National Institute Of Materials Physics - Romania

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.

The present work reports the magnetic properties and Mossbauer spectra of soft ferrites MnaZnbFe1-a-b2+Fe23+O4 with Ti4+ substitutions. The addition of Ti4+ to manganese-zinc ferrites induces considerable improvements of properties such as the linear dependence of permeability versus temperature, and small disaccommodation of the permeability. Samples with Mn0.57Zn0.34Fe0.09+x2+TixFe2-2x3+O4 composition (where x = 0.00, 0.02, 0.04, 0.06, 0.08, 1.00) were prepared by the usual ceramic technology. The presence of Ti ions strongly influences the magnetic anisotropy, as evidenced by the temperature dependence of the permeability. A sublattice assigned to Ti4+Fe2+ pairs appeared in the Mossbauer spectra, which also gave information on the local surroundings of iron ions. The appearance of a secondary maximum of permeability (SMP) at negative temperatures or a constant slope over a wide temperature range is typical for mu(i)(t) curves. Increasing Ti4+ contents shift the SMP to higher temperatures because of a positive contribution to K1 of the Ti4+Fe2+ pairs at octahedral sites, in agreement with the hyperfine magnetic fields observed for ferrites with large titanium substitutions.

A strong non-stoichiometry of pure f c c CeO2 was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO2 reduction to A-Ce2O3 by laser irradiation was demonstrated. Particles of stable Ce7O12 phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce11O20 and cubic Ce12O22 phases was found. The controversial C-Ce2O3 Phase was detected at the limits of a b c c particle. An unknown b c c phase of acicular morphology, strongly related to C-Ce2O3, was also registered. The dose dependence of CeO2 structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce3+ ions).

Adherent and transparent unhydrogenated DLC layers were deposited by means of r.f. and d.c. magnetron sputtering at low deposition temperatures. The optical gap value increases from 0.7 to 2.2 eV when deposition pressure is increased from 10(-2) to 10(-1) Torr. In high gap materials, large amounts of sp3 hybridized carbon atoms are present. When Al contacts are evaporated in coplanar configuration on these high gap materials, rectifying and photoelectrical effects are observed.

The homogeneous-inhomogeneous transitions of a two-dimensional electron gas in a strong perpendicular magnetic field are studied in the particular case when half of the states in a spin splitted Landau level are occupied. To this end the dielectric susceptibility in the homogeneous states as well as the free energy in the modulated states are calculated. Unless the g factor dos not exceed a certain value the charge-density waves are stable in an interval of finite temperatures.