, were bound to the modified surface, and the affinity of endothelial cells for the latter was investigated.

Many compounds with general chemical formula A 2 BB′X 6 and with tolerance factor value t

The effect of M3+ ion substitution on the physical properties of Rb2KM3+F6 (M3+ =Ga, Fe, Sc, In, Lu, Er, Ho, Tb) series with elpasolite structure was studied by DTA under hydrostatic pressure, calorimetric, X-ray, elastic and inelastic neutron scattering measurements. There exists a strong dependence of both type and succession of ferroelastic phase transitions on the M3+ ion size. Hydrostatic pressure effect was found partially inconsistent with chemical pressure. Possible models of phase transition mechanism were considered. It was supposed that anharmonic F atom vibrations could play a great role in phase transitions of the series.

Measurements of the heat capacity of the elpasolite Rb2KGaF6 and the solid solutions Rb2KGaxSc1−x F6 with a gallium content in the range x=0.6–0.95 are performed. The nature of the influence of the chemical composition of the compounds and the hydrostatic pressure on the temperature and entropy of the ferroelastic phase transitions is established.

This chapter deals with the description of the most important structural types of transition-element fluorides and oxide-fluorides, pointing out the relationships occurring between these networks. The peculiar nature of M–F bonds generally leads to outstanding physical behavior, such as ferri- and ferromagnetism, piezoconductivity, multiferroism, F-induced high-critical-temperature superconductivity, high ionic conductivity, and optical properties, and the knowledge of the precise structural characteristics of these phases appears to be essential for a better approach of these properties. Some new elaboration methods are described as far as they provide access to materials with specific features (nanosized particles, etc.) and significantly improved properties.

International audience; For the first time, structural phase transitions induced by the temperature were found in AxMxIIM(1−x)IIIF3 fluorides with the defect pyrochlore structure (FdView the MathML sourcem, Z=8). The room temperature structure of CsFe2F6 was determined using X-ray powder diffraction technique. This phase was found to be ordered with the Pnma space group. The study of the temperature stability of orthorhombic structure by differential scanning calorimeter between 100 K and 700 K has shown a succession of phase transitions. The Pnma (Z=4)→Imma (Z=4)→I41/amd (Z=4)→FdView the MathML sourcem (Z=8) structural sequence was proposed to occur within a rather narrow temperature range 500-560 K. The mechanism of structural transition has been mainly associated with the rotation of (MF6) octahedra and small displacements of some Fe atoms. This assumption is in good agreement with the low experimental entropy value, which is characteristic for displacive transformations.

The different types of CF interactions have been evidenced from XPS investigations on fluorinated carbonaceous materials. The respective amount of the C1s and F1s components depends on several factors, including the nature of the host material, the thermal treatments which increase the graphitization level and the fluorination conditions. TEM images and corresponding Fourier transforms of high temperature-treated pitch-based and PAN-based fibres have shown that only stage-1 compounds and graphitic-type domains are detectable in the experimental conditions. In particular, no interlayer spacing corresponding to higher stage materials has been noticed. The observed results can be correlated to the ratio of semi-ionic to covalent FC bonding determined by XPS.

An enhancement of the in-plane thermal conductivity in the superconducting state is observed in most high-T-c superconductors. Here the observation of a similar enhancement of the thermal conductivity in copper-oxide compounds, which is not related to the superconducting transition, is reported. Since this anomaly is also accompanied by a significant variation of the electrical conductivity it indicates that a strong electron-phonon coupling exists and that thermal-conductivity enhancements in copper-oxide compounds should be attributed to an increase of its lattice component.

The heat capacities of Rb2KHoF6 and Rb2KErF6 have been investigated over a wide temperature range 150-430 K including the temperatures at which the phase transitions Fm3m → P21/n occur. The pressure dependence of phase transition temperatures, studied up to 0.6 GPa was found to be nonlinear. Effect of B3+ ion size both on entropy of structural transformation and dT0dp is discussed for Rb2KB3+F6 series.

The X-ray photoelectron spectroscopy study of fluorine-intercalated carbon fibers has shown the formation of semi-ionic bonding between carbon and intercalated fluorine species, leading to an increase in the positive charge on the involved carbon atoms and a subsequent increase in C 1s binding energy. Covalent CF bonding is present at the external surface of the fibers. The relative amounts of these species are dependent on the intercalation rate and on the nature of the host fibers: polyacrylonitrile or pitch based, either as received or high temperature treated.

The radiofrequency plasma technique involving mixtures of CF4+O2 gases has been applied to the surface treatment of high Tc superconducting oxides (YBa2Cu3O7−δ). Investigation of the various experimental parameters of the process has shown that the improvement of the critical current density, Jc, mainly depends on the inlet precursor composition CF4+τ%O2, on the total pressure and on the reaction time. The presence of fluorine in the bulk of the ceramics has been observed from electron microprobe analysis, together with an increase of the “Cu3+” content. The plasma-enhanced fluorination (PEF) treatment improves the superconducting properties of the materials: both values of the resistivity in the normal state and of the superconducting transition width are reduced and the critical transition temperature is improved by about 1 K.

The peculiar reactivity of fluorine with graphite can be related to some characteristic parameters of this element. The competition between strong C-F bonds in covalent graphite fluorides and weak semi-ionic bonds in graphite intercalation compounds is pointed out. The different experimental routes for the intercalation of fluorine and metal fluorides into graphite are reviewed. The oxidation state, the environment of the metal and the nature of the bonding between the intercalated species and carbon can be approached by various techniques, including XANES, EXAFS, and XPS spectroscopies. The structural relationships between the graphene sublattice and the intercalate sublattice can be pointed out on the basis of single crystal data. Concerning the transport properties, acceptor-GICs have proved to be very appropriate to investigate the effects of disorder and reduced dimensionality. The charge transfer which accompanies the insertion of fluorinated species between the graphene layers can be evalu...

ISBN 978-0-444-72002-3; The surface characteristics of materials dominate the performance of the final products in many applications. For instance the mechanical behavior of composite materials are strongly dependent on the nature of the fiber/matrix interface, the lubricant properties are influenced by surface tribological effects, the success of bio-implants is mostly determined by biocompatibility, etc.

International audience; Treatments involving fluorinated gases and radio-frequency plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying at low temperature the surface properties of materials. These techniques are adapted to yield nanosized functionalized materials and will be illustrated by several examples, including carbon-based electrode materials, functionalized silica and silicates, switchable hydrophobic-hydrophilic porous alumina. X-ray photoelectron spectroscopy is a preferred method for characterizing the resulting surface modifications.

The influence of fluorination on ceramic samples of La1.85-xNdxSr0.15CuO4 solid solutions has been studied. The presence of small amounts of fluorine species in the interior of the grains which has been evidenced by Auger electron spectroscopy significantly influences the structural and transport properties. It has been shown that the intercalation of fluorine into the structure reinforces the metallic-like behaviour and superconducting properties by hindering the tendency to the low-temperature structural transition and by increasing the charge carrier density.

The influence of Ba substitution for La and of the fluorination on the thermal conductivity, the electrical resistivity, the thermoelectric power and the specific heat of La2-xBaxCuO4 has been investigated. In the La2-xBaxCuO4 system, either the electron-phonon interactions or phonon-defect interactions may explain the temperature dependence of the thermal conductivity. The intercalation of the fluorine atom into the lattice leads to a superconducting transition with an onset higher than 30 K for all compositions investigated. The effect of fluorine intercalation in these oxides is similar to an increase in barium concentration. Indeed the presence of fluorine leads to an increase of the free carriers concentration and of the lattice disorder. For the particular composition x congruent-to 0.125, the fluorination seems to suppress the low-temperature structural transition (LTO-->LTT) and to eliminate all the anomalies usually observed in the transport properties.

The structural and magnetic properties of Na 2 CoMF 7 compounds (M = Al, Cr, Fe), showing a new structure deriving from that of weberite have been investigated. The monoclinic structure, favored by cobalt (II) ions, is intermediate between the orthorhombic type (Na 2 NiFeF 7 ) and the trigonal type (Na 2 MnFeF 7 ). It exhibits (CoF 5 ) n 3n− chains which run along the [110] and [110] directions, forming a sequence of double layers [Co 2 MF 10 ] n 3n− . The influence of the trivalent cation within the layers and between them on the magnetic behavior is demonstrated.

The influence of fluorination on the thermal conductivity, the electrical resistivity and the thermoelectric power of YBa2Cu3O7-delta has been investigated. It was found that the presence of fluorine does not drastically modify the thermal conductivity of the YBa2Cu3O7-delta compounds. The sip of the thermoelectric power was found to be negative between 92 and approximately 200 K for the sample without fluorine and positive above 92 K for the YBa2Cu3O7-delta compound treated in fluorine atmosphere. Also, a small increase of the electrical resistivity was found after treatment with fluorine. These results are consistent with the fact that the fluorine treatment generates passivation layers around the ceramic grains without affecting the inner structure of the grains.

The influence of a fluorine-gas treatment on both structural and superconducting properties of the La 2− x Sr x CuO 4 (0≤ x ≤0.1 5) solid solution has been studied. This treatment leads to the occurence of superconductivity at 40 K and 36 K for x =0 and x =0.02, respectively. In contrast, only a small change of the critical temperature is observed for x =0.10 and x =0.15 after fluorination of the compounds. Such a behaviour is consistent with the increase of the hole concentration arising from the incorporation of anionic species into the lattice. The study of the F 2 -treated La 2 CuO 4 by neutron diffraction reveals that the occupancy rate of the anionic interstitial site O(4))( 1 4 , 1 4 , 0.2646) is n =0.089(6), corresponding to the formula La 2 Cu(O,F) 4, 18 for the resulting oxyfluoride. The analysis of the EXAFS spectra recorded at the La L III -edge shows an increment of the anionic species around lanthanum ions.

The superconductivity properties previously described for La 2 CuO 4 -derived materials under reactive atmosphere (F 2 or Cl 2 gas) and for fluorinated La 2− x Ba x CuO 4 (0 x ⩽0.15) have been monitored by electron spin resonance (ESR). The intensity of the zero magnetic field microwave absorption depends on the halogenation temperature and pressure. This absorption in characteristic of the superconducting volume fraction. In addition to the zero field resonance, the Cu 2+ ESR signal gives information on localized copper species belonging to non-superconducting phases or decomposition products obtained by halogenation at high temperature or pressure.

International audience; A study of amorphous silica generated from mineral materials by the fluoride method and by different procedures of analysis., i.e., x-ray diffractio, x-ray fluorescence, IR spectroscopy, and nuclear magnetic resonance, is reported...

International audience; Conventional lithographic printing processes using porous alumina for offset applications generally use “wet” routes. Recently “dry” processes have been developed which are based on a heat-induced hydrophilic/oleophilic conversion of one or more layers of the coating so that a stronger affinity to-wards ink or water fountain is created at the exposed areas with respect to the surface of the unex-posed coating. Treatments involving rf plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying the surface properties of materials. Many advantages of these techniques can be indeed outlined, when compared to more conventional methods: room-temperature reactions, chemical modi-fications limited to surface only without changing the bulk properties, possible non-equilibrium reac-tions. The influence of PEF treatments on porous alumina layer used in printing plates has been tested with various fluorinated gases (CF4, C3F8 and C4F8) and characterized by XPS. The hydrophobic prop-erties of the fluorinated layer have been deduced from contact angle measurements. Using C4F8 rf-PEF treatment, the outmost surface of the hydrophilic alumina substrate used for lithographic printing is hydrophobized, or in other words, the hydrophilic substrate is converted into a support with hydro-phobic properties. Once being hydrophobized, the surface layer may be rendered hydrophilic using a heat pulse, thus giving rise to switchable hydrophobic-hydrophilic properties of the material.

The physical properties of Rb2KScF6 crystal characterized by the medium size of B3+ atom in Rb2KB3+F6 series are studied. The two successive phase transitions from cubic into tetragonal and then into monoclinic phases are found. The results are discussed in view of the influence of B3+ atom size on structural transformations.

A superconducting transition is observed in La2CuO4 treated under chlorine pressure. The resulting compound obtained at 300°C and for Pcl2 ≈ 6 bar shows an X-ray diffraction pattern which can be indexed in an orthorhombic symmetry with the following unit-cell parameters: a = 5.354 A , b = 5.378 A and c = 13.190 A . The diamagnetic susceptibility of ZFC samples is about −1×10−3 emu g−1 at 5 K under 100 Oe applied magnetic field and the superconducting transition occurs at 32±1 K. Parasitic phases, present after chlorine treatment, are easily removed by a washing process which does not alter the critical temperature of the sample. When air-annealed above 150°C, the Cl2-treated compound is irreversibly transformed into stoichiometric La2CuO4 with a consequent loss of superconductivity. Similarities with high oxygen pressure treatments have been pointed out.

The influence of a flourine gas treatment on both structural and superconducting properties of the solid solution La1.85−xNdxSr0.15CuO4 has been studied. A single-phase domain of solid solution deriving from the K2NiF4-type structure is observed for 0⩽x⩽1.0 and superconducting compounds are evidenced only for x⩽0.4. After fluorination the materials become superconducting on the whole range of composition, i.e. for 0⩽x⩽1.0, in which the symmetry of the unit cell of the compounds goes from tetragonal to orthorhombic with rising x. The dependence of the critical temperature with x is practically linear from 40 K (x=0) down to 11 K (x=1.0) and is strongly correlated with the corresponding decrease of the unit cell volume. Both unit cell parameters and volume of the fluorinated compounds suggest an anionic intercalation in the structural network.

International audience; Aluminum fluoride hydrate was synthesized by a microwave hydrothermal process. The structure derives from the ReO3 type structure, that is, the high temperature structure of α-AlF3. The cubic symmetry adopted by this compound arises from water molecules, stabilized as ligand surrounding Al3+ cations which induces cationic vacancies as revealed by Rietveld refinement. The following chemical formula Al0.82◻0.18F2.46(H2O)0.54 is supported by chemical analysis and TGA measurements. This represents the first example of aluminum vacancy compound in the Al-based fluorides chemistry. High field 27Al NMR spectroscopy enabled identification and quantification of the following species: AlF6 and AlF6−x(H2O)x with x = 1, 2, 3, and showed that vacancies are mainly surrounded by water molecules but also by a low content of fluoride ions as also evidenced by 19F NMR. The hydrogen bonding network, which takes place in the vicinity of the cationic vacancies, was characterized by FTIR and 1H NMR spectroscopies. A 2:1 complex X···H−O−H···X where X is a proton acceptor, which is related to a shift of the νsym(OH) and νasym(OH), was detected. This complex appears to be stable up to 773 K. The phase transition into the rhombohedral form occurs at around 573 K, but at such a temperature a large amount of water molecules associated to cationic vacancies still remained thanks to the 2:1 complex. The acidic properties have been revealed by pyridine and CO probe molecules. At 573 K, the solid exhibits both strong Lewis and Brønsted acidities with an equivalent number of sites providing bifunctionality. The strong acidic behavior highlights the effect of water molecules/cationic vacancies on the surface structure. Whereas the Lewis strength acidity progressively decreases with dehydration, Brønsted acidity remains strong even at 773 K.

This paper is dealing with YBa2Cu3O7 − δ ceramics obtained by solid state sintering (TS ≤ 920 °C). The presence of carbonates in the starting powder or the existence of carbon dioxide traces in the gas phase during the heat treatment leads to the formation of a thin amorphous layer at solid-gas interfaces which has been observed by TEM. That layer acts as an oxygen diffusion barrier. Therefore, the reoxidation of the ceramics at low temperature is inhibited, explaining for a part the low values of Jc measured in bulk ceramics. The influence of fluorination treatments is also investigated.

The first-order ferroelastic structural phase transition Fm3m ⇔ R3 occurring in CoZrF6 is studied by means of calorimetric, neutron diffraction, Raman and Brillouin scattering measurements. A phenomenological model based on Landau theory is proposed.

When La 2 CuO 4 is treated with pure F 2 gas at 200°C, the X-ray diffraction pattern of the resulting product is characteristic of a new single phase derived from the K 2 NiF 4 -type structure. An enhancement of the orthorhombic distortion relative to the starting oxide is observed with: a = 5.342 A; b = 5.436 A and c = 13.192 A. Both weight uptake and increase of the c unit cell constant could be consistent with an incorporation of fluorine atoms in the lattice. This new compound is superconducting at T c = 40 K with H c1 ⋍ 700 Oe and exhibits a strong diamagnetic susceptibility ( χ g = -6.13 × 10 -3 emu / g ) at 6 K under an applied field of 1 Oe

The electrochemical performances of raw and surface- fluorinated graphite powders used as anode in Li-ion battery have been studied. The fluorination obtained from F2 gas mainly concerns the surface of the particles for TF2≤300 {degree sign}C. For higher temperature, important modifications of the bulk structure were observed. When they are used as anode in Li-ion battery, fluorinated graphite powders exhibit better electrochemical performances than raw graphite notably due to an increase of the surface area and surface defects which allow the storage of a higher amount of lithium into the host lattice. These results have been confirmed by impedance measurements performed in delithiated state: a drastic decrease of both the charge transfer resistance and the resistance related to the SEI layer was observed for TF2 lower than 300 {degree sign}C.

During the fluorine evolution reaction occurring in the electrolysis of molten KF-2HF, a thin, solid fluorocarbon layer is formed on the carbon anode. The high anodic overvoltage which characterized this process is mainly due to the presence of such a film which drastically changes the wettability of the electrode by the melt. Activation of carbon at 40 V in KF-2HF modifies strongly the surface morphology (scanning electron microscopy) and composition (energy-dispersive X-ray and X-ray photoelectron spectroscopy) of the surface fluorocarbon film. In addition, it enhances the electrochemical performances of the carbon electrodes vs fluorine evolution. Local microconductivity measurements carried out by atomic force microscopy have revealed heterogeneities of composition of the carbon-fluorine film due to the presence of conducting fluorine-graphite intercalation compounds and insulating graphite fluorides (CF x ) within the film. Finally, the peculiar shape of the fluorine bubbles observed during electrolysis of molten KF-2HF was studied, taking into account the presence of these two fluorocarbon compounds on the surface.

The crystal structure of the low-temperature forms of Rb2KCrF6 and Rb2KGaF6 has been solved on single crystal. The symmetry is tetragonal with F4/m space group; the unit cell parameters are: a = 19.407 ( 3 ) A , c = 8.833 ( 2 ) A for Rb2KCrF6 at T = 90 K and a = 19.375 ( 4 ) A , c = 8.782 ( 2 ) A for Rb2KGaF6 at T = 90 K . The relationships between the parameters of the prototype cubic elpasolite, which is stable at high temperature, and the tetragonal superlattice of the low temperature form have been established. Considering the general formulation A2BB′F6, the cationic positions in the A and (B,B′) sublattices remain identical in the two allotropic varieties. The main originality of the structure concerns the environment of 4/5 of the potassium atoms (B sublattice) which is transformed from octahedra into pentagonal bipyramids sharing edges with adjacent B′F6 octahedra containing Cr or Ga. The displacive phase transition is simply explained by the rotation of 45° in the (a,b) plane of 1/5 of the B′F6 (B′=Cr, Ga) octahedra. The similarity of this phase transition and the transformation of perovskite into tetragonal tungsten bronze (TTB) will be discussed.