Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics

Barium lanthanum bismuth titanate (Ba1−(3/2)xLaxBi4Ti4O15, x = 0–0.4) ceramics were fabricated using the powders synthesized via the solid-state reaction route. X-ray powder diffraction analysis confirmed the above compositions to be monophasic and belonged to the m = 4 member of the Aurivillius family of oxides. The effect of the partial presence of La3+ on Ba2+ sites on the microstructure, dielectric and relaxor behaviour of BaBi4Ti4O15 (BBT) ceramics was investigated. For the compositions pertaining to x ≤ 0.1, the dielectric constant at both room temperature and in the vicinity of the temperature of the dielectric maximum (Tm) of the parent phase (BBT) increased significantly with an increase in x while Tm remained almost constant. Tm shifted towards lower temperatures accompanied by a decrease in the magnitude of the dielectric maximum (εm) with an increase in the lanthanum content (0.1 < x ≤ 0.4). The dielectric relaxation was modelled using the Vogel–Fulcher relation and a decrease in the activation energy for frequency dispersion with increasing x was observed. The frequency dispersion of Tm was found to decrease with an increase in lanthanum doping, and for compositions corresponding to x ≥ 0.3, Tm was frequency independent. Well-developed P(polarization)–E(electric field) hysteresis loops were observed at 150 °C for all the samples and the remanent polarization (2Pr) was improved from 6.3 µC cm−2 for pure BBT to 13.4 µC cm−2 for Ba0.7La0.2Bi4Ti4O15 ceramics. Dc conductivities and associated activation energies were evaluated using impedance spectroscopy.

Thermodynamic Properties of LaCrO4, La2CrO6, and La2Cr3O12, and Subsolidus Phase Relations in the System Lanthanum-Chromium-Oxygen

In the system La-Cr-O, there are three ternary oxides (LaCrO4, La2Cr3O12, and La2CrO6) that contain Cr in higher valence states (V or VI). On heating, LaCrO4 decomposes to LaCrO3, La2Cr3O12 to a mixture of LaCrO4 and Cr2O3, and La2CrO6 to LaCrO3 and La2O3 with loss of oxygen. The oxygen potentials corresponding to these decomposition reactions are determined as a function of temperature using solid-state cells incorporating yttria-stabilized zirconia as the electrolyte. Measurements are made from 840K to the decomposition temperature of the ternary oxides in pure oxygen. The standard Gibbs energies of formation of the three ternary oxides are derived from the reversible electromotive force (EMF) of the three cells. The standard enthalpy of formation and standard entropy of the three ternary oxides at 298.15K are estimated. Subsolidus phase relations in the system La-Cr-O are computed from thermodynamic data and displayed as isothermal sections at several temperature intervals. The decomposition temperatures in air are 880 (+/- 3)K for La2Cr3O12, 936 (+/- 3)K for LaCrO4, and 1056 (+/- 4)K for La2CrO6.

Use of Composition-Graded Bi-Electrolyte Cells for Thermodynamic Studies on Lanthanum Aluminates

Innovative bi-electrolyte solid-state cells incorporating single crystal CaF2 and composition-graded solid electrolyte (LaF3) y (CaF2) 1-y (y = 0 to 0.32) were used for measurement of the standard Gibbs energy of formation of hexagonal La0.885Al11.782O19 and cubic LaAlO3 from component binary oxides La2O3 and alpha-Al2O3 in the temperature range from 875 to 1175 K. The cells were designed based on experimentally verified relevant phase relations in the systems La2O3-Al2O3LaF3 and CaF2-LaF3. The results can be summarized as: 5.891 alpha-Al2O3 + 0.4425 La2O3 (A-rare earth)-> La0.885Al11.782O19 (hex), Delta G(f(ox))(degrees)(+/- 2005)/Jmol(-1) = -80982 + 7.313(T/K); 1/2 La2O3 (A-rare earth) + 1/2 a-Al2O3 -> LaAlO3 (cubic), Delta G(f(ox))(degrees)(+/- 2100)/Jmol(-1) = -59810 + 4.51(T/K). Electron probe microanalysis was used to ascertain the non-stoichiometric range of the hexaaluminate phase. The results are critically analyzed in the light of earlier electrochemical measurements. Several imperfections in the electrochemical cells used by former investigators are identified. Data obtained in the study for LaAlO3 are consistent with calorimetric enthalpy of formation and entropy derived from heat capacity data. Estimated are the standard entropy and the standard enthalpy of formation from elements of hexagonal La0.885Al11.782O19 and rhombohedral LaAlO3 at 298.15 K. c 2014 The Electrochemical Society. All rights reserved.

Study of Methane Oxidative Coupling over Ti-La-Li Mixed Oxides

The composition and structures of Li-Ti-La mixed oxides as well as their catalytic activity for methane oxidative coupling have been studied by means of XRD XPS, IR, SEM and so on. The results indicate that by changing x value in Li-La1-xTixO2 oxides phas

MAGNETIC-PROPERTIES OF LANTHANUM MANGANITE AND VALENCE EQUILIBRIA OF MANGANESE

The paramagnetic susceptibility of lanthanum manganite has been measured over a wide temperature range (100-1073 K). On the basis of the thermodynamic equilibria between the various manganese ions with different valence and spin states and the magnetic interactions between the various manganese ions, a semiempirical formula has been proposed to calculate the paramagnetic susceptibilities of lanthanum manganite at different temperatures. The results indicate that most of the discrepancies between the calculated and experimental reciprocal susceptibilities of lanthanum manganite are less than 10% and that the relative contents of the various manganese ions in lanthanum manganite vary with temperature. The relative content of the trivalent manganese ion with a high spin state is dominant over the whole temperature range, while be relative content of the tetravalent manganese ion with a high spin state decreases monotonously with increasing temperature. At 300 K the calculated relative content of the tetravalent manganese ion in lanthanum manganite is about 34%, which is in good agreement with the experimental result (30%). There are some divalent manganese ions present in lanthanum manganite from low temperature to high temperature. The ratio of the relative contents of the tetravalent and divalent manganese ions in the compound varies with temperature. Above 750 K the relative content of the tetravalent manganese ion is less than that of the divalent manganese ion. The variation in the electrical resistivity of lanthanum manganite with temperature has also been interpreted reasonably.

Investigations on the promoting effect of metal oxides on La-V-O catalyst in propane oxidative dehydrogenation

The addition of reducible metal oxides as promoters shows a positive effect on the catalytic behavior of lanthanum vanadate (LaVO4). A C3H6 yield increase of 6.5% is observed at 500 degreesC on molybdenum-promoted LaVO4, which can be attributed to the change of the redox properties, the blocking of the strong oxidation sites of the catalysts and to an increase of the accessibility of the labile oxygen toward the reactant. The influence of the catalyst preparation method and of the Mo loading as well as the additional promoting effect of CO2 in the gas feed was also examined.

Catalysis by some metal oxides modified with phosphate ions

Surface acidity of phosphate modified La2O3,CeO2 and SnO2 has been estimated by titrimetric Method using Hammett Indicators.Mixed Oxides of tin and lanthanum have also been prepared and subjected to phosphate modification.Surface characterizartion of the samples has been carried out using XRD, surface area,thermal analysis and IR spectroscopy. Phosphate content in the samples has been chemically estimated. The catalytic activity for benzylation and esterification reaction has also been investigated.

Catalytic activity of lanthanum oxide for the reduction of cyclohexanone

Lanthanum oxide, La2O3 has been found to be an effective catalyst for the liquid phase reduction of cyclohexanone. The catalytic activities of La2O3 activated at 300, 500 and 800·C and its mixed oxides with alumina for the reduction of cyclohexanone with 2-propanol have been determined and the data parallel that of the electron donating properties of the catalysts. The electron donating properties of the catalysts have been determined from the adsorption of electron acceptors of different electron affinities on the surface of these oxides.

Studies on the catalysis by sulfated mixed oxides of tin with some rare earth elements

A comparative study of acid-base properties and catalytic activity of Sn-La and Sn-Sm mixed oxides and their corresponding sulfate modified analogues are reported in this thesis. The catalytic activity and product selectivity in the decomposition of alcohols are correlated with the acid-base and redox properties of the catalyst systems under study The effect of catalyst preparation, pretreatment and various reaction parameters on the catalytic activity of sulfate modified oxides is investigated in the oxidative dehydrogenation reactions The experimental conditions are optimised to synthesise industrially important organic chemicals viz. 2,6 xylenol, o-cresol, N-methylanilne and N,N-dimethylaniline employing the mixed oxide systems. The effect of sulfate treatment on the catalytic activity of these systems in the alkylation reactions of phenol, anisole and aniline is also investigated and the merits and demerits of sulfate treatment are highlighted.

Cobalt catalysts promoted with cerium and lanthanum applied to partial oxidation of methane reactions

Catalysts of Co/Mg/Al promoted with Ce and La were tested in the catalytic partial oxidation of methane (POM) reaction. The addition of promoters was made by anion-exchange. X-ray diffraction (XRD) confirmed the formation of hydrotalcite phase for precursors. The mixed oxides were characterized as a mixture of Co3O4, periclase (Co, Al)MgO and/or spinel structure (Mg, Co)Al2O4. In the catalytic POM reaction over the promoted catalysts, a reduction in the carbon formation rate was found. (c) 2008 Elsevier B.V. All rights reserved.

Oxygen reduction reaction activity and structure of La-based perovskite oxides

1706 files in 35 folders, containing 388MB. Comprises plots, figures, and manuscripts. The data contains x-ray diffraction patterns and electrochemical data of lanthanum based perovskite oxides (e.g. 9 different perovskite compositions e.g. LaNiO₃, LaCoO₃, LaFeO₃, LaMnO₃, LaCrO₃, LaNi_0.5Co_0.5O₃ and LaNi_0.5Fe_0.5O₃, LaNi_0.5Mn_0.5O₃ and LaNi_0.5Cr_0.5O₃) characterized using rotating ring disk electrodes.

Synthesis and characterization of lanthanum- and yttrium-doped Fe2O3 pigments

MELO, D. M. A. et al. Synthesis and charactezarion of lanthanum and yttrium doped Fe2O3 pigments. Cerâmica, São Paulo, v. 53, p. 79-82, 2007.

Evaluation of rare earth oxides doping SnO2.(Co1/4,Mn3/4)O-based varistor system

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Characterization of dense lead lanthanum titanate ceramics prepared from powders synthesized by the oxidant peroxo method

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Lanthanum-based perovskites obtained by the polymeric precursor method

Cobalt oxides, specially the ones with perovskite structure, are of a high technological interest, due to their interesting optical, electrical and magnetic properties. La(1 -x)Ca(x)CoO(3) powder samples were synthesized by the polymeric precursor method, with x varying from 0 to 0.4. The powder precursors were characterized by TG/DTA, XRD and IR. The TG curves showed several thermal decomposition steps; the first one is ascribed to the loss of water and the remaining steps are related to the combustion of the organic matter. The XRD patterns indicated only the presence of the perovskite phase. Moreover, the structure changes from rhombohedral to cubic, as calcium is added to the perovskite and the calcination temperature increases.