Formula La0 Research Articles

Influence of Gd-doping in La0.7Ca0.3MnO3 on its structural and magneto-electrical properties

Bulk samples of gadolinium doped manganites with compositional formula La0.7−xGdxCa0.3MnO3 (x=0.0 and 0.1) were prepared by conventional solid state reaction method. After characterizing the samples by X-ray diffraction, scanning electron microscope and energy dispersive X-ray spectrometer, a systematic investigation of electrical and magneto-transport properties has been investigated. The replacement of La ion by Gd results in a decrease of the metal–insulator transition temperature TMI and the magnetoresistance as well as the resistivity are found to increase. The electrical resistivity in the entire temperature range fit well with the phenomenological percolation model, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions.

Structural Characterization and Photoluminescent Properties of (La1–xSmx)2Ti2O7 Solid Solutions Synthesized by a Sol–Gel Route

AbstractA series of compounds with the general formula (La1–xSmx)2Ti2O7 (0 ≤ x ≤ 1.0) has been synthesized by a sol–gel method and characterized by XRD, Raman spectroscopy, and scanning electron microscopy (SEM). These compounds are structurally isomorphic to perovskite‐type La2Ti2O7 until a substitution rate of x = 0.8. Above this substitution rate (x > 0.8), a biphasic mixture is obtained between the substituted perovskite layered phase and the pyrochlore Sm2Ti2O7 phase. The unusual phosphor (La1–xSmx)2Ti2O7 has been elaborated and its luminescent properties were investigated for low rates of substitution. The emission and excitation spectra were used to study the luminescent properties. The (La1.9Sm0.1)2Ti2O7 powders emit bright red­–orange lightunder UV excitation. The emission properties show thatLa2Ti2O7 is a suitable host where the La3+ ions can be substituted by other trivalent lanthanide ions that induce luminescence properties in the visible region and lead to phosphor materials. The electronic structure calculations of (La1.9Sm0.1)2Ti2O7 shows that this compound is an insulator with a band gap of about 2.77 eV. The valence band mainly consists of oxygen states with hybridized titanium and samarium states; whereas, the conduction band mainly consists of titanium states with hybridized lanthanum and samarium states.

Structural Stability and Catalytic Activity of Lanthanum-Based Perovskites

Perovskite-type oxide materials with a general formula La1−xAxFe1−yCoyO3−δ, where A is an alkaline earth metal Sr or Ba, have been studied as cathode materials for catalytic reduction of oxygen in solid oxide fuel cells (SOFCs), as well as combustion catalysts. In this study, we use a combination of temperature-programmed reduction measurements, X-ray diffraction, carbon black catalytic oxidation measurements, and first-principles, density-functional-theory (DFT) calculations to elucidate the main processes that contribute to the structural stability and catalytic activity for soot oxidation of these materials. In particular, we investigate the dynamics of the structure reconstruction with oxygen loss during the regulated increase of the temperature. The calculations are in good qualitative agreement with catalytic experiments and allow us to identify special combinations of the perovskite chemical composition and local surface structures for which one could expect the highest catalytic activity for the s...

Immobilization of “Mo6+” in Monazite Lattice: Synthesis and Characterization of New Phosphomolybdates, La1−xCaxP1−yMoyO4, Where x=y=0.1-0.9

In the process of high-level nuclear waste (HLW) disposal, monazite, LaPO4 is considered as one of the potential phosphate ceramics to immobilize lanthanides and actinides. HLW along with hazardous long-lived nuclides contains elements such as Zr, Mo, etc. in higher wt%. In the current investigation, an attempt was made to introduce “Mo” into the monazite lattice. The phosphomolybdates of the formula La1−xCaxP1−yMoyO4, x, y=0.1–0.9 were synthesized by a solution route and characterized by powder X-ray diffraction, FT-IR, TGA/DTA, and scanning electron microscopic techniques. The phases with compositions La0.9Ca0.1P0.9Mo0.1O4, La0.8Ca0.2P0.8Mo0.2O4, and La0.7Ca0.3P0.7Mo0.3O4 were found to be crystallized in monazite structure. The composition La0.1Ca0.9P0.1Mo0.9O4 with X=Y=0.9 crystallized in the scheelite structure. The compositions indicate the immobilization of ∼4.1, 8.5, 12.9, and 42.5 at. wt% of “Mo.” With x, y=0.6, 0.5, 0.4, 0.3, and 0.2, along with the monazite phase, formation of a secondary phase LaP3O9 was observed.

Synthesis and Study of a Ce-Doped La/Sr Titanate for Solid Oxide Fuel Cell Anode Operating Directly on Methane

The possibility to introduce cerium in the perovskite-type titanate with formula La0.33Sr0.67TiO3+δ (LST) was investigated. Pure-phased La0.23Ce0.1Sr0.67TiO3+δ (LCST) was only obtained by synthesis at high temperature in reducing (diluted hydrogen) atmosphere. The material exhibits the same orthorhombic symmetry with Immm space group as LST and nearly the same cell volume. When exposed to oxidizing atmosphere at 1200 °C, Ti3+ and Ce3+ oxidation leads to the decomposition of LCST and the growth of several nanoscaled Ce-rich phases, as highlighted by backscattered electron microscopy. Shifting the gas back to a reducing atmosphere, but at lower temperature, only involves partial reversibility, ensuring the presence of nanoparticles of (electro)catalytically active phase within an electronically conducting n-type network. The catalytic tests in methane steam reforming at 900 °C (CH4/H2O = 10/1) show that the properties of the partially decomposed phase are greatly improved, what could make it a promising ano...

High-frequency heating and dynamic susceptibility of small La1 − x Ag y MnO3 + δ particles in the range of the magnetic phase transition

The nanoparticles of the lanthanum-silver manganites with the general formula La1 − x Ag y MnO3 + δ (LAMO) that exhibit the Curie temperature T C in the range 35…50°C, which is important for medical applications, are synthesized and studied. The heating kinetics of the LAMO aqueous suspensions under the action of the high-frequency (HF) magnetic field is experimentally studied and the self-stabilization in the vicinity of the Curie temperature is demonstrated. A sharp increase in the HF-absorption (specific absorption rate) is observed for the materials under study when approaching the Curie temperature from below. This effect is proven by direct measurements of the dynamic magnetic susceptibility in the critical range. The physical interpretations of the HF absorption are based on the hysteresis and oscillations of domain boundaries. It is demonstrated that the materials under study are promising for the medical hyperthermia with controlled heating temperature.

Synthesis and Impedance Spectroscopy of Ga-Doped La<sub>9.33</sub>(SiO<sub>4</sub>)<sub>6</sub>O<sub>2</sub> Apatite Oxide Ion Conductor

Apatite materials are interesting alternative solid oxide fuel cell (SOFC) electrolytes because of their open structure including tunnels for the diffusion of oxide ions and their good chemical stability. However, the conductivity was still relative low when it was used as SOFC in the intermediate operating temperature range. So the apatite-type lanthanum silicates/germanates and doping were widely researched for enhancing conductivity recently. This study reports the influence of Ga doping on conductivity of the apatite phase La9.33(SiO4)6O2 according to the formula La9.33+x/3(SiO4)6-x(GaO4)xO2 where 0  x  2. Samples were prepared by a sol-gel process. The conductivities of all compositions have been obtained by impedance spectroscopy analysis. The conductivity enhancement observed for moderate x values and illustrates further influenced by the tetrahedral cation sublattice and the La vacancies in optimizing the conductivity.

Catalytic properties of La0.8Sr0.2Co0.5M0.5O3 (M Co, Ni, Cu) in methane combustion1

A set of perovskite-type catalysts of general formula La0.8Sr0.2Co0.5M0.5O3 (M = Co, Ni, Cu) were prepared by alanine solution combustion method and used successfully for the methane combustion. The property of these materials were characterized by XRD, and TPR measurements. The effects of transition-metal ions on site B on structure and performance of the catalysts were studied. The result indicated that the structure and catalytic activities of the catalysts can remarkably affect by transition-metal ions on site B, the decrease of the electrons filled in d orbitals in the atom on site B is propitious to increase the catalytic activity for methane combustion, and the sequences of catalysts activities are La0.8Sr0.2CoO3 τ; Ce0.8Sr0.2CoO3 τ; Nd0.8Sr0.2CoO3.

Correlation of Fuel Cell Anode Electrocatalytic and ex situ Catalytic Activity of Perovskites La0.75Sr0.25Cr0.5X0.5O3−δ (X = Ti, Mn, Fe, Co)

The performance of a series of perovskite oxides having the mutual chemical formula and structure La0.75Sr0.25Cr0.5X0.5O3−δ (X = Co, Fe, Ti, Mn) as solid oxide fuel cell anode electrocatalysts depends on the nature of the substituent element X. The electrocatalytic activity for methane oxidation in a fuel cell correlates well with ex-situ temperature programmed catalytic conversion of CH4, X = Co > Mn ∼ Fe > Ti, under temperature programmed reaction conditions in 5% CH4/He. The total conductivity of the materials in air decreases X = Co > Fe > Mn > Ti. Within the series of catalysts, the order of maximum fuel cell power density depended on feed: CH4, X = Fe > Mn > Ti; H2, X = Fe > Mn > Ti; and 0.5% H2S/CH4, X = Ti > Fe > Mn. The Co-containing catalyst was unstable under reducing conditions. A process is proposed to explain the difference in catalyst order and enhanced activities in H2S/CH4 as fuel compared to CH4 alone.

Comparative study of La–Sr–Fe–O perovskite-type oxides prepared by ceramic and surfactant methods over the CH4 and H2 lean-deNOx

Mixed oxides of the general formula La0.8Sr0.2FeO3−x were prepared by ceramic and surfactant methods and tested towards the NO/H2/O2 and NO/CH4/O2 lean-deNOx reactions in the 200–450°C range. The materials were characterized for their bulk and surface composition by XRD, Mössbauer and XPS techniques. The main crystal phases detected were perovskites LaFeO3 and SrFeO3−x, and α-Fe2O3. The Mössbauer technique allowed the precise determination of the composition of Fe-containing phases in the solids prepared by both the ceramic and surfactant methods. Significant differences in the composition of the phases and the surface chemical states of Fe, Sr and La were revealed among the two solid compositions. For the CH4 lean-deNOx reaction at 350°C, the La0.8Sr0.2FeO3−x solid composition (LSF-2) prepared by the surfactant method exhibited an increase by 50% in the rate of NO conversion and by 30% in the rate of N2 formation (per gram basis) as compared to the solid prepared by the ceramic method (LSF-1). An opposite relative catalytic activity behavior was observed for the two solids when tested towards the H2 lean-deNOx reaction. At 375°C (maximum activity observed) the solid prepared by the ceramic method (LSF-1) exhibited an increase by 83% in the rate of N2 formation as compared to the solid prepared by the surfactant method (LSF-2). It is suggested that the mobility of surface lattice oxygen, the concentration of oxygen vacant sites, and the mixed valences of metal cations present in the La0.8Sr0.2FeO3−x solid all influenced by the preparation method used are the main parameters that largely influenced the catalytic behavior observed over the LSF-1 and LSF-2 solids.

Magnetoelectric behavior of sodium doped lanthanum manganites

Nanocrystalline samples of sodium doped manganites with compositional formula La1−xNaxMnO3 (0.025⩽x⩽0.25) were prepared by polyvinyl alcohol assisted precursor method. After characterizing the samples by x-ray diffraction and transmission electron microscopy a systematic investigation of electrical, magnetic, and thermopower properties has been undertaken. The resistivity data were analyzed using effective medium approximation. From the analysis it has been found that the metallic fraction is increasing up to x=0.10 and remains constant with further doping. A close examination of the resistivity data clearly indicates that the sodium doped samples are slowly transformed from colossal magnetoresistance behavior to charge ordering behavior. Thermoelectric power data at low temperatures were analyzed by considering the magnon drag concept, while the high temperature data were explained by small polaron conduction mechanism.

Diffusion Path and Conduction Mechanism of Oxide Ions in Apatite-Type Lanthanum Silicates

Apatite-type lanthanum silicates of general formula La9.33+2x/3(SiO4)6O2+x have appeared recently as a new promising class of oxide ion conductors with potential applications as electrolytes for solid oxide fuel cells (SOFCs). They have been shown to demonstrate relatively high oxide ion conductivity at moderate temperatures as well as at low oxygen partial pressures. In this paper, the diffusion pathways and the conduction mechanism of oxide ions in these phases are reinvestigated. This is done by means of atomic scale computer modeling techniques with both semiempirical and bond valence methods. Our results support that oxide ion conduction along the c-axis proceeds by an interstitial mechanism. They also support the presence of interstitial sites located within the conduction channel. However, contrarily to recent research, it is shown that the channel oxide ions are involved in the conduction process by a push−pull type mechanism. This mechanism brings into play a cooperative movement of both two adjacent interstitial oxide ions forming a complex defect and the channel oxide ions. This complex defect is shown to move along the c-axis via a nonlinear pathway different from the conduction path proposed in literature to date. The calculated migration energy of this mechanism is found to be equal to 0.32 eV, which compares well with activation energy measured along the c-axis for Nd9.33(SiO4)6O2 single crystals.

A comparative kinetic study on the oxidative coupling of methane over LSCF perovskite-type catalyst

A gas-phase heterogeneous kinetics is described over perovskite with formula La0.6Sr0.4Co0.8Fe0.2O3−δ (LSCF) for the oxidative coupling of methane under differential conversion conditions in a microcatalytic fixed-bed reactor. The ethane (C2H6) and carbon oxides (COX) formation and methane conversion rates were obtained as a function of methane and oxygen partial pressure under experimental conditions of: 0.20<PCH4<0.82atm, 0.04<PO2<0.15atm, and 1073 < T < 1173 K. The different kinetic models were examined and two Eley–Rideal mechanisms (a and b) were successfully performed to fit the experimental data into the theoretical relations from which two mechanisms having a first step of reversible adsorption of molecular oxygen on the catalyst surface with a subsequent reaction of methane molecule with the adsorbed oxygen were selected for data analysis. The parity graph of the ethane formation rate and the F-test statistical analysis were indicated mechanism “a” involving the formation of CH3O2 (methylperoxy) intermediate species can be suggested as the favorable (main) model for the analysis of kinetic parameters of OCM reaction over LSCF perovskite catalyst. Furthermore, the kinetic rate constants and activation energies were estimated for the selected models. Eventually the best model was introduced in course of conceptual and statistical analysis.

Local Defect Structures and Ion Transport Mechanisms in the Oxygen-Excess Apatite La9.67(SiO4)6O2.5

Apatite-type oxides of general formula La9.33+x(SiO4)6O2 + 3x/2 have been attracting considerable interest recently because of their observed high oxide-ion conductivity and potential use in solid oxide fuel cells (SOFCs), oxygen sensors, and ceramic membranes. In this paper, computer modeling techniques are used to investigate, at the atomic level, the energetics of defect formation, oxide-ion migration, and cation migration in the oxygen-excess apatite silicate, La9.67(SiO4)6O2.5. Recent research has suggested that oxide-ion conduction in these apatite systems proceeds by an interstitial mechanism. Our results support this view and have revealed how the flexibility of the SiO4 substructure plays a crucial role in facilitating oxide-ion migration: the presence of interstitial oxide ions creates pseudo-“SiO5” units, which can effectively pass along the c direction by oxygen transfer. La vacancy migration is also examined and, as expected, found to have a high energy barrier.

Structural and elastic properties of Ca-substituted LaMnO3 at 300 K

The compositional dependence of structural and elastic properties of Ca2+ substituted manganite perovskite system with a general formula La1−xCaxMnO3 (x = 0.00, 0.25, 0.33, 0.50 and 1.00) have been studied by means of x-ray diffraction, scanning electron microscopy and ultrasonic pulse transmission technique at 300 K. The elastic moduli of the specimens have also been computed and these are corrected to the void free state. The lattice parameters calculated theoretically are in agreement with those determined experimentally. The structural–elastic properties correlations have been studied. The observed enhancement of elastic constants up to x = 0.33 and reduction with a further increase in calcium concentration has been explained in the light of change in the strength of interatomic bonding caused by change in interatomic distances, microstructure and electronic configuration. The applicability of the heterogeneous metal mixture rule for theoretical estimation of elastic constants has been tested.

Effect of Co Content Upon the Bulk Structure of Sr- and Co-doped LaFeO3

The bulk structure was investigated for Fe-based perovskite-type oxides with the formula La0.6Sr0.4CoyFe1−yO3−δ (y = 0.1, 0.2, and 0.3). The materials were confirmed to be stoichiometric with respect to oxygen under ambient conditions and the structural features were then further characterized under different environments as a function of temperature. Under reducing atmospheres, the degree of reduction increased with Co content, suggesting the presence of preferential oxidation of Fe over Co. Under milder conditions, oxygen vacancy formation was not proportional to Co content, which was likely caused by an electronic structure transition. The unit cell parameters were also shown to strongly depend upon Co content, temperature, and environment. A rhombohedral to cubic transition occurred at lower temperatures for higher Co content, but showed less dependence upon environment. A change in the thermal expansion behavior occurred at the temperature where oxygen vacancies formed leading to two regions of linear thermal expansion. The use of lattice parameters compared to dilatometry allowed for the simultaneous monitoring of unit cell symmetry and expansion behavior so the link between thermal properties and unit cell symmetry could be firmly established.

Influence of B site Substituents on Lanthanum Calcium Chromite Nanocrystalline Materials for a Solid‐Oxide Fuel Cell

Highly reactive and nanocrystalline powders of LaCrO3based compositions, having the general formula La0.9Ca0.1Cr1−xMxO3−δ (0≤x≤0.1, and M=Al, Co, or Mg), suitable for solid‐oxide fuel cell (SOFC) applications, have been synthesized using an auto‐combustion technique with ammonium dichromate as the chromium source. Owing to very fine crystallite size (ranging from 10 to 50 nm) and the high reactivity of the powders (surface area as high as 25 m2/g), the sintering temperature reduces drastically and a highly dense, uniform, and fine‐grained microstructure is obtained. A dramatic improvement in densification (nearly theoretical density) is observed for aluminum substitution, when sintered at as low a temperature as 1300°C. The microstructure shows a uniform distribution of grains having an average grain size of ∼0.5 μm. Depending on the substituent, the electrical conductivities of the sintered samples in air, at 1000°C, were found to be in the range of 10–45 S/cm, and are more than that of the values required for SOFC application. The thermal expansion coefficients, as obtained, are also comparable with the other SOFC cell components.

Elastic behavior of La0.67Ca0.33MnO3:ZrO2 composites

Abstract A series of Colossal Magneto Resistance materials, with compositional formula (1 − x) La0.67Ca0.33MnO3 + xZrO2 (where x = 0%, 10%, 20%, 40%, 60%, 80%) were prepared by sol–gel technique. When characterized structurally by X-ray diffraction they are found to have cubic structure. After measuring their bulk densities, the ultra sonic longitudinal (Vl) and shear velocities (Vs) were measured at room temperature using the pulse transmission technique. Using the ultrasonic data, the values of Young's and rigidity moduli along with Poisson's ratio and Debye temperatures have been calculated. As the materials are porous, zero porous elastic moduli have also been arrived at using a well-known model. The observed variation of elastic moduli with varying ZrO2 concentration has been explained qualitatively.

Catalytic Activity for Methane Flameless Combustion and Thermal Stability of Nano-Sized Lanthanum Cobaltites Doped with Ce, Pr and Tb

High surface area nano-sized catalysts with the general formula La0.9M0.1CoO3 (M=Ce, Pr, Tb) were synthesised by a novel flame-spray-pyrolysis method. All the prepared samples possessed the LaCoO3 perovskite-like structure and consisted of 30-60 nm particles, but existing as large (80-200 nm) agglomerates. Their surface area ranged from 45 to 60 m2/g. All the catalysts showed a very high activity for methane flameless combustion, attaining 100% conversion at temperatures Tf, between 495 and 515 0C, depending on the nature of the M doping ion. The results of life-tests did not show any decreasing of catalytic activity after 50 h under the reaction conditions at Tf. The thermal stability under high temperature application was investigated by overheating the catalysts twice at 800 0C for 1 hr in a flowing reacting gas mixture, fast deactivation cycles, and then measuring methane conversion at Tf. All the catalysts showed a significant decrease of activity, depending on the chemical composition. Taking account of the results of SEM analysis, this was attributed to sintering, connected with the Gibbs instability of nano-sized materials.

Structure and electrical transport of some Cd‐doped La0.67Sr0.33MnO3manganites

AbstractA systematic investigation of cadmium‐doped manganites with compositional formula La0.67(Sr0.33–yCdy) MnO3(y= 0–0.33) has been undertaken mainly to understand their structural as well as their electrical transport behavior. The materials were prepared by the citrate‐gel route and were characterized by X‐ray diffraction (XRD). By analyzing the XRD data using the Rietveld refinement technique, it has been concluded that these materials have rhombohedral structure with R$ \bar 3 $c space group. It has been found that the ferro–paramagnetic transition temperature (TC) values decrease continuously with increasing Cd concentration, while the metal–insulator transition temperatures (TP1), except in the cases of two samples (withy= 0.066 and 0.132), are found to decrease. A qualitative explanation for the observed behavior is offered. With a view to understanding the conduction mechanism, electrical resistivity and thermopower measurements were carried out and the data were analyzed using various theoretical models. It has been concluded that the resistivity (ρ) data in the ferromagnetic metallic part (T&lt;TP1) is in agreement withρ(T) =ρ0+ρ2T2+ρ4.5T4.5, indicating the importance of grain/domain boundary effects, electron–electron and two‐magnon scattering processes in the conduction mechanism, while, at high temperature (T&gt;TP1), the paramagnetic insulating regime, the adiabatic small polaron model fits well, thereby indicating that polaron hopping might be responsible for the conduction mechanism. Finally, the low‐temperature thermopower (T&lt;TP1) data were analyzed by considering the spin‐wave fluctuation (∼T4) in addition to the magnon scattering, whereas the high‐temperature (T&gt;TP1) data were analyzed by Mott's equation. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)