Photo luminescence: A probe for short, medium and long-range self-organization order in ZrTiO4 oxide

Photoluminescent disordered ZrTiO4 powders were obtained by the polymeric precursor soft-chemical method. This oxide system (ordered and disordered) was characterized by photoluminescence, Raman spectroscopy, X-ray diffraction, differential scanning calorimetry and UV vis absorption experiments. The UV absorption tail formation in the disordered oxides was related to the diminution of optical band gap. In the disordered phase, this oxide displayed broad band photoluminescence caused by change in coordination number of titanium and zirconium with oxygen atoms. The gap decreased from 3.09 eV in crystalline oxide to 2.16 eV in disordered oxide. The crystalline oxide presented an orthorhombic alpha-PbO2-type structure in which Zr4+ and Ti4+ were randomly distributed in octahedral coordination polyhedra with oxygen atoms. The amorphous-crystalline transition occurred at almost 700 degrees C, at which point the photoluminescence vanished. The Raman peak at close to 80-200 cm(-1) indicated the presence of locally ordered Ti-O-n and Zr-O-n polyhedra in disordered photoluminescent oxides. (c) 2006 Elsevier B.V. All rights reserved.

Synthesis and reactivity in salt metathesis reactions of trivalent [La(Tp(Me2))(2)X] (X = Cl, I) complexes: crystal structures of [La(Tp(Me2))(2)Cl] and [La(Tp(Me2))(2)(kappa(2)-pz(Me2))]

Reaction of LaX3(THF)(n) (X = Cl, 1) with two equiv. of K(Tp(Me2)) gave good yields of the bis-Tp complexes [La(Tp(Me2))(2)X] (X = Cl (1); I (3)). However, the formation of 1 and 3 is always accompanied by significant amounts of La(Tp(Me2))(2)(kappa(2)-pz(Me2)) ([pz(Me2)](-) = 3,5-dimethyl-pyrazolato) (2). The pyrazolato complex 2, which presumably arises from decomposition of the [Tp(Me2)](-) moiety during salt metathesis, was independently prepared in good yield from 1 and in situ generated [pz(Me2)](-). The solid-state structures of 1 and 2 were determined by single-crystal X-ray diffraction studies. Subsequent reactions of halogeno-Tp(Me2) complexes 1 and 3 with various alkali metal salts MR (M = Li, R = CH2SiMe3, Ph, N(SiMe3)(2); M = K, R = OAr) gave M(Tp(Me2)) as the major product. Alternatively, the mono-Tp bis(aryloxide) derivatives [Ln(Tp(Me2))(OC6H2-2,6-'Bu-4-Me)(2)] (Ln = La (4); Nd (5)) were obtained in high yields by salt metathesis of [Ln(OC6H2-2,6-'Bu-4-Me)(3)] with one equiv. of K(Tp(Me2)). (C) 2004 Elsevier Ltd. All rights reserved.

Evolution of rheological properties and local structure during gelation of siloxane-polymethylmethacrylate hybrid materials

Hybrid siloxane-polymethylmethacrylate (PMMA) nanocomposites with covalent bonds between the inorganic (siloxane) and organic (polymer) phases were prepared by the sot gel process through hydrolysis and polycondensation of 3-(trimethoxysilyl)propylmethacrylate (TMSM) and polymerization of methylmethacrylate (MMA) using benzoyl peroxide (BPO) as initiator. The effect of MMA, BPO and water contents on the viscoelastic behaviour of these materials was analysed during gelation by dynamic rheological measurements. The changes in storage (G') and loss moduli (G), complex viscosity (eta*) and phase angle (6) were measured as a function of the reaction time showing the viscous character of the sot in the initial step of gelation and its progressive transformation to an elastic gel. This study was complemented by Si-29 and C-13 solid-state nuclear magnetic resonance (NMR/MAS) measurements of dried gel. The analysis of the experimental results shows that linear chains are formed in the initial step of the gelation followed by a growth of branched structures and formation of a three-dimensional network. Near the gel point this hybrid material demonstrates the typical scaling behaviour expected from percolation theory.

Photoluminescence in disordered Zn2TiO4

In this work, the polymeric precursor method Was used to obtain disordered Zn2TiO4 powders, either Undoped or doped with Sn4+, Cr3+ and V5+, to be applied its photoluininescent material. The characterization was undertaken by means of thermal analysis (TG and DTA), X-ray diffraction (XRD), infrared spectroscopy (IR) and photoluminescence (PL). Previous works stated that titanate octahedra containing a short Ti-O distance show efficient luminescence at roorn temperature if these octahedra are isolated from each other. In the present work, the phenomenon was observed in condensed octahedra, sharing edges. The room temperature PL noticed in undoped Zn2TiO4 had its intensity increased by the dopant addition-the increase was of about 300% for V5+ doping 400% for Cr3+ and 800% for Sn4+. (c) 2005 Elsevier B.V. All rights reserved.

THEORY OF NON-STEADY-STATE ELECTRICAL CHARACTERISTICS OF METAL-INSULATOR-METAL STRUCTURES WITH SCHOTTKY BARRIERS AND UNIFORMLY DISTRIBUTED INTERFACE IMPURITY STATES

The metal-insulator (or amorphous semiconductor) blocking contact is still not well understood. In the present paper, we discuss the non steady state characteristics of Metal-lnsulator-Metal Structure with non-intimate blocking contacts (i.e. Metal-Oxide-Insulator-Metal Structure). We consider a uniform distribution (in energy) of impurity states in addition to impurity states at a single energy level within the depletion region. We discuss thermal as well as isothermal characteristics and present expressions for the temperature of maximum current (T-m) and a method to calculate the density of uniformly distributed impurity states. The variation of mobility with electrical field has also been considered. Finally we plot the theoretical curves under different conditions. The present results are closing into available experimental results.

Thermal studies of solid 4-chlorobenzylidenepyruvates of heavy lanthanides(III) and yttrium(III)

Solid-state compounds of general formula LnL(3).2H(2)O, where Ln is heavier trivalent lanthanides and yttrium, L is 4-chlorobenzylidenepyruvate have been synthetised.On heating these compounds decompose in steps. They lose the hydration water in the first step and the thermal decomposition of the anhydrous compounds occurs with the formation of oxochloride (Eu, Gd); mixture of oxide and oxochloride that decrease with increasing of atomic number of metal (Tb-Tm); or oxide (Yb, Lu, Y) as final residue, up to 900degreesC. The dehydration enthalpies found for terbium, holmium, ytterbium and yttrium compounds were: 34.93, 42.40, 57.39 and 62.24 kJ mol(-1), respectively.

Thermal studies on solid compounds of 4-chlorobenzylidenepyruvate of some alkali earth metals

Solid-state M-4-Cl-BP compounds, where M stands for bivalent Mg, Ca, Sr, Ba and 4-Cl-BP is 4-chlorobenzylidenepyruvate, have been synthesized. Simultaneous thermogravimetry-differential thermal analysis (TG-DTA), differential scanning calorimetry (DSC), infrared spectroscopy, elemental analysis and complexometry were used to characterize and to study the thermal behaviour of these compounds. The results led to informations about the composition, dehydration, thermal stability and thermal decomposition of the isolated complexes.

Thallium-arene contacts in a rare yttrium tris(pyrazolyl)hydroborate ate complex

A salt elimination reaction between [YCl3(THF)(3.5)] and 1 or 2 equiv. of Tl(Tp(Ms*)) [Tp(Ms*) = HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)(-)] leads in both cases to single metathesis, giving a mixture of the mono-Tp(Ms*) complex [YCl3(Tp(Ms*))Tl] (1) and another complex, [YCl2(Tp(Ms* *))] (2) [Tp(Ms* *) = HB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)(2)(-)], that results from the transfer of a second mesityl group to the 5-position of the pyrazolyl ring. The solid-state structure of 1 shows a unique ate dimeric structure with the TV cations coordinated by two mu(2)- and two mu(3)-bridging Cl atoms as well as two eta(3)-mesityl ligands. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).

Gelation of siloxane-poly(oxypropylene) composites

The viscoelastic properties of siloxane-poly(oxypropylene) (PPO) nanocomposites prepared by the sol-gel process has been analyzed during gelation by dynamic rheological measurements. The changes of storage and loss moduli, complex viscosity and phase angle has been measured as a function of time showing the newtonian viscosity of the sol in the initial step of gelation, and its progressive transformation to a viscoelastic gel. The rheologic properties have been correlated to mass fractal, nearly linear growth models and percolation theory. This study, completed by quasi-elastic light scattering and Si-29 solid state nuclear magnetic resonance measurements, shows that the mechanisms of gelation of siloxane-PPO hybrids depend on the molecular weight of the polymer and on the pH of the hybrid sol. For hybrids prepared in acid medium, a polymerization involving silicon reactive species located at the extremity of the polymer chains and presenting a functionality f = 2 occurs, forming a fractal structure during the first stage of sol-gel transition. For samples prepared under neutral pH, the fractal growth is only observed for hybrids containing short polymer chains (M-w similar to 130 gmol(-1)). The fractal dimensionality determined from the change in the rheological properties, indicates that the fractal growth mechanism changes from reaction-limited to diffusion-limited aggregation when the molecular weight of the PPO increases from 130 to 4000 gmol(-1) and as catalyst conditions change from acidic to neutral. Near the gel point, these hybrid gels have the typical scaling behavior expected from percolation theory. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Synthesis, characterization and thermal studies on solid compounds of 2-chlorobenzylidenepyruvate of heavier trivalent lanthanides and yttrium(III)

Solid-state compounds of general formula LnL(3)center dot nH(2)O, where Ln represents heavier lanthanides and yttrium and L is 2-chlorobenzylidenepyruvate, have been synthesized. Chemical analysis, simultaneous thermogravimetry-differential analysis (TG-DTA), differential scanning calorimetry (DSC), X-ray powder diffractometry, elemental analysis and infrared spectroscopy have been employed to characterize and to study the thermal behaviour of these compounds in dynamic air atmosphere.On heating these compounds decompose in four (Gd, Tb, Ho to Lu, Y) or five (Eu, Dy) steps. They lose the hydration water in the first step and the thermal decomposition of the anhydrous compounds up to 1200 degrees C occurs with the formation of the respective oxide, Tb4O7 and Ln(2)O(3) (Ln=Eu, Gd, Dy to Lu and Y) as final residue. The dehydration enthalpies found for these compounds (Eu, to Lu and Y) were: 65.77, 55.63, 86.89, 121.65, 99.80, 109.59, 131.02, 119.78, 205.46 and 83.11 kJ mol(-1), respectively.

Photoluminescent BaMoO4 nanopowders prepared by complex polymerization method (CPM)

The BaMoO4 nanopowders were prepared by the Complex Polymerization Method (CPM). The structure properties of the BaMoO4 powders were characterized by FTIR transmittance spectra, X-ray diffraction (XRD), Raman spectra, photoluminescence spectra (PL) and high-resolution scanning electron microscopy (HR-SEM). The XRD, FTIR and Raman data showed that BaMoO4 at 300 degrees C was disordered. At 400 degrees C and higher temperature, BaMoO4 crystalline scheelite-type phases could be identified, without the presence of additional phases, according to the XRD, FTIR and Raman data. The calculated average crystallite sizes, calculated by XRD, around 40 nm, showed the tendency to increase with the temperature. The crystallite sizes, obtained by HR-SEM, were around of 40-50 nm. The sample that presented the highest intensity of the red emission band was the one heat treated at 400 degrees C for 2 h, and the sample that displayed the 'highest intensity of the green emission band was the one heat treated at 700 degrees C for 2 h. The CPM was shown to be a low cost route for the production of BaMoO4 nanopowders, with the advantages of lower temperature, smaller time and reduced cost. The optical properties observed for BaMoO4 nanopowders suggested that this material is a highly promising candidate for photoluminescent applications. (C) 2005 Elsevier B.V. All rights reserved.

Synthesis, characterization and thermal behaviour of solid 4-methoxybenzoates of heavier trivalent lanthanides

Solid-state Ln-4-MeO-Bz compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and 4-MeO-Bz is 4-methoxybenzoate, have been synthesized. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA), differential scanning calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy and complexometry were used to characterize and to study the thermal behaviour of these compounds. The results led to information about the composition, dehydration, phase transition, coordination mode, structure, thermal behaviour and thermal decomposition of the isolated compounds. The phase transition observed in the some compounds has been reported for the first time. (C) 2006 Elsevier B.V. All rights reserved.

EQCM study during lithium insertion/deinsertion processes in Nb2O5 films prepared by polymeric precursor method

The electrochemical quartz crystal microbalance (EQCM) technique was used to study two chemically distinct Nb2O5 electrochromic thin films (one pure and the other lithium-doped) during the lithium electroinsertion reaction. In the initial cycles, the electrode showed an irreversible mass variation greater than expected for Li+ insertion/deinsertion processes, which was attributed to the wettability effect (allied to the porous morphology) that emerged as the dominant process in apparent electrode mass changes. As the cycles progressed, the mass variation stabilized and the changes in apparent mass became reversible, showing a good correlation with the charge variations.The results generally indicated that the Li+ insertion/deinsertion process occurred more easily in the Nb2O5-doped film, which also displayed a greater capacity for Li+ insertion. However, a total mass/charge balance analysis revealed that the stoichiometry of the Li+ solid state insertion/deinsertion reaction was similar in the two electrodes under study. © 2005 Elsevier B.V. All rights reserved.

The Role of the Eu3+ Concentration on the SrMoO4:Eu Phosphor Properties: Synthesis, Characterization and Photophysical Studies

SrMoO4 doped with rare earth are still scarce nowadays and have attracted great attention due to their applications as scintillating materials in electro-optical like solid-state lasers and optical fibers, for instance. In this work Sr1-xEuxMoO4 powders, where x = 0.01; 0.03 and 0.05, were synthesized by Complex Polymerization (CP) Method. The structural and optical properties of the SrMoO4:Eu3+ were analyzed by powder X-ray diffraction patterns, Fourier Transform Infra-Red (FTIR), Raman Spectroscopy, and through Photoluminescent Measurements (PL). Only a crystalline scheelite-type phase was obtained when the powders were heat-treated at 800 A degrees C for 2 h, 2 theta = 27.8A degrees (100% peak). The excitation spectra of the SrMoO4:Eu3+ (lambda(Em.) = 614 nm) presented the characteristic band of the Eu3 + 5L6 transition at 394 nm and a broad band at around 288 nm ascribed to the charge-transfer from the O (2p) state to the Mo (4d) one in the SrMoO4 matrix. The emission spectra of the SrMoO4:Eu3+ powders (lambda(Exc.) = 394 and 288 nm) show the group of sharp emission bands among 523-554 nm and 578-699 nm, assigned to the D-5(1)-> F-7(0,1and 2) and D-5(0)-> F-7(0,1,2,3 and 4), respectively. The band related to the D-5(0)-> F-7(0) transition indicates the presence of Eu3+ site without inversion center. This hypothesis is strengthened by the fact that the band referent to the D-5(0)-> F-7(2) transition is the most intense in the emission spectra.

Synthesis, characterization and thermal behaviour of solid 2-methoxybenzoates of trivalent metals

Solid-state Ln(L)(3) compounds, where Ln stands for trivalent Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y and L is 2-methoxybenzoate have been synthesized. Simultaneous thermogravimetry and differential thermal analysis (TG-DTA), differential scanning calorimetry (DSC), X-ray powder diffractometry, infrared spectroscopy and complexometry were used to characterize and to study the thermal behaviour of these compounds. The results provided information on the composition, dehydration, coordination mode, structure, thermal behaviour and thermal decomposition.