Oxygen Chemical Diffusion Coefficient in Manganite Thin Films by Isothermal Electric Resistivity Measurements

In this paper we report the determination of oxygen diffusion coefficient for optimally doped manganite thin films of La0.85Na0.15MnO3 (LNMO). The study was performed by means of isothermal conductivity measurements on two different epitaxially oriented thin films grown on SrTiO3 (100) and SrTiO3 (110). The values found for the oxygen diffusion coefficients are around 10-14 cm2s-1. The diffusion seems to be faster for the c-oriented film with respect to the other one (growth on SrTiO3 (110)). The activation energy for the diffusion process is 0.35 eV for the LNMO film on SrTiO3 (100) and 0.66 for the LNMO film on SrTiO3 (110). A possible reason to account for the different behaviour between the two films is proposed.Comment: 20 Pages and 4 Figures. final version to appear in J. Phys. Chem.

Kinetics of MgWO4 Formation in the Solid State Reaction between MgO and WO3

The solid state reaction of MgWO4 formation from MgO (single crystals) and WO3 was investigated in the temperature range 800 - 985 °C under both air and argon atmosphere. The techniques employed for the kinetic study (contact and thermogravimetric methods) allowed to point out that the process follows the linear rate law in the early stages and the parabolic one in the subsequent stages. From inert marker experiments, kinetic determinations under the two different atmospheres and conductivity measurements, it was possible to state that the reaction is governed by a cation counterdiffusion mechanis

On the Mechanism of CaWO4 Formation in the Solid State from CaO and WO3

The formation in the solid state of CaWO4 from CaO (single crystal) or Ca3WO6 and WO3 was investigated by the contact method between 800 and 1000 °C in air and in argon. The results obtained indicate that these reactions are governed by the diffusion of the W6+ and O2- ions. From the CaWO4 structure, along with the lattice disorder predominant in this compound (0 vacancies and 0 interstitials) and the values of ionic mobilities, it was possible to state that the oxygen diffusion, via V0 2·, represents the rate - determining step in the considered reaction

Kinetics of Formation of SrWO4 in the Solid State Reaction Between WO3 and SrCO3

The solid state reaction WO3 + SrCO3 → SrWO4 + CO2 was studied with the reactants in form both of separate pellets and of powder mixtures. As for pellets, by an application of Wagner's method it was possible to state that the governing process is the cation counterdiffusion involving W6+ and Sr2+. Thermogravimetric measurements in the temperature range 663° - 755 °C on equimolar powder mixtures and with SrCO3 in a specially prepared spherulitic form were interpreted using Dünwald- Wagner's equatio

Kinetics and Mechanism of Solid State Reactions in the Agl-TlI System

The kinetics of the solid state reactions AgI + TiI → AgTlI2 (I) Agl + AgTl2I3 → 2 AgTlI2 (II) AgTlI2 + Til → AgTl2I3 (III) are investigated with reactant pellets by the contact method. With the aid of inert marker experiments and transport number determinations it was possible to state that the governing mechanism is the cation counter-diffusion in all cases and that the rate determining step is the diffusion of Tl+. The experimental reaction rate constants agree reasonably with those calculated on the basis of the Wagner's thermodynamic theory

Absence of Long Range Magnetic Order in the La1.4Sr0.8Ca0.8Mn2O7 Bilayered Manganite

In this work we studied, by means of high-resolution neutron diffraction as a function of temperature, the La1.4Sr0.8Ca0.8Mn2O7 bilayered manganite for two different annealing treatments. Out data allowed us to shown, for the first time, the absence of long-range magnetic order in this optimally doped bilayered manganite where the A-site of the structure is doped with equal proportions of different isovalent cations (Ca and Sr). The system, however, presents defined IM transitions which suggest that the transport properties are not linked to the evolution of long-range order and that two dimensional spin ordering in the layers of the perovskite blocks may be sufficient to 'assist' the hole hopping. Possible reason for the suppression of magnetic order induced by the Ca doping is a size effect coupled to the cation size mismatch between the Sr and Ca ions.Comment: 24 pages, 7 figure

Sodium Doped LaMnO3 Thin Films: Influence of Substrate and Thickness on Physical Properties

In this paper we report the results about the synthesis and characterization of optimally doped La1-xNaxMnO3 thin films grown onto SrTiO3 (100), NdGaO3 (100) and NdGaO3 (110) for thickness ranging from 11 to 82 nm. The effect of substrate nature and orientation, film thickness and annealing procedure was investigated in order to optimize their magnetoresistance (MR). We obtained very smooth films displaying MR values greater than 70%, near to room temperature.Comment: 31 pages, 9 figures Final version to appear in J. Phys. Chem.

Unexpected effect of Ru-substitution in lightly doped manganites

In this Communication we report about the unexpected effect of ruthenium doping in sodium ligthly-doped manganites. This effect seems to be in contrast with the usual model applied to describe the effect of this magnetic ion into the manganite structure. We propose a possible compensation mechanism which seems also able to describe other peculiar features encountered in these materials.Comment: 3 pages, 2 Figures to appear in ChemCom