Phonon dispersion and low energy anomaly in CaC6_6

We report measurements of phonon dispersion in CaC$_6$ using inelastic X-ray and neutron scattering. We find good overall agreement, particularly in the 50 meV energy region, between experimental data and first-principles density-functional-theory calculations. However, on the longitudinal dispersion along the $(1 1 1)$ axis of the rhombohedral representation, we find an unexpected anti-crossing with an additional longitudinal mode, at about 11 meV. At a comparable energy, we observe also unexpected intensity on the in-plane direction. These results resolve the previous incorrect assignment of a longitudinal phonon mode to a transverse mode in the same energy range. By calculating the electron susceptibility from first principles we show that this longitudinal excitation is unlikely to be due to a plasmon and consequently can probably be due to defects or vacancies present in the sample.Comment: Accepted for publication in Physical Review

Superconductivity in Li3Ca2C6 intercalated graphite

In this letter, we report the discovery of superconductivity in Li3Ca2C6. Several graphite intercalation compounds (GICs) with electron donors, are well known as superconductors. It is probably not astonishing, since it is generally admitted that low dimensionality promotes high superconducting transition temperatures. Superconductivity is lacking in pristine graphite, but after charging the graphene planes by intercalation, its electronic properties change considerably and superconducting behaviour can appear. Li3Ca2C6 is a ternary GIC, for which the intercalated sheets are very thick and poly-layered (five lithium layers and two calcium ones). It contains a great amount of metal (five metallic atoms for six carbon ones). Its critical temperature of 11.15 K is very close to that of CaC6 GIC (11.5 K). Both CaC6 and Li3Ca2C6 GICs possess currently the highest transition temperatures among all the GICs.Comment: 5 pages, 3 figure

The rose of the Sainte-Chapelle in Paris: sophisticated stained glasses for late medieval painters

The restoration of the rose (15th century) of the Sainte-Chapelle in Paris, France, offered a unique opportunity to investigate the color and chemical composition of these emblematic medieval French stained glasses with non-destructive analyses. The obtained results are aimed at complementing the knowledge from art historians and thus together trying to compensate for the total absence of archives on the construction of the rose. Comparison with the glasses of the nave (13th century) reveals an important evolution of the aesthetics based on new types of glasses: new colors and extensive use of flashed glass. The systematic study of the chemical composition of both sides of each glass piece revealed that about half of the studied glasses were flashed. For non-flashed glasses, this comparison allowed evaluating the influence of glass surface weathering, although very moderate, on the composition variability. In light of the variability criteria, the multivariate analysis of the chemical composition allowed inferring that most glasses originate from the same production glasshouse. The new colors result from the original composition of flashed glass, allowing superimposing otherwise incompatible redox states of the coloring transition elements. The comparison with the glasses of the nave reveals the glass technology evolution that occurred over two centuries and allowed the production of new glasses for the medieval glaziers at the eve of the Parisian Renaissance

The rose of the Sainte-Chapelle in Paris: sophisticated stained glasses for late medieval painters

The restoration of the rose (15th century) of the Sainte-Chapelle in Paris, France, offered a unique opportunity to investigate the color and chemical composition of these emblematic medieval French stained glasses with non-destructive analyses. The obtained results are aimed at complementing the knowledge from art historians and thus together trying to compensate for the total absence of archives on the construction of the rose. Comparison with the glasses of the nave (13th century) reveals an important evolution of the aesthetics based on new types of glasses: new colors and extensive use of flashed glass. The systematic study of the chemical composition of both sides of each glass piece revealed that about half of the studied glasses were flashed. For non-flashed glasses, this comparison allowed evaluating the influence of glass surface weathering, although very moderate, on the composition variability. In light of the variability criteria, the multivariate analysis of the chemical composition allowed inferring that most glasses originate from the same production glasshouse. The new colors result from the original composition of flashed glass, allowing superimposing otherwise incompatible redox states of the coloring transition elements. The comparison with the glasses of the nave reveals the glass technology evolution that occurred over two centuries and allowed the production of new glasses for the medieval glaziers at the eve of the Parisian Renaissance

Recherches sur la réduction des composés d'insertion du graphite avec les chlorures de fer III et de cobalt II par les métaux alcalins lourds en phase vapeur

Non disponible / Not availableLa première partie est une analyse des travaux antérieurs (une centaine) qui sont résumés dans des tableaux récapitulatifs. Le travail expérimental consiste en la réduction du trichlorure de fer et du déchlorure de cobalt insérés dans le graphite. Ces composés ont été complètement réduits par les métaux alcalins lourds en phase vapeur entre 100 et 300. L'emploi d'un graphite précurseur formé de particules monocristallines (diamètre moyen 10, épaisseur inferieure à 0,1 micron) facilement orientables sur un support plan, et l'association de techniques variées ont permis la description des produits réduits. Chaque particule contient une série de domaines de composition et de structure différentes: graphite pur, domaines contenant des nodules de fer alpha ou de chlorure alcalin aux cristallites désorientées, domaines où les cristallites de chlorure ont des orientations précises par rapport aux axes cristallographiques de la matrice; enfin domaines contenant du fer intercale, dont les mailles planes dépendent de la nature du réducteur: ces différentes mailles évoluent par recuit vers une maille carrée unique de paramètre 348 pm, que l'on retrouve pour le cobalt. L'influence très nette de la nature du réducteur sur la structure des produits de réduction explique au moins en partie les différences existant dans la littérature pour la réduction d'un même chlorure intercale

Chemical, structural and electrical resistivity of two first stage arsenic-potassium-graphite intercalation compounds

International audienceThe action on graphite of liquid arsenic-potassium alloys leads to the formation of a new series of graphite intercalation compounds. Two ternary phases of first stage have been observed and isolated. The alpha-type phase is obtained by action on graphite of composition alloys close to that of defined K3As compound. On the other hand, the beta-phase is easily obtained by action on graphite of K-As alloys whose chemical stoichiometry is equal or near to the 37 at.% As eutectic point composition. The Fourier transform analysis of the 001 reflexions family of these two phases showed that the intercalated metallic sheets between graphitic gaps in these phases are generally three-layered stacking according to K/As/K sequence along c-axis. An exam, by means classical X-ray diffraction and transmission electronic microscope (TEM) of the in-plane structure of these compounds revealed that the intercalated metallic sheets are perfectly crystallised and an equatorial stratum was defined for each of them. Electrical conductivity measurements have been carried out on these phases along both axis of graphite and showed that they are strongly anisotropic and exhibit a metallic character in parallel to graphitic plans

Comparative study of ternary graphite-potassium-metal (M=Tl, Hg, Au) intercalation compounds.

International audiencePotassium-thallium, potassium-mercury, and potassium-gold alloys can be intercalated into graphite, leading to the formation of poly-layered ternary compounds. Different ways of synthesis are involved for the preparation of the corresponding Graphite Intercalation Compounds (GICs). The as-prepared bulk GICs are described in a comparative study, regarding their chemical formula, their formation mechanism, and their structural and superconducting properties. Analogies and differences are pointed out