Etude par spectrometrie de photoelectrons de la structure electronique des ynals et des ynones conjugues

This study proposes an interpretation of the UV photoelectron spectrum of propynal mainly with semi-empirical calculations. The electronic structure of 16 other conjugated ynals and ynones has been studied by the same techniques, and the variations of the first ionization potential, associated with the exceptionally deep oxygen lone pair, may be interpreted in terms of polar effect and intramolecular interactions. The discussion about the set π_-, π_≡ that cannot be separated yields only some qualitative results.     

Etude par spectrometrie de photoelectrons de la structure electronique des enynes conjugues

We have measured by UV excited photoelectron spectroscopy the ionization potentials of 22 conjugated alkynes, in particular, those associated with three π orbitals (π_, π_, π₊. The corresponding bands appear in this order in the spectrum. We have also calculated the orbital energies of these compounds by two semi-empirical quantum mechanical methods (INDO and SPINDO).Discussion of the ionization potential variations in terms of polar effects confirms the spatial distribution of the orbitals predicted by calculation.The vibrational structure of the photoelectron bands confirms their attribution and the role predicted for the orbitals associated with the chemical bonds of these compounds.     

Etude radiocristallographique et par resonance Mössbauer de CaLaFeO4

CaLaFeO₄ has a K₂NiF₄ type structure. Its critical exponent β determined by Mössbauer spectrometry shows a crossover at T/T_N = 0, 955.     

Etude par resonance paramagnetique electronique des verres du systeme B2O3-Na2O-Na3PO4

An EPR study of fast Na⁺ ion vitreous conductors of the system B₂O₃?Na₂O?Na₃PO₄ has been carried out. After X-ray irradiation two types of paramagnetic centers have been observed. The first one is of boron-oxygen hole center (B.O.H.C.) type and similar to that earlier observed for other alkali borate glasses. The second one is of PO₄^2? type. A simulation of its spectrum has been achieved and a defect model discussed.     

Etude par RMN et rayons X de sesquicarbure d'uranium

We have measured the position and linewidth of the ¹³C NMR signal in U₂ C₃ between 4 and 300 K. This study confirms the existence of an electronic transition around 50 K which brings possibly a non magnetic fundamental state. This transition involves no crystallographic distorsion and no lattice parameter anomaly.     

Etude cristallographique et magnetique de MnGeN2 par diffraction neutronique

The position parameters of Mn and N atoms in MnGeN₂ have been improved using neutron diffraction data. The magnetic structure is each Mn being antiparallel to the moments of its four nearest neighbours.     

Etude de la cinetique de la reaction de polymerisation de resines epoxy et polyester par analyse du comportement dielectrique

In this piece of research, we have chosen to study the dielectric behaviour of different resins while in process of polymerization and determine the dielectric parameters most relevant to the monitoring of the kinetic process taking place; our aim being to design electromagnetic sensors which will enable us to monitor the polymerization process thoroughout.Consequently we have confined ourselves to carrying out radiofrequency measurements by using only a few set frequencies, without attempting to monitor, as many researchers have tried to, the evolution of dielectric relaxation [1, 5].We have studied two types of resins, one of them being epoxy-based and the other polyester-based. We have succeeded in demonstrating that the perception of the real terms of the complex dielectric constant as well as of the evolution of the loss tangent angle, both complementary elements, are essential for a thorough analysis of the kinetic process.Whereas ε is most suitable for the monitoring of the concluding phase of the polymerization, tan δ is more appropriate to the study of the opening stage of the process, while allowing us to define with great accuracy the exothermicity point reached after gel time [6, 7, 8].     

Etude cristallographique et magnetique de MnSiN2 par diffraction des neutrons

The Neel temperature (180°C) and magnetic structure of MnSiN₂ have been obtained by neutron diffraction as well as better atomic coordinates than those deduced previously from X-ray data only.     

Evolution de quelques proprietes physiques de phases de structure “bronzes de tungstene quadratiques” par remplacement de l'element Alcalino-Terreux par le plomb

Substitution of lead for the alkaline-earth element in Sr₂KNb₅O₁₅ and Ba₂NaNb₅O₁₅ leads to the solid solutions Sr_2(1?xPb_2xKNb₅O₁₅ and Ba₂(_1?xPb_2.05xNa_1?0.1xNb₅O₁₅ for 0? x? 1 which have the tetragonal Magnéli bronze structure. The ferroelectric Curie temperature and the harmonic efficiency in nonlinear optics show minima for x_srPbK = 0.15 and x_BaPbNa = 0.65. For compositions poorer in lead the polar axis is along the O_z direction. When X_SrPbK > 0.15 and x_BaPbNa > 0.65 the polar axis becomes parallel to the O_y direction, and partially coupled ferroelastic-ferroelectric properties simultaneously appear.     

Etude par diffraction neutronique de la forme haute pression de FeOOH

A powder sample of iron oxyhydroxide (FeOOH_HP) synthesized under high pressure-high temperature conditions, has been studied by neutron diffraction. The magnetic structure has been determined. The magnetic and chemical unit cells are the same and the antiferromagnetic arrangement is collinear with the spins parallel to the c-axis. The crystallographic structure affinement by the Rietveld method has confirmed that FeOOH_HP is isostructural with InOOH (space group P2₁nm) and has established the positional atomic parameters. The magnetic moment of iron (4.7μ_B at 4.2 K) and the O-H distance in the hydrogen bond (1.11 Å) are discussed with regard to the values obtained for the other forms of iron oxyhydroxide.     

Etude par spectrometrie Raman des complexes de l'iode dans les alcools et les ethers

Laser Raman spectra of iodine-alcohol and iodine-ether solutions have been studied. It was found a new line at 167 cm^?1 for alcohols and at 170 cm^?1 for ethers which is tentatively attributed to an intermediate Hassel complex.     

Etude par correlation angulaire γ-γ perturbee des aspects statiques et dynamiques de la structure du heptafluorohafniate de sodium

We have studied the temperature dependence of the perturbation of the differential γ-γ angular correlation of the ¹⁸¹Ta 133–482 keV cascade in sodium heptafluorohafnate. In the 20–300°C temperature region, we observed a drastic variation of the quadrupole interaction parameters when raising the temperature from 100 to 180°C. Such a variation is due to the existence of a phase transition corresponding to an orthorhombic-quadratic transformation of the lattice. In the low temperature phase (T < 120°C) the perturbation is static and the nuclear probe is located at two equally populated sites. In the high temperature phase (T > 180°C) the perturbation arises from the combination of static and dynamic interactions. The dynamic part of the interaction could be analysed within the isotropic perturbation model and was ascribed to jumps of the fluorine atoms onto the vertices of the cubic coordination polyhedra of the hafnium in the HfF₇^3? ion. The activation energy of the process is 0.13 ±0.03 eV. The idea of an order-disorder phenomena occuring in the course of the phase transition is consistently supported by these observations. Our results yield also valuable informations about the nature of the hafnium-fluor bond in this compound.     

Proprietes optiques et structure electronique de MnAu2

The optical spectrum of the helical antiferromagnetic compound MnAu₂(t_N = 90°C) has been measured, using a scanning ellipsometric method, in UHV, between 0.47 and 5.7 eV, at temperatures ranging from 88 to 700 K. Below 0.6 eV the experimental data can be fitted to a Drude-like intraband model. The maximum of the interband absorption occurs at 5.1 eV, while the onset of interband absorption may be placed at 0?.4eV as is suggested by the rapid rise of $\text{ε}{}_2\text{(ω)}\text{λ}$. below 0.5 eV. In the absence of theoretical work, the analysis of the optical spectrum leads to a preliminary rough model of the electronic structure; the proposed local density of d states is represented. The 5.1eV peak is attributed to d → E_F transitions (parabolic edge at 2.7 eV similar to Au), originating in the lower part of the band, associated mainly with Au sites. To account for the moment $\text{(}\text{3.6μ}{}_{\mathrm{S}}\text{at Mn}\text{)}$, the upper d band (mainly Mn sites) is split: the d↑ band is below E_F (interband edge at 0.4eV), while the d↓ band contains 1.4 electrons. ESCA measurements tend to confirm this model. An important unusual fact is the sharp anomaly of /~ε(ω) in the infrared, around T_N; attempts to correlate this with magnetic (s-d) interactions have been initiated.     

Influence des impuretes et de l'inhomogeneite d'un plasma sur la mesure de sa temperature electronique

The effect of impurities on soft X-ray continuum radiation emitted from a high-temperature deuterium plasma has been studied. The results of these calculations are useful in the determination of the electronic temperature of a laser-created plasma. Such a plasma is inhomogenous and contains impurities.     

Caracterisation et Etude par effet Mössbauer d'une nouvelle variete haute pression de FeOOH

A new high pressure form of FeOOH, isostructural with InOOH, has been prepared by hydrothermal synthesis under very high pressure conditions. Characterization of the new phase was performed by D.T.A., T.G.A., infrared and Mössbauer spectroscopy. The compound behaves as an antiferromagnet with a Néel temperature of approximately 570° K. The hyperfine magnetic field at 0° K is about 525 kOe. Thermal decomposition occurs below the Néel temperature. Comparison with other FeOOH phases is made.     

Etude des interactions quadrupolaires dans quelques tungstates par resonance Mössbauer de 182W

The tungstates having a double-perovskite, trirutile, scheelite or a wolframite type structure have been studied by means of Mössbauer ¹⁸²W spectroscopy. It is shown that the electric field gradient at the W nucleus is essentially due to a lattice term in the scheelite structure while the valence term seems to be preponderant in the case of the wolframite structure.     

Etude par spectroscopie de photoelectrons et d'electrons auger des etapes initiales de l'oxydation du chrome polycristallin

The joint application of various techniques of surface analysis (XPS, UPS, AES and work function measurements) allowed us to pinpoint the initial stages in the oxidation of polycrystalline chromium in the temperature range 180–720 K. At weak exposures ( < 10 L) the oxygen was dissociatively adsorbed without providing any information on the amount of oxygen involved or the sites of adsorption (chemisorption vs. physical interaction due to the roughness of the surface). At stronger exposures the oxide Cr₂ O₃ is observed to form, and this is covered by a layer of chemisorbed oxygen at ordinary temperatures and below. The maximum thickness of the oxide layer depends on the temperature, as would be expected from the logarithmic oxidation kinetics. The change in shape of the Auger peaks of chromium during oxidation has been interpreted in terms of interatomic transitions made via the 2p levels of oxygen.     

Etude de la pression de radiation de l'onde evanescente par absorption de lumiere resonnante

In this paper we show how the technique of measuring the deflection of atomic beams by absorption of resonant light could be applied to study the density momentum of the evanescent wave.