Publications

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.

In comparison with the ''classical'' Kim-Anderson model for the current-voltage characteristics of type-II superconductors, another model with a less sharp pinning potential is presented. In addition to the approximate solution, for currents close to the critical one, a complete solution of this model is derived, which better fits the experimentally obtained current-voltage characteristics.

Ceramic materials of the ZrO2-SnO2-TiO2 system, modified by La2O3 and ZnO additions, were investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry. All the samples sintered at 1330, 1360 and 1400-degrees-C contain a (Zr(x)Sn(z))TiO4 (x+z = 1, 0 < z less-than-or-equal-to 0.4) solid solution coexisting with ZrTiO4, the last gradually disappearing with rising temperature. At 1400-degrees-C, the (Zr0.8Sn0.2)TiO4 solid solution matrix was not quite homogeneous, containing small amounts of a grain-boundary phase assumed to be La2(Zr,Sn,Ti)2O7 and the expected TiO2-basis solid solution rich in Zn and La. The evolution of the chemical composition of (Zr(x)Sn(z))TiO4 solid solution to (Zr0.8Sn0.2)TiO4 and the simultaneous disappearance of ZrTiO4 are thought to explain the variation trends of lattice parameters with increasing sintering temperature.

Amorphous Ge/Si and crystalline Mo/V superlattices have been grown by dual-target d.c. magnetron sputtering with modulations according to finite approximants of various generalized Cantor sequences. The samples were investigated with both conventional powder X-ray diffractometry and high-resolution X-ray diffractometry as well as transmission electron microscopy. The experimental X-ray results were compared with derived analytical expressions for the locations and scalings of the diffraction peaks. Results from the X-ray measurements and the analytical expressions were also compared with numerical reflectivity simulations. The analytical and numerical calculations show good agreement with the experimental results.

Electron spin resonance (ESR) investigations with a polycarbonate solid state nuclear detector, irradiated with oxygen ions, are reported. The nature of the paramagnetic defects induced by irradiation is discussed. The temperature dependence of resonance line parameters is studied. From the experimental data, obtained by ESR, spectroscopy, the activation energy for defect recombination, the average isotropic exchange integral between paramagnetic defects as well as the average distance between defects, are estimated. Correlations with latent tracks structure are discussed.

A three-dimensional lattice gas model for enantiomeric phase separation is introduced. The enantiomeric molecules (d and 1) are the two nonsuperimposable mirror images having the molecular structure C(AB)2, where C is a tetrahedrally bonded carbon atom with one bond to each end of two AB groups. The lattice gas model consists of a body-centered cubic lattice, each site of which can be either vacant or occupied by a molecule oriented so that the A and B groups point toward neighboring lattice sites. Pairs of molecules interact with short-range, orientationally-dependent interactions. For a domain of interaction parameters, the Pirogov-Sinai extension of the Peierls argument is used to prove that d-rich and l-rich phases exist in the model at sufficiently low temperature. For another domain of interaction parameters, at sufficiently high chemical potential there is an infinite number of ground states, each containing a racemic mixture of d and l molecules.

Different iron hydroxide precipitation processes simulating radioactive waste treatment have been investigated by Mossbauer spectroscopy at room temperature and at 80 K. Magnetic oxides (hematite or magnetite) partially affected by superparamagnetic relaxation have been observed. The crystallization degree and the particle size depend on the concentration and the addition order of chemicals. Much smaller particles were precipitated with Ca(OH)2 than with NaOH as neutralization reagent.

We have studied the spectroscopic properties of Pb2+ in a CdCl2 host crystal. By excitation in the absorption band due to the 1A1g --> 3T1u (P-3(1)) transition, two emission bands are observed. At very low temperature a strong uv luminescence with a very long lifetime (2.5 ms) is detected, As the temperature increases, the lifetime of the uv emission drastically decreases to a few nanoseconds at 100 K, and the uv intensity declines. Meanwhile a strong green fluorescence appears. It is assumed that this emission is of the impurity-trapped exciton type, as in the case of CdCl2:Cu+ and BaF2:Eu2+. A detailed study of the kinetics of both fluorescences is presented.

The stereochemistry of ring-opening metathesis polymerization of several cycloolefins (cyclopentene, cyclooctene, 1,5-cyclooctadiene, cyclododecene) in the presence of WCl6-based catalysts (WCl6/iBu3Al/epichlorohydrin, WCl6/iBu3Al/Chloranil, WCl6/phenylacetylene, WCl6/Ph4Sn, WCl6/allyl4Si, WCl6/iBu2AlOAliBu2) has been carefully examined. The product stereoselectivity and polymer microstructure were greatly dependent on the nature of the catalytic system and reaction conditions; the nature of the cycloolefin also influenced the steric course of the reaction. An important stereochemical consequence was that the cis and trans content of the polyalkenamer greatly depended on the oxidation state of the W-atom, catalyst activity and reaction temperature. The relationship between the microstructure of several polyalkenamers (polypentenamer, polyoctenamer, polydodecenamer) obtained with the above catalytic systems and cycloolefin reactivity or catalyst nature is fully illustrated. The role of the ligand as well as of the cocatalyst and activator is also shown. The particular stereochemical behaviour of cycloolefins in the presence of the studied WCl6-based catalytic systems is discussed in terms of the metallacarbene-metallacyclobutane mechanism. The steric effects induced by the coordinating cycloolefin and the existing ligands at the transition metal are also considered.