M.E. Torres

Cooperative energy transfer in Yb3+–Tb3+ codoped silica sol-gel glasses

Optical properties of Yb3+–Tb3+ codoped silica sol-gel samples have been studied after the gel to glass transition. Different upconversion emissions have been observed under near infrared excitation at about 1 μm. The Tb3+ ions are excited by means of energy transfer processes from Yb3+ ions. The temporal evolution of the blue-green upconversion emissions coming from Tb3+ ions and their dependence on the excitation intensity at about 1 μm has been studied. The experimental results are in good agreement with a cooperative resonant energy transfer mechanism from Yb3+ ions. An efficient backtransfer process is observed from Tb3+ ions towards Yb3+ ions. The upconversion efficiency, which is limited by this backtransfer process, has been obtained and compared with other upconversion results in similar matrix.

Luminescent properties of transparent nanostructured Eu3+ doped SnO2–SiO2 glass-ceramics prepared by the sol–gel method

Nanostructured silica based ceramic samples of composition (100− x)SiO2–xSnO2, doped with 0.4 mol% of Eu3+ and with x from 1 to 10, have been developed after a thermal treatment of precursor sol–gel glasses. A structural analysis has been performed by x-ray diffraction and high resolution transmission electron microscopy. The mean radius of the obtained SnO2 nanocrystals, ranging from 2 to 5 nm, is comparable to the exciton Bohr radius, corresponding to wide bandgap semiconductor quantum dots in an insulator SiO2 glass. A spectroscopy study in terms of emission and excitation spectra has been carried out as a function of SnO2 concentration. Moreover, time-resolved fluorescence measurements have also been performed in order to discern the emission of ions in glassy and nanocrystalline environments. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations.

Structural and dielectric characterization of cadmium tartrate

Polycrystalline samples of dimeric cadmium tartrate, [(Cd,C4H4O6)2H2O)]3H2O [labeled CdT(I)], were studied using impedance measurements and x-ray powder diffraction. The dependence of the real part of the dielectric constant on temperature showed a sharp peak at about 65 °C, revealing a structural phase transition, while the other broad peak in the temperature range (70<T<85 °C) was due to the loss of water molecules. The x-ray powder diffraction patterns at three temperatures (25, 60, and 70 °C) are consistent with three nonequivalent space groups. According to these results, it seems that this compound undergoes two successive phase transitions: P212121→P21→Pnmn, suggesting an intermediate ferroelectric behavior, labeled CdT(II) between a paraelectric CdT(I) and an anhydrous phase, labeled CdT(III).

Characterization and thermal and electromagnetic behaviour of gadolinium-doped calcium tartrate crystals grown by the solution technique

Energy dispersive analysis by X-rays (EDAX), X-ray diffraction (XRD), infrared (IR), thermogravimetric (TG), differential thermogravimetric (DTG), differential scanning calorimetric (DSC) and electromagnetic studies carried out on crystalline materials obtained by diffusion of gadolinium and calcium ions through silica gel impregnated with tartaric acid and grown by means of slow cooling between 72°C and 40°C are reported. EDAX confirmed the presence of gadolinium. X-ray diffraction data did not show any appreciable differences between the structures of doped and undoped calcium tartrate. IR in the range of 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis (TG, DTG and DSC) indicated that the material is thermally unstable. Dielectric and magnetic measurements were determined in the frequency range 45 MHz to 2 GHz.

Electrical conductivity of doped and undoped calcium tartrate

The electrical conductivity of polycrystalline samples of calcium tartrate tetrahydrate ([CaC4H4O6 2H2O] 2H2O) in pure form and doped with barium and with strontium were studied in the temperature range (65<T<95°C). According to these results, it seems that two types of conduction exist in these compounds, one at low temperature and the other at high temperature, by the way of extrinsic and intrinsic conduction, respectively. This behavior may be attributed to the rotation of the tartrate ions by thermal energy.

Characterization and thermal and electromagnetic behaviour of manganese tartrate crystals grown by the silica gel technique

We report here on the dielectric properties (dielectric constant, ɛ′ and dielectric losses ɛ″), magnetic properties (μ′ and μ″), infrared spectroscopic, thermal studies (thermogravimetric analysis (TGA)), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC) and X-ray diffraction (XRD) for the polycrystalline powdered samples of manganese tartrate MnC4H4O6·2H2O obtained by diffusion of manganese ions through silica gel impregnated with tartaric acid. The aforementioned properties are used as a probe for the detection and study of the different transitions that have been found in this salt. Energy dispersive analysis of X-rays (EDAX) confirmed the presence of manganese. X-ray diffraction data giving 2gq, intensity andd-values are reported for the first time. Infrared wavelengths in the range 500–4000 cm−1 and the description of peaks recorded for the material are given. Results of thermal analysis indicated that the material is thermally unstable. Dielectric and magnetic measurements were determined in the frequency range of 45 MHz to 2 GHz.

Structural investigation of the negative thermal expansion in yttrium and rare earth molybdates

The Sc(2)(WO(4))(3)-type phase (Pbcn) of Y(2)(MoO(4))(3), Er(2)(MoO(4))(3) and Lu(2)(MoO(4))(3) has been prepared by the conventional solid-state synthesis with preheated oxides and the negative thermal expansion (NTE) has been investigated along with an exhaustive structural study, after water loss. Their crystal structures have been refined using the neutron and x-ray powder diffraction data of dehydrated samples from 150 to 400 K. The multi-pattern Rietveld method, using atomic displacements with respect to a known structure as parameters to refine, has been applied to facilitate the interpretation of the NTE behavior. Polyhedral distortions, transverse vibrations of A· · ·O-Mo (A = Y and rare earths) binding oxygen atoms, non-bonded distances A· · ·Mo and atomic displacements from the high temperature structure, have been evaluated as a function of the temperature and the ionic radii.

Polymeric aqua-1κo-bis[μ-(R, R)-tartrato-1κ2o1, o2 :2κ2o3, o4]dicadmium(II) trihydrate

The structure of the title compound, {[Cd 2 (C 4 H 4 O 6 ) 2 -(H 2 O)].3H 2 O} n , consists of corrugated polymeric sheets of dimeric [Cd 2 (C 4 H 4 O 6 ) 2 (H 2 O)] units and three water molecules of crystallization. Both cadmium ions are coordinated by two (R,R)-tartrate ligands in a cis arrangement, and the octahedral geometry for each cation is completed by two carboxyl O atoms of different neighbouring dimers or by one carboxyl O atom and a water molecule. Additional water molecules are held in the crystal lattice, forming a hydrogen-bonding network to keep the dimers together in the non-planar sheets.

Barium L-tartrate

Barium L-tartrate, Ba 2+ .C 4 H 4 O 2- 6 , has been grown in silica-gel medium for the first time and its structure has been solved. A new coordination of the cation with the tartrate anion is observed. The cation exhibits ninefold coordination without the presence of water molecules and the tartrate groups are linked through Ba…O contacts to form a three-dimensional network.

Polymorphism in Ho2(MoO4)3

1 Dpto. Fisica Fundamental II, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 2 Dpto. Fisica Fundamental y Experimental, Electronica y Sistemas, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 3 Dpto. Fisica Basica, Universidad de La Laguna, Avda. Astrofisico Fco. Sanchez, s/n, 38206 La Laguna, Tenerife, Spain 4 C.A.I. Difraccion de Rayos X, Universidad Complutense de Madrid, 28040 Madrid, Spain 5 Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France