Fluorogermanium(IV) salts of graphite. A system in equilibrium with elemental fluorine

Pyrolytic graphite is not intercalated by GeF{sub 4} alone but is intercalated by GeF{sub 4}/F{sub 2} mixtures to yield, at the intercalation limit, at 20{sup o}, a first-stage material, C{sub 12}GeF{sub 5-6}, which is in equilibrium with gaseous fluorine. GERMANIUM tetrafluoride is a superior fluoride ion acceptor since it can stabilize the O{sub 2}{sup +} ion and the NF{sub 4}{sup +} ion in salts. It is this superior fluoride-ion acceptor capability which must account for our observation that pyrolytic graphite readily intercalates GeF{sub 4} in the presence of fluorine, but does not do so, even with high pressures of GeF{sub 4}, if fluorine is absent, Similar experiments, in which SiF{sub 4} was substituted for GeF{sub 4}, failed to bring about any silicon fluoride intercalation. It has long been known that GeF{sub 4} is a superior fluoride ion acceptor to SiF{sub 4} since, with SF{sub 4}, the former yields the salt (SF{sub 3}{sup +}){sub 2}GeF{sub 6}{sup 2-}, whereas the silicon analogue is not stable at ordinary temperatures and pressures (in spite of the lattice energy of the silicon analogue being slightly more favorable by virtue of the smaller size of SiF{sub 6}{sup 2-})

Etude cristallochimique de quelques composés oxyfluorés, hydroxyfluorés et fluorés des éléments III B

L'utilisation croissante de l'aluminium dans les applications les plus diverses a suscité un développement important des recherches consacrées à ses composés fluorés qui jouent un rôle essentiel dans l'élaboration du métal. Il n'en est pas de même pour ses homologues de la colonne III B : lorsque nous avons commencé ce travail la chimie des composés fluorés du gallium, de l'indium et du thallium n'avait pratiquement pas été explorée. Cette lacune s'expliquait par la relative rareté des ces éléments, mais également par les difficultés inhérentes à la chimie du fluor...non disponibl

Role of metal fluoride octahedra in the mechanism of phase transitions in A2BMF6 elpasolites

Sequences of phase transitions (PTs) occurring in (AI)2BIMIIIF6 elpasolites have been studied via X-ray diffraction, differential scanning calorimetry (DSC) and adiabatic calorimetry at low temperature (LT). The substitution of NH4+ ions for alkaline ions in sites with either 6 or 12 coordination number considerably affects both transition temperature and entropy change associated with the transition(s). Two main mechanisms are responsible for the PTs: displacive transitions, as for the Rb2KMIIIF6 series, in which the transitions are associated with small octahedral tilts and are followed by a rather small entropy change; order-disorder transitions of rigid octahedra, as for (NH4)2NH4MIIIF6. In this latter case, the fluoride atoms located at the apices of the octahedra are disordered onto several possible positions in the cubic..

Holistic optimisation of noise reducing devices

International audienceThe work presented in this paper is part of the QUIESST European project, in which one of the objective is to perform multi-objective holistic optimisations of noise reducing devices. We present here optimisation results concerning the extrinsic performances of noise barriers. The performances under interest are acoustical, economical and environmental. The variety of noise barriers considered is very wide, ranging from straight and flat barriers, to rough or capped barriers. A total number of nine noise reducing device families are optimised. Acoustical performances are obtained from numerical calculations: the Boundary Element Method (in 2D) is used to obtain relative sound pressure levels at a set of receivers in different situations. Theses situations include road and rail sources; rural and urban cases; flat, embanked and depressed topographies. The economical performance is calculated according to the maintenance cost of the different materials in use in the barrier. Four environmental performances indicators are considered; their calculation is based on a life-cycle assessment analysis. All performances are expressed as a gain (or loss) relative to a reference screen. It is shown that the optimisation procedure allows one to obtain a wide variety of optimised noise reducing devices, and hence provides a helpful design tool by allowing one to focus on specific parameters