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International audience; Comparison of the crystal structures of Cu(OH)2 and gamma- FeO(OH) shows a structural analogy between these two solid phases, despite the difference of formulas. That is due especially to the way in which polyhedra are connected, giving rise to a two-dimensional structure composed of wavy sheets. Gamma-FeO(OH) named lepidocrocite, is obtained probably by a topotactic reaction which we propose a hypothesis, from divalent iron hydroxide Fe(OH)2 whose structure displays also a two-dimensional framework of brucite type. The lepidocrocite structural type obtained by this way, allows strong deformations of octahedral surroundings of cations, contrarily to the brucite type. Deformations can be due to the loss of hydrogen atoms in the oxi-hydroxide phase or to the strong Jahn-Teller effect which affects divalent copper in the case of copper hydroxide. If an analogy between the structures of Cu(OH)2 and brucite is not possible, this analogy exists with lepidocrocite even if copper has a pentahedral surrounding whereas trivalent iron one is octahedral. In the other hand, in the two crystal structures, an oxygen atom has a peculiar surrounding composed of only two cations and one hydrogen. That explains why this structural type presents a low stability.; Malgré leur stoechiométrie différente, Cu(OH) 2 et gamma-FeO(OH) présentent une analogie structurale. Elle se manifeste essentiellement dans le mode d'enchaînement des polyèdres qui aboutit à une structure bidimensionnelle composée de feuillets ondulés. L'oxy-hydroxyde gamma-FeO(OH) appelée lépidocrocite, est obtenue probablement par une réaction topotactique, dont nous proposons une hypothèse, à partir de l'hydroxyde de fer divalent Fe(OH) 2, qui possède également une structure feuilletée, isotype de la brucite. La structure de type lépidocrocite ainsi obtenue peut tolérer des déformations importantes de l'environnement octaédrique des cations contrairement au type structural de la brucite. Cette déformation est due à une dissymétrie entre les oxygènes, induite par l'absence d'un hydrogène pour l'oxy-hydroxyde ou à l'effet Jahn-Teller important présenté par le cuivre divalent dans la structure de l'hydroxyde de cuivre. Si une analogie structurale est impossible entre Cu(OH) 2 et la brucite, elle existe en revanche avec la lépidocrocite même si le cuivre divalent se trouve dans un environnement pentaédrique alors que le fer trivalent se trouve dans un environnement octaédrique. Les deux structures sont caractérisées par un atome d'oxygène possédant un environnement exotique constitué par deux cations seulement et un hydrogène. Cela a pour conséquence de rendre relativement instable ce type structural.

Bi 0.7 La 0.3 O 1.5 crystallizes with rhomboedral symmetry (space group R 3 m ) and hexagonal unit cell parameters: a = 0.4043(2) nm, c = 2.7557(7) nm, Z = 9. Its structure has been solved at room temperature by using both single-crystal X-ray diffraction and powder neutron diffraction techniques. It can be described as a regular repetition along the c axis of identical M 3 O 4.143 fluorite-like sheets. Each sheet comprises three close-packed cationic layers: one internal layer of Bi La atoms in distorted cubic eightfold coordination, surrounded by two external layers of Bi atoms in fourfold coordination. Extra anions partially occupy delocalized sites within vacant octahedra of the interlayer space. Relations with the fluorite type and possible diffusion pathways for anions are proposed.

The general inertia defect of nonplanar XY6 type of molecules and ions has been evaluated for the first time by applying the method suggested by Herschbach and Laurie. The centrifugal distortion constants have also been calculated for the above system., Dhanalakshmi A., Lalitha M. Ground vibrational inertia defect and centrifugal distortion constants of octahedral XY6 molecules and ions. In: Bulletin de la Classe des sciences, tome 68, 1982. pp. 249-258.

From among the minerals of the montmorillonite group, hectorite, montorillonite and nontronite have been studied by means of electron microscopy and selected area diffraction methods. It was possible to demonstrate that hectorite has a strong resemblance to the ideal structure attributed to the layer and to determine the symmetry of the projection of the basic layer on to its plane (bidimensional space group) in the case of montmorillonite and nontronite. The symmetry c 1 m 1 (equivalent to the tridimensional group C 2) was assigned to montmorillonite (WYOMING mineral). Nontronite corresponds to the symmetry c 2 m m (equivalent to the tridimensional group C 2/m). In the latter case it was shown that the structure was obtained from the ideal structure by a slight rotation of the tetrahedrons of the SiO2 layer and by a certain distorsion of the octahedral layer . The investigations by means of selected area diffraction also made it possible to establish that the particles observed under the electron microscope were in certain cases aggregates of basic particles in reciprocal orientation. Hence, certain varieties of montmorillonite (e. g. CAMP BERTEAUX mineral) show the presence of particles consisting of a mosaic of small elements in definite reciprocal orientation at a multiple of 60°. Whereas hectorite appears to be formed by elongated basic particles along the axis a, associated in a parallel direction with each other with a fluctuation of orientation around the axis a., Méring Jacques, Oberlin Mathieu-Sicaud Agnès. Etude en microscopie et microdiffraction électroniques de l'hectorite, de la montmorillonite et de la nontronite. In: Bulletin du Groupe français des argiles. Tome 14, nouvelle série n°9, 1964. pp. 147-148.

International audience; The structures of the phases occuring in the FeF3 - FeF2 - MF (M = Li, Na, K, Rb, Tl) systems differ in the manner in which the FeF6 octahedra are connected. These structures may assume the form of: a)tridimensional (FeF3)n networks, b) double layers of (Fe2F7)n, c) single layers of (FeF4)n, d) bridged chains of (Fe3F14)n, e) isolated chains of (FeF5)n or f) isolated FeF6 octahedra. This classification, which can be extended to other compounds of transition elements, aluminium and gallium, accounts for the magnetic properties of the iron fluorocompounds.