Publications

Laser excited site-selective spectroscopy of Nd3+ ions in Ca3Ga2Ge3O12 garnet crystals has been investigated in the F-4(3/2)-->I-4(11/2) and F-4(3/2)-->I-4(9/2) regions. Six non-equivalent sites were observed for the Nd3+ in this material. The energy levels of some relevant multiplets were obtained for each Nd3+ site. The evolution of the non-equivalent Nd3+ CryStal field sites in Ca3Ga2Ge3O12 as a function of dopant concentration has been investigated in the range up to 8 at%. The nature of the Nd3+ sites in terms of the possible charge compensating defects is discussed. The formation of Nd3+-Nd3+ pairs is suggested.

The formation of nickel oxide from Ni-Al takovite-like compounds has been investigated. Information about the stoichiometry of the NiO phase obtained by Diffuse Reflectance (DR) spectroscopy is discussed in connection with TPR and solubility data.

SiO2-Sm2O3 vitreous films 120 nm thick were obtained via the sol-gel method. Infrared, energy-dispersive analysis of X-rays, spectroscopic ellipsometry and absorption spectroscopy showed a total inclusion of samarium in the silica matrix, which remains amorphous up to 1000-degrees-C. The inclusion of the samarium ion increases the porosity. The optical properties of films were investigated by spectro-ellipsometry, and the parameters obtained were compared with those obtained by other techniques.

A class of periodic one-dimensional models with nearest-neighbor interactions is presented in which particles can hop between adjacent sites with symmetric jump rates. The dc conductivity is calculated exactly for a wide choice of the factor describing the influence of the nearest-neighbor interaction on the jump rates. By allowing the cell size to become infinite, the result is extended to several nonperiodic chains including the random ones.

The results of real-time neutron diffraction measurements during the superconducting phase formation process in the Bi(Pb)-Sr Ca-Cu-O system are reported. A Sr-Ca-Cu-O type precursor, with the same stoichiometry as the 2223 phase, was used as starting material, and the temperature range favorable to the formation of the 2223 phase was investigated. The diffraction patterns were processed by a multiphase Rietveld refinement. The formation and decomposition of the 2201 and 2212 phases were directly observed. Experimental evidence on the existence of a partially melted phase in the range 855-860-degrees-C, involved in the formation of the 2223 phase, is discussed.

The dissipation mechanism in high-T(c) superconducting ceramics was studied by analyzing the magnetic field dependence of the transport critical current and the form of the current-voltage characteristics. It was found that the actual magnetic field dependence of the transport critical current is significantly slower than that predicted by the weak-link-quenching model. The low-voltage-level current-voltage characteristics were described in terms of thermally activated flux creep at grain boundaries, taking into account collective pinning of intergranular Josephson vortices. For the investigated samples, a low-field, high-temperature mean pinning energy barrier of a few tens of meV was determined.

The operational range of the gas counters used in conversion electron Mossbauer spectroscopy (CEMS) is at the limit between the proportional counter and Geiger Muller domains. Starting from the theory of phenomena in gas detectors under space charge conditions, a general expression for the multiplication factor was obtained. The theoretical results, presented graphically, are discussed in connection with experimental data.

We present stationary-state Monte Carlo experiments evaluating the dc conductivity of the one-dimensional lattice gas with binary energetic disorder. The resulting correlation factor is compared to previous analytic calculations.

Pd-Ho/gammaAl2O3 catalysts exhibit higher activity for styrene hydrogenation than Pd/gammaAl2O3 ones. The addition of holmium to Pd catalysts changes the place of the palladium impregnation front depending on the state of the supported palladium. As a consequence of the interaction between holmium and Pd/gammaAl2O3 support, oxidized forms as holmium aluminate were formed. These forms are catalytically inactive but represent adsorption centers for styrene. On holmium containing catalysts, hydrogenation takes place between the styrene adsorbed on basic centers and the hydrogen spilled over from palladium particles.