National Institute Of Materials Physics - Romania

The electron paramagnetic resonance spectra of Cr3+ in LiNbO3 co-doped with ZnO or CaO (approximate to 6%) have been studied. Both impurities give rise to the same effect as that observed in MgO-doped samples: a strong diminution of the anisotropic (axial) spectrum reported for LiNbO3:Cr and the appearance of an intense and isotropic line. The origin of this line in the co-doped samples is discussed.

In this work, we perform a study of the off-center effect: on the diffusion coefficient and we propose a new mechanism for fast diffusion. The model assumes that the equilibrium positions of the cationic impurities are noncentral and that the diffusion proceeds by hopping across the potential barrier along the nonlinear paths with the highest probability [1]. The nonlinear paths between two consecutive positions characterize even the selfdiffusion and furthermore, in the off-center diffusion this path is appropriate. The main results is that the off-center position considerably enhances the diffusion. The theoretical diffusion coefficients have been obtained by modeling the potential barrier. Changes of the configuration entropy and the vibration spectra due to the presence of the noncentral impurity are included in the model. We emphasize the good agreement of the model with the experimental data for KCl:Cu+ and show that if the impurity is placed close to the central site, the appropriate diffusion coefficient is close to that for the cationic self-diffusion: We proceeded in the Li+ cation case as in the case of Cu+ cation.

Preparation of modified ruthenium molecular sieves has been investigated in two steps deposition of ruthenium and modifying of ruthenium molecular sieves in presence of ligands. As support sieve there were used two molecular sieves with large pore apertures and low acidity strength (zeolite L and APO-34). Correlation of in situ W-VIS ruthenium deposition measurements with catalyst characterisation revealed that ruthenium deposition takes place not only through ionic exchange but also through adsorption of ruthenium hydrolysed species. Modifying of the ruthenium molecular sieves catalysts with ligands has as effect a diminution of electronic charge on metal. The experimental data indicate that the presence of ligand favours an enantioselective hydrogenation of D-fructose to D-mannitol even if the yields are not very high.

A sequential algorithm, leading to three-dimensional polytetrahedral dense random packing structures is employed to simulate the atom-by-atom growth of relatively small clusters, under a large variety of physical assumptions. The computer model developed can provide information about the influence of experimental growth conditions on the topological and energetical parameters of the cluster : average coordination number, atomic pair distribution function, the degree of local tetrahedral perfection and specific bonding energy as a function of cluster size. The degree of icosahedral order built-in during the simulated growth process can be estimated from the pair distribution function and is of particular interest, due to its relevance to quasicrystalline thin films investigation.

Films of C-60, at different stages of annealing of T-c = 200 degrees and 300 degrees C have been electrical characterized over the temperature domain from -130 degrees C to T-c. X-ray diffraction revealed a random polycrystalline fee structure with stacking defects of an intrinsic nature, due to deposition conditions. The value of room-temperature conductivity was found to be in the range (6.3-1.0)*10(-10) (Omega cm)(-1). In the stable annealed state the conductivity showed an activated temperature dependence above 423 K and a non-activated dependence below 330-280 K. The activation energies E(2) 0.8 eV (film thickness 0.70 mu m) and E(2) = 1.0 eV (film thickness 2.40 mu m) were in good agreement with the energy gap values (1.63 eV and 2.08 eV) which were deduced from the al;sorption spectral dependence. Annealing decreased the non-activated contribution to conduction, extending the intrinsic conduction temperature range.