Etude sur les elargissements par des gaz etrangers comprimes des raies du spectre de rotation pure de la molecule HF—II. Perturbation par les molecules lineaires homopolaires

As part of a study of spectral-line broadening of the HF molecule by compressed foreign gases, the influence of linear homopolar perturbers is analysed. The separation of the calculation in two parts allows resonant collisions to be seen and gives a simple explanation of the experimental observations.     

Etude comparative des structures des alumines β stoechiometrique et non stoechiometrique: Transition de phase dans l'alumine β stoechiometrique (II)

The usual preparation methods of β alumina lead to a non stoichiometric compound $\text{(β“N.S.”)}$ of formula 11Al₂O₃?(1 + x) B₂O with x ≈-0.3; a metastable phase with a composition close to stoichiometry $\text{(β“S”}\text{and}\text{x ? 0)}$ can however also be obtained. X-Ray diffuse scattering studies of this stoichiometric form of silver β alumina reveals a sharp order—disorder phase transition at about 307 K. The low temperature ordered state of the silver ions is found to correspond to a 3D hexagonal superstructure with the lattice constants $\text{a}\text{3}\text{, a}\text{3}\text{, c}$. Above the transition temperature 3D short range order is observed up to about 315 K, where a cross over occurs towards a higher temperature 2D short range state, similar to that previously observed at low temperature in $\text{β}{}_{\mathrm{<mtext>Ag</mtext>}}\text{“N.S.”}$. Above 500 K the conducting silver ions are found to be in a 2D quasi liquid state. A similar type of order—disorder phase transition seems to occur in stoichiometric sodium β alumina at lower temperature. It is concluded that the very particular behaviour namely the absence of phase transition in the usual forms of β alumina is a direct consequence of non stoichiometry.     

Etude de l'adsorption de monocouche a l'interface solide-solution diluee par spectroscopie de photoelectrons (XPS): Application a l'adsorption de phospholipides sur des fibres damiante

The adsorption of phospholipids on asbestos chrysotile in a solid-dilute-solution system has been studied by X-ray photoelectron spectroscopy (XPS). Two batches with different particle sizes were used: with thin and long fibres (SFA), and with short fibres (FCC). The fibres were analysed on a portion of filter after filtration. This method of sample preparation results in good reproducibility of the absolute intensities of the XPS peaks. Under these conditions, the surface coverage for monolayer adsorption was determined using an overlayer model. Comparison of the results deduced from the XPS data with those obtained from chemical analysis shows that for low initial concentrations of phospholipids, a different mode of adsorption occurs on the FCC fibres.     

Determination of the phase transitions in BaTiO3 type crystals by using the theory of symmetry breaking: Etude des transitions de phase dans les cristaux du type BaTiO3 par la theorie des symetries brisees

Crystaks of the BaTiO₃ family showing the following sequence of phase transitions : tetragonal, orthorhombic, rhombohedral, are investigated. From the MASON-MATHIAS model based on a TiO₆ octahedron in which a Ti ion can be displaced along the different pseudo-cubic axes, these different polar structures are found by considering the possible lowering of the O_h symmetry associated with the non polar phase. By the appropriate orientation of the basis of the vector representation connected with the occurence of the polarisation $\text{P}$, the order-parameter of the phase transitions are determined for three cases (O_h → C_4v, C_2v, C_3v). The order of the transition is discussed. These parameters can be used for a thermodynamical investigation of the crystal based on an expansion of its free energy.