Publications

Haematite particles of four different morphologies (polyhedral, platelike, needlelike and disk shaped) were synthesized by the hydrothermal method. The morphology and average particle diameter (1.4, 7.4, 0.2, and 0.12 mm, respectively) were determined by transmission electron microscopy combined with electron diffraction. The haematite samples were studied by transmission Mossbauer spectroscopy in the temperature range 4.2-300 K. In all cases, a weak ferromagnetic (WF) phase was present above the Morin temperature of 230 K and found to coexist with an antiferromagnetic (AF) phase below this temperature. However, the populations of the two phases at 230 K were demonstrated to depend on the morphology of the particles. Moreover, the WF and AF phases exhibit a different dependence of the magnetic texture on temperature and particle morphology. (C) 1999 American Institute of Physics. [S0021-8979(99)66908-0].

High-quality single crystals of Rb2ZnCl4 and K2ZnCl4, pure or doped with Cu, Mn, Cd, Tl, Sn, Pb and In cations, were grown by Czochralski technique in argon atmosphere, using an experimental setup that allows direct visual access to the whole growth zone. Slowly cooled crystals exhibit excellent cleavage properties. Fastly cooled crystals do cleave poorly. As shown by X-ray diffraction studies, such K2ZnCl4 samples exhibit inclusions of the high-temperature Pmcn phase with lattice parameters a = 7.263(2) Angstrom, b = 12.562(2) Angstrom and c = 8.960(4) Angstrom in the P2(1) cn room temperature stable phase. ESR and optical spectroscopy studies revealed the localization and valence state of the cation dopants. (C) 1999 Elsevier Science B.V. All rights reserved.

Trinuclear carboxylate compound [Fe3O(CH3COO)(6)(H2O)(3)](2)[PtCl6].8H(2)O has been studied by Mossbauer spectroscopy, magnetochemistry, thermal analysis and X-ray crystallography. Crystal data: monoclinic, space group P2(1)/n a = 10.505(2), b = 14.278(3), c = 19.885(4) Angstrom, beta = 96.00(3)degrees and Z = 2. The final R-value is 0.027 for 4606 reflections with I greater than or equal to 2 sigma(I). The crystal consists of the complex [Fe3O(CH3COO)(6)(H2O)(3)](+) cations, centrosymmetric [PtCl6](2-) anions and water molecules. The [Fe3O(CH3COO)(6)(H2O)(3)](+) cation has the typical structure of a trinuclear iron(III) compound with mu(3)-O bridge. All crystal components are connected via a system of hydrogen bonds into a 3D network. Mossbauer spectrum displays at room temperature a single quadrupole doublet with an isomer shift of 0.70 mm/s and quadrupole splitting of 0.48 mm/s, consistent with high-spin Fe(III). mu(eff) per Fe atom (3.25 mu(B) at 293 K and 2.23 mu(B) at 120 K), indicate the antiferromagnetic coupling between the paramagnetic iron ions with J = -31 cm(-1).

The effects of Mo substitution in Pr2Fe17-xMox and PrFe12-xMox compounds were investigated by X-ray diffraction, magnetic measurements and Mossbauer spectroscopy. Different fitting procedures, using magnetic sextets at low temperature, magnetic sextets and quadrupole doublets at room temperature and paramagnetic doublets well above the Curie points, had to be considered in order to analyze the Mossbauer spectra. The Mossbauer data obtained on the Mo-containing compounds stand for distributions of the hyperfine fields due to the random distributions of Mo over the 6c sites in the 2:17 and 8i sites in the 1:12 compounds. The variations in the isomer shifts and hyperfine fields with composition are discussed in relationship to the local environment details at the different iron sites. Site-specific information on the electronic effects due to Fe(Sd)-Mo(4d) states hybridization are derived. (C) 1999 Elsevier Science S.A. All rights reserved.

Cadmium sulphide (CdS) thin films were deposited on glass substrate by precipitation from aqueous solution technique. The films were doped with copper using the direct method consisting in the addition of a copper salt (CuCl(2)) in the deposition bath of CdS. The doped films were annealed in air, at 300 degrees C, for 1 h. We report some structural, optical, electrical and photoelectrical properties of CdS thin films before and after Cu doping, correlated with investigation of trapping levels by Thermally Stimulated Currents (TSC) method. (C) 1999 Elsevier Science S.A. All rights reserved.

In this paper, we present Surface Enhanced Raman Scattering (SERS) experiments performed successfully on some conducting polymers and carbon nanotubes deposited in thin films on rough metallic surfaces. The enhancement mechanism in SERS has a twofold origin: electromagnetic and chemical. The electromagnetic enhancement is, however, the dominant mechanism and consists in the excitation of localized and delocalized surface plasmons (SP). The second enhancing mechanism for SERS, not yet convincingly supported by experimental data, is of chemical origin. This mechanism is due to the increase of the polarizability of the molecules on the metal surface under the action of the incident radiation, leading to the formation of new chemical bonds between the molecules and the metal surface. Unfortunately, the experimental data available to argue the presence of a chemical process are scarce and scattered. In this paper, we try to present some details regarding this aspect. In order to do so, we studied poly 3-hexylthiophene (3-PHT) and polyaniline (PAN) as a function of the type of the rough metallic support (Ag, Au or Cu), the oxidization state and thickness of the polymer layer. We studied also carbon single walled nanotubes. Our experiments reveal the existence of a chemical surface effect. The results obtained for 3-PHT show that SERS spectra depend on the oxidizing properties of the metal surface and on the nature of the solvent. This dependence is explained by the existence of some interfacial reactions that lead to the formation of interface compounds. The SERS measurements reported here reveal an increase of the intensities of the Raman lines, accompanied by a modification of the corresponding intensity ratios, when the degree of doping is increased. We observe for the first time by SERS spectroscopy that doping of 3-PHT with FeCl3 leads to the appearance of a state of disorder in the structure of the macromolecular chain, as a result of steric hindrance effects. The type of the rough metallic support can modify SERS spectra and such an effect is clearly shown for the polyaniline case. No such dependence on metallic support type is observed on the SERS spectra of carbon nanotubes. (C) 1999 Published by Elsevier Science S.A. All rights reserved.

This paper discusses the formation of "corals" arranged silver nanocrystals in a KCl matrix starting from Ag- ions ( similar to 10(17) ions/cm(3)) by subsequent thermal annealing at temperatures between 400-700 degrees C and 250 degrees C. In the last case only, by illumination with total radiation of a 100W Xe-lamp, spherical separated silver clusters were obtained. The stepwise annealing processes were followed by the recording of the optical absorption spectra at 20 degrees C. The sizes of silver clusters were determined through transmission electron microscopy. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

The photo-structural transformations induced by ultraviolet (UV) radiation in Ge27Sb13Se60 amorphous thin films prepared by thermal evaporation method have been investigated. The mechanical, optical and structural data were investigated before and after irradiation of the fresh and annealed films. The modifications of the film structure and properties depend essentially on the state of the films: fresh or annealed below glass transition temperature.