National Institute Of Materials Physics - Romania

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.

An experimental and theoretical investigation is made on the current pulse evolution, from a very low amplitude toward the steady-state shapes, if a sequence of alternative voltage pulses is applied to a ZnS:Mn metal-insulator-semiconductor-insulator-metal structure. The influence on this evolution of the voltage pulse maximum amplitude, of its rising slope, of pulse shape, and of the frequency is investigated. A quantitative model is developed which explains the current pulse evolution taking into account the competition between the multiplication of the carriers in the ZnS layer, and their recombination involving deep hole traps. The model describes the shapes of current pulse evolution and their peculiarities corresponding to different experimental conditions.

From the magnetic and Mossbauer spectroscopy study of LiNi1-xFexO2 With x = 0, 0.01, 0.05, and 0.10 evidence for strong ferromagnetic correlations and spin freezing at low temperatures was found, reminding many characteristic features of spin glass behavior in variance with models describing the ground state of LiNiO2 as a quantum liquid.

A magnetic and Mossbauer spectroscopy study of nonoriented and magnetically oriented samples of Nd6Fe13Sn was performed. At 4.2 K a metamagnetic transition takes place in a field of 7.1 T. Strong evidence for antiferromagnetic spin arrangements in the iron magnetic sublattice and antiferromagnetism of the Nd moments with only a small contribution to the net magnetization in fields up to 14 T was found.

Bimetallic Pd-Fe/gamma-Al2O3 catalysts with different concentration of palladium and 1 wt.% of iron were prepared via impregnation of Pd/gamma-Al2O3 catalysts with Fe (NO3)3 solution and compared with corresponding monometallic Pd and Fe catalysts. The state of the supported components was studied by hydrogen chemisorption, ESR and Mossbauer spectroscopy; hydrogenation of styrene was also used as a test reaction. The hydrogenation was shown to proceed selectively to ethylbenzene on monometallic Pd catalysts and unselectively to the mixture of ethylbenzene and ethylcyclohexane on the Fe catalysts. Presence of 0.1 wt.% of palladium in the Pd-Fe catalysts directed the hydrogenation exclusively to ethylbenzene while selectivity drops again upon increasing Pd loading to 0.3 and 0.5 wt.%. Possible reasons for such a non-trivial effect from the second metal are discussed.