Columnar Liquid Crystals in Cylindrical Nanoconfinement

Axial orientation of discotic columnar liquid crystals in nanopores of inorganic templates, with the columns parallel to the axis of the nanochannels, is considered desirable for applications such as production of molecular wires. Here, we evaluate experimentally the role of the rigidity of the LC columns in achieving such orientation in nanopores where the planar anchoring (i.e., columns parallel to wall surface) is enforced. We studied the columnar phase of several discotic compounds with increasing column rigidity in the following order: dendronized carbazole, hexakis(hexyloxy)triphenylene (HAT6), a 1:1 HAT6-trinitrofluorenone (TNF) complex, and a helicene derivative. Using 2-D X-ray diffraction, AFM, grazing incidence diffraction, and polarized microscopy, we observed that the orientation of the columns changes from circular concentric to axial with increasing column rigidity. Additionally, when the rigidity is borderline, increasing pore diameter can change the configuration from axial back to circular. We derive expressions for distortion free energy that suggest that the orientation is determined by the competition between, on the one hand, the distortion energy of the 2-d lattice and the mismatch of its crystallographic facets with the curved pore wall in the axial orientation and, on the other hand, the bend energy of the columns in the circular configuration. Furthermore, the highly detailed AFM images of the core of the disclinations of strength +1 and +1/2 in the center of the pore reveal that the columns spiral down to the very center of the disclination and that there is no amorphous or misaligned region at the core, as suggested previously

Réflectivité de HfSe2, NiTe2, TiTe2 de 12 eV à 42 e

The reflectivity of the layer structure compounds : HfSe2, NiTe 2, TiTe2 has been measured between 12 eV and 42 eV, using synchrotron radiation at Orsay (L.U.R.E.). The Hf, Te and Ti core level excitation spectra are discussed in terms of the conduction band densities of states.La réflectivité des composés lamellaires HfSe2, NiTe2 et TiTe2 a été mesurée entre 12 eV et 42 eV en utilisant le rayonnement synchrotron d'Orsay (L.U.R.E.). Les structures observées dues à l'excitation de niveaux profonds de Hf, Te et Ti servent de base pour un modèle qualitatif de densité d'états de conduction

Evolution des paramètres de maille de LiNb03 dopé par le potassium ou le magnésium

L'évolution des paramètres de maille a et c de la phase nonstoéchiométrique LiNbO3 du niobiate de lithium est comparée aux résultats de la littérature. L'addition d'oxyde de potassium au bain de croissance induit la diminution des deux paramètres montrant une évolution vers la stoéchiométrie, sans incorporation de potassium dans le cristal. Au contraire, dans le cas d'ajout d'oxyde de magnésium au bain de croissance, l'augmentation des paramètres indique une substitution du lithium par le magnésium dans le cadre de la solution solide LiNbO3-Mg4Nb2O9 qui suit la loi de Végard.The evolution of a and c lattice constants of nonstoichiometric LiNbO3 lithium niobate is compared with litterature results. Addition of potassium oxide to the melt induces shortening of both parameters towards the stoichiometry, without incorporation in the crystal of potassium. On the contrary, with magnesium oxide addition in the melt, a lengthening of both parameters is connected with substitution of lithium by magnesium in a LiNbO3-Mg4Nb2O9 solid solution which obeys Végard's law

Conservation d'aragonite dans des tests de mollusques du Campanien de Provence

Preservation of aragonite in shells of molluscs from the Campanian of Provence. Shells of Unio from the Campanian beds of the Arc syncline in Provence are made up of original aragonite. Very small amounts of calcite (usually no more than 3%) detected using X ray diffraction are evidences of limited mineralogical transformations. All samples were taken underground in a coal mine, either from areas submitted to water seepages or from dry zones. The preservation of aragonite cannot be explained by absence of circulation of water. The aragonite crystals may have been protected by layers of amino-acids and by organic matter included in the whole series.Dans les tests d'Unio du Campanien du synclinal de l'Arc l'aragonite originelle est très largement conservée. L'analyse fine par diffractométrie d'échantillons prélevés dans des ouvrages miniers montre que la teneur en calcite dépasse rarement 3 %. Ce phénomène ne peut pas s'expliquer par une simple absence de circulations d'eaux. Un confinement par des films de matière organique à l'échelle des cristaux lamellaires constituant les tests mais aussi à l'échelle de la série sédimentaire doit être envisagé.Gaviglio Patrick, Redondo Claude Pierre, Cerclier O., Monteau Raymond. Conservation d'aragonite dans des tests de mollusques du Campanien de Provence. In: Géologie Méditerranéenne. Tome 22, numéro 2, 1995. pp. 85-91