Scientific publications

We present results on the structural and magnetic properties of highly crystalline α-Fe2O3 nanoparticles of average size similar to 200 nm, synthesized from a novel sol-gel method using metal alkoxide precursor. These particles are multi-domain, showing the weak ferromagnetic antiferromagnetic (WF-AF) transition (i. e., the Morin transition) at T-M = 256 (2) K. Mossbauer measurements revealed a jump in hyperfine parameters (HP's) at T similar to T-M, which also displays thermal hysteresis upon cooling or heating the sample. The analysis of HP's as a function of temperature allowed us to discard temperature gradients as well as the coexistence of WF/AF phases as possible origins of this hysteretic behaviour. Instead, the hysteresis can be qualitatively explained by the small size and high-crystallinity of the particles, which hinder the nucleation of the WF or AF phases yielding metastable states beyond T-M.

Mixed-metal oxides with the composition Y3ScxGa5-xO12 (x = 2.0, 2.1, 2.2, 2.25, 2.3, 2.4, 2.5, and 3.0) have been prepared by an aqueous sol-gel method. The effects of scandium substitution on the garnet phase formation were studied by IR spectroscopy and X-ray powder diffraction (XRD). The XRD data indicate that single-phase Y3ScxGa5-xO12 ceramic samples were obtained for x = 2.0, 2.1, 2.2, 2.25, 2.3, and 2.4. The results also show that the formation of Y3ScxGa5-xO12 garnets depends on the molar ratio of scandium and gallium in the investigated composition, and consequently on the mean cationic radius at the Al3+ sites. The variation of lattice parameters for the Y3ScxGa5-xO12 phases with different x is reported.

Structures comprising a xerogel, doped with lanthanide ions (erbium, terbium and europium), embedded in porous anodic alumina (PAA) have been fabricated and their optical and electrical characteristics have been studied. Erbium photoluminescence at 1.53 µm from titania xerogel/PAA was found to increase with the number of xerogel layers and erbium. concentration for the excitation wavelength 532 nm, matching the area of transparency of both titania xerogel and PAA. Visible green and red electroluminescence was observed for terbium- and europium-doped In2O3 and SnO2 xerogels embedded in porous anodic alumina. The improvement of the electrical properties of the xerogel/PAA cell is discussed, taking into account the observed ability of conducting ln2O3:Sn (ITO) nanoparticles to penetrate into the anodic alumina pores.

We report here on dialkyldithiocarbamates complexes of the type [C6H3(CH3)S2MS2CNR2] (M = As and Bi: R = CH3, C2H5, and CH2-CH2), prepared by the reaction of toluene-3,4-dithiolatoarsenic(III) and -bismuth(III) chlorides [C6H3(CH3)S2MCl] (M = As and Bi) with sodium/ammonium salts of dialkyldithiocarbamates [XS2CNR2] (X = Na and NH4: R = CH3, C2H5 and CH2-CH2). The new derivatives were characterized by melting point, elemental analysis (C, H, N, S and As/Bi) and spectroscopic studies (IR, 1H and 13C NMR). Single crystal X-ray diffraction analyses of toluene-3.4-dithiolatoarsenic(III) dimethyldithiocarbamate and toluene-3,4-dithiolatoarsenic(III) pyrrolidine-dithiocarbamate revealed a monodentate chelating mode of the dimethyl as well as the pyrrolidine-dithiocarbamate moieties in these complexes.

A BaSnO3 powder with a crystallite size of 27.6 nm has been prepared through a hydrothermal reaction of a peptised SnO2∙xH2O and Ba(OH)2 at 250°C and the following crystallization of this hydrothermal product at 330°C. The peptisation of the SnO2∙xH2O gel is dependent on the pH value. Through peptisation the mean particle size of SnO2∙xH2O in the aqueous solution has been decreased by a factor of 100 to 8 nm. A limited agglomeration in the sol-prepared powder has been observed under the microscope. The structure evolution and crystallisation behaviours of the sol-prepared powders were investigated by TG-DTA, IR and XRD. The BaSn(OH)6 phase in the as-prepared powder transforms into an amorphous phase at 260°C, from which the BaSnO3 particles nucleate and grow with an increase in temperature. The single-phase BaSnO3 powder has been obtained at a temperature as low as 330°C. This sol-prepared powder is more sinter-reactive than the get-prepared powder and can be sintered to a ceramic with 90.7% of the theoretic density.

Nanocrystalline NiGa2O4 films were deposited on silicon substrates by the CVD of a new heterometal alkoxide, [NiGa2(OtBu)8]. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) showed a single-step decomposition behavior for the molecular precursor at low temperature (240°C), suitable for a CVD process. [NiGa2(OtBu)8] is monomeric in solid state with a tetrahedral Ni2+ center coordinated by two monoanionic {Ga(OtBu)4}-moieties. Despite an adequate vapor pressure, the gas-phase transport of [NiGa2(OtBu)8] is susceptible to the distance and geometry of the effective diffusion path. Investigations on CVD deposits obtained using different transport pathways (reservoir –> substrate) show that gas-phase travel through a long (39 cm) and angular reactor tube induces fragmentation of the heterometal compound, which yields minor amounts of Ni, NiO, and Ga2O3 (amorphous), besides the target (NiGa2O4) composition. Shortened reservoir –> substrate path length (13 cm) produced stoichiometric NiGa2O4 films, apparently due to reduced collision probabilities and a laminar flow. In both cases, the chemical composition was determined using energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS), whilst the structure was evaluated using powder X-ray diffraction (XRD) and scanning electron microscopy (SEM). From these, the advantages and limitations of the single molecular source in the growth of NiGa2O4 films could be determined.

To obtain yttrium-gallium garnet (Y3Ga5O12, YGG) a simple "chimie douce" method has been developed. This sol-gel method yielded excellent starting gel precursor for the fabrication of YGG phase, which could be used as host material for optical applications. The pattern of X-ray diffraction analysis of the ceramic sample sintered for 10 h at 1000°C showed the formation of monophasic Y3Ga5O12 phase. The phase transformations, composition and micro-structural features in the gels and polycrystalline sample were studied by thermoanalytical methods (TGA/DTA), powder X-ray diffraction analysis (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The quality of the resulting products (homogeneity, crystallisation temperature, grain size, grain size distribution, etc.) is discussed.

The effects of annealing temperature and hydrolysis catalyst on the crystalline form and crystallite size of TiO2 prepared by a sol-gel process were investigated. Three types of TiO2 were synthesized by hydrolysis of titanium(IV)-iso-propoxide with water in n-propanol without catalyst (Type-A) and with HCl as catalyst, where the H+/Ti4+ mol ratio was 0.54 for Type-B and 0.2 for Type-C. Hydrolysis products were thermally treated at 100, 200, 300, 400 and 500 °C for 1 h after drying at room temperature. Characterization of the particles was carried out using XRD, BET, TG/DTA and SEM analysis. The pore size distributions were computed by the DFT plus method. The results showed that the acid catalyst and catalyst/alkoxide ratio have a large effect on the formation of anatase TiO2. In particular, it was found that anatase phase TiO2 particles occur at 400 and 500 °C for Type-A TiO2, while they do so at nearly 100 °C for Type-B and Type-C TiO2. The crystallile size of Type-A, Type-B and Type-C increased from 17.96 to 19.24 nm, from 12.38 to 15.12 nm and from 10.60 to 12.20 nm, respectively, when the thermal treatment temperature was raised from 400 to 500 °C.