Lanthanum Substitution Research Articles

Sinterability and conductivity of barium doped aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5 electrolyte of solid oxide fuel cells

Apatite ceramics are interesting alternative solid oxide fuel cells (SOFCs) electrolytes because of their open structure for the transportation of oxide ions and their good chemical stability. This study reports the influence of barium doping on the microstructure, sinterability and oxide conductivity properties of the aluminium lanthanum oxyapatite La9.5Ba0.5Si5.5Al0.5O26.5. SEM results show that lanthanum substitution with barium improves the sinterability of apatite ceramics. The barium doping also enhances the conductivity of the aluminium lanthanum silicates. The oxygen ion conductivity of La9.5Ba0.5Si5.5Al0.5O26.5 sintered at 1600°C is 2.21×10−2Scm−1 at 800°C, higher than 9.81×10−3Scm−1 of La10Si5AlO26.5 sample prepared under the same conditions. The results in the present study demonstrate that doping Ba on the La site for aluminium lanthanum oxyapatite reduces the sintering temperature and improves the ion conductivity. The enhancement of Ba dopant is mainly on the improvement of the densification and thus substantially reduced grain boundary resistance of aluminium lanthanum oxyapatite particularly at low temperatures

Ferromagnetic Ordering in Lanthanum Substituted Nano-Cobalt Ferrites at Room Temperature

The effect of Fe3+ ions substitution by rare earth La3+ ions on magnetic properties of nano crystalline cobalt ferrite is investigated and reported in this paper. Co-precipitation method was used to prepare ultrafine particles of CoLaxFe2-xO4 (x = 0, 0.10, 0.15). The samples have cubic spinel structure and average crystallite size of x = 0, x = 0.1 and x = 0.15 are 49.84 nm, 27.73 nm and 47.97 nm respectively. Structure has been investigated by XRD and TEM while magnetic properties have been studied using VSM and Mossbauer spectrometer. La3+ ions modulate significantly the magnetic properties of cobalt spinel ferrites.

Effect of lanthanum substitution at A site on structure and enhanced properties of new Aurivillius oxide K0.25Na0.25La0.5Bi2Nb2O9

Aurivillius ferroelectrics K(0.25)Na(0.25)La(0.5)Bi(2)Nb(2)O(9) (KNBN-La) and K(0.25)Na(0.25)Bi(2.5)Nb(2)O(9) (KNBN-Bi) were prepared by using solid-state reaction process. Rietveld refinements for the KNBN-La and KNBN-Bi were carried out by using powder X-ray diffraction at room temperature and they were confirmed to be two-layer Aurivillius oxides with orthorhombic space group A2(1)am. The lattice parameters are a = 5.50468(10) Å, b = 5.49217(10) Å, and c = 25.05108(35) Å for KNBN-La and a = 5.48867(6) Å, b = 5.47895(6) Å, and c = 25.10591(25) Å for KNBN-Bi. Lanthanum (La(3+)) substitution for bismuth (Bi(3+)) led to an enhancement in relaxation behavior for the KNBN-La ceramics, with a ferroelectric to paraelectric phase transition temperature (T(c)) of about 360 °C. The KNBN-La ceramics had a high remnant polarization (P(r)) of 13.6 μC cm(-2) and a field-induced strain of up to 0.031%. Particularly, the decrease in P(r) for the KNBN-La ceramics after 10(8) cumulative switching cycles was only 6%.

Colossal negative thermal expansion in BiNiO3 induced by intermetallic charge transfer

The unusual property of negative thermal expansion is of fundamental interest and may be used to fabricate composites with zero or other controlled thermal expansion values. Here we report that colossal negative thermal expansion (defined as linear expansion <−10−4 K−1 over a temperature range ~100 K) is accessible in perovskite oxides showing charge-transfer transitions. BiNiO3 shows a 2.6% volume reduction under pressure due to a Bi/Ni charge transfer that is shifted to ambient pressure through lanthanum substitution for Bi. Changing proportions of coexisting low- and high-temperature phases leads to smooth volume shrinkage on heating. The crystallographic linear expansion coefficient for Bi0.95La0.05NiO3 is −137×10−6 K−1 and a value of −82×10−6 K−1 is observed between 320 and 380 K from a dilatometric measurement on a ceramic pellet. Colossal negative thermal expansion materials operating at ambient conditions may also be accessible through metal-insulator transitions driven by other phenomena such as ferroelectric orders.

La1−xCaxMnO3 perovskites as catalysts for total oxidation of volatile organic compounds

La1−xCaxMnO3 perovskites were prepared by the citrate method and characterized by means of X-ray diffraction, temperature programmed desorption of oxygen, temperature programmed reduction, and measurement of specific surface area. The characterization results indicated that pure perovskite phases were obtained and their specific surface areas are appropriate for the application in catalytic oxidation. The unsubstituted perovskite, LaMnO3, could present a certain amount of Mn4+ and cationic vacancies, which is generally called oxidative nonstoichiometry. The electronic unbalance yield by the partial substitution of lanthanum by calcium, a cation of lower oxidation state, caused an oxidation state increase of part of the manganese occupying the B sites of the structure. Concurrently, the amount of cationic vacancies decreased with the increase of calcium amount. The catalytic activity was evaluated in the combustion of ethanol and n-hexane. Both reactions would occur by means of a suprafacial reaction mechanism in which the adsorbed oxygen species are relevant.

Ab initio energetics of lanthanum substitution in ferroelectric bismuth titanate

Using first principles calculations and atomistic thermodynamicsthe bulk and defect properties of orthorhombic bismuth titanate(Bi4Ti3O12) and bismuthlanthanum titanate (Bi3.25La0.75Ti3O12) have been investigated. Heats of formation, valid chemical conditions for synthesis,lanthanum substitution energies and oxygen and bismuth vacancy formation energies havebeen computed. The study improves our understanding of how native point defects andsubstitutional impurities influence the ferroelectric properties of these layered perovskitematerials. It is found that lanthanum incorporation could occur on either of the twodistinct bismuth sites in the structure and that the effect of substitution is to increase theformation energy of nearby native oxygen vacancies. The results provide directatomistic evidence over a range of chemical conditions supporting the suggestionthat lanthanum incorporation reduces the oxygen vacancy concentration. Oxygenvacancies contribute to ferroelectric fatigue by interacting strongly with domainwalls, and therefore a decrease in their concentration is beneficial. The conditionsthat favor the greatest reduction in oxygen vacancy concentration are described.

Relation between defects and catalytic activity of calcium doped LaFeO 3 perovskite

Perovskite-type compounds La 1 − xCa xFeO 3 (x = 0.0, 0.1, 0.2, 0.3, 0.4) were prepared by the amorphous citrate method and tested in the catalytic combustion of methane. The solids were characterized by nitrogen adsorption, XRD, TPR and O 2-TPD techniques. It was found that La 0.6Ca 0.4FeO 3 has the highest catalytic activity for methane combustion. Physical properties and catalytic results are interpreted in terms of the defect structure, giving a strong physical–chemical support to the observed results. The higher catalytic activity of the doped perovskites is driven by a fraction of the iron ions in unusual oxidation states (+ 4) which are formed by the partial substitution of lanthanum by calcium. At low substitution levels, x < 0.1, the charge compensation is achieved fundamentally by the oxidation of a part of the iron ions from Fe 3+ to Fe 4+. At higher doping levels instead, the formation of oxygen vacancies account for the main charge-compensating mechanism. The linear dependence of T 50 on the amount of Fe 4+(%) suggest that the active sites for the methane combustion are directly related to this unusual oxidation state of iron.

Improvement in magnetic properties of La substituted BaFe12O19 particles prepared with an unusually low Fe/Ba molar ratio

In this study, effect of lanthanum substitution on the phase composition, lattice parameters and magnetic properties of barium hexaferrite has been studied in samples synthesized in ammonium nitrate melt. Samples, prepared with different lanthanum amount and having various initial Fe/(Ba+La) ratios in between 12 and 2 {(Ba1−xLax)·n Fe2O3, where 0≤x≤1 and 1≤n≤6)}, are sintered at temperatures from 800 to 1200°C. The lattice parameters, both a and c, decreases with increasing La amount which results in a decrease of the unit cell volume. The scanning electron microscope micrographs show that the pure and La-substituted sample with x=0.3, both calcinated at 1000°C, have grain sizes smaller than 1μm. The coercivities of the La-substituted samples increase with increasing La amount and reaches to a maximum value of 5.73kOe, when x=0.3. Sintering at higher temperatures (above 1000°C) decreases the coercivity, resembling a transition from single to multi-domain behavior of the particles, while saturation magnetization of the samples continues to increase due to the increasing grain size. Magnetization measurements of the samples prepared with different Fe/(Ba+La) molar ratios, n's, revealed that the specific saturation magnetization slightly increases with decreasing n, while coercivities fluctuates around 5.5kOe. However, a sharp increase in the saturation magnetization has been observed in the sample having n=1 and washed in HCl. It was measured as 59.2emu/g at 15kOe, which is higher than that of the pure sample (57.5emu/g). Thus, the magnetic parameters are optimized in the sample Ba0.7La0.3Fe12O19 so as to maximize both coercivity and specific saturation magnetization in the HCl-washed sample synthesized by starting with an unusually low Fe/(Ba+La) molar ratio of 2 (or n=1).

Tunability of third order nonlinear absorption in (Pb,La)(Zr,Ti)O3 thin films

This paper reports the demonstration of the tunability of the third order nonlinear optical absorption in (Pb1−3x/2Lax)(Zr0.53Ti0.47) (PLZT) thin films. The optical nonlinearity is investigated employing the open aperture z-scan technique at the off-resonant wavelength of 532 nm. By altering the defect chemistry in PZT through A-site aliovalent lanthanum substitution, the nonlinear optical (NLO) absorption can be effectively tuned. The results of the influence of lanthanum concentration on leakage current and microstructure have also been combined to confirm the defect-mediated mechanism of NLO absorption in PLZT thin films.

Evolution of the Spin Resonance of CeCoIn5as a Function of Magnetic Field and La Substitution

We report the evolution of the spin resonance in CeCoIn$_{5}$ as a function of magnetic field and lanthanum substitution. In both cases, the resonance peak position shifts to lower energy and the lineshape broadens. For La doping, it is found that the ratio $\Omega_{res}/k_{B}T_{c}$ is almost constant as a function of $x$. Under magnetic field the decrease of the excitation energy is similar for H// [1,$\bar{1}$,0] and [1,1,1] and faster than the decrease of $T_{c}(H)$. The Zeeman effect found for the field applied along [1,$\bar{1}$,0] corresponds to the ground state magnetic moment.

In situ XANES cell used for the study of lanthanum strontium cuprate deNO x catalysts

The potential NO x -reduction electro catalyst lanthanum strontium cuprate has been characterized with an in situ X-ray spectrochemical gas reaction cell. In a series of samples with increasing substitution of trivalent lanthanum by divalent strontium ions, the oxidation state of copper increased, maintaining charge neutrality, with the concentration of oxygen vacancies likely increasing at substitution ratios larger than Sr/La > 0.08. During heating in air, the valence of copper ions in the structure increased. Upon exposure to NO at 500 °C the valence of copper ions in a lanthanum strontium cuprate sample decreased, whereas it remained unchanged in the strontium-free lanthanum cuprate sample.

Enhanced dielectric and ferroelectric properties of Pb1−3x/2Lax(Zr0.5Ti0.5)O3 thin films with low lanthanum substitution

Effects of lanthanum (La) substitution (0.003≤x≤0.015) on the dielectric and ferroelectric properties of Pb(Zr0.5Ti0.5)O3 thin films have been investigated. The films were synthesized on the Pt (111)/Ti/SiO2/Si (100) substrates by a sol–gel method. Large dielectric constants of the films are obtained within range of 800–1600 which are almost comparable to those observed in bulk ceramics. The films also show improved remnant polarization values and reduced coercive field values with the increasing addition of La substitution. Our results suggest that low La substitution contributes to enhance film electric properties due to the improvement of non-180° domain wall mobility as well as the stabilization of tetragonal phase.

Structural, Dielectric and Piezoelectric Properties of PLZT (x/60/40) Ceramics

The paper reports the influence of lanthanum substitution on the structural, dielectric and piezoelectric properties of PZT. Polycrystalline samples were synthesized by solid state reaction technique. XRD analysis shows all the samples to be single phase with perovskite structure, initially showing rhombohedral structure and then transforming into rhombohedral. The transition temperature (Tc) was observed to decrease monotonically with increasing x. The results showed that, after poling, the dielectric constant decreased for rhombohedral composition but increased for tetragonal compositions. Electromechanical coupling factor (kp) was found to increase with x and becomes saturate at x = 0.07 mol%. Composition with x = 0.07 gives maximum value of kp = 52% and d33 = 450 pC/N. All the samples depict well defined ferroelectric behavior.

ChemInform Abstract: Effects of Substitution in Perovskites La2-xSrxNiO4-λ on Their Catalytic Action for the Nitric Oxide + Carbon Monoxide Reaction.

Abstract The catalytic activity of the perovskite series La 2− x Sr x NiO 4−γ ( x = 0.00, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.50) has been studied for the reduction of nitric oxide by carbon monoxide in equimolecular mixture. The conversion ( X %) of reactants approaches the value X NO / X CO = 2 at low temperatures and low degree of substitution of lanthanum by strontium but tends to unity at high temperatures and high degree of substitution. This behaviour is due to the change of the rate determining step of the reaction which at low temperature is the oxygen formation through decomposition of nitric oxide while at high temperatures it is the reaction between the adsorbed oxygen species and carbon monoxide. The apparent activation energies for the nitric oxide elimination drop with the process of substitution from x = 0.00 to x = 1.00 and increase thereafter. The controlling factor of the catalytic activity of the series seems to be the strength of adsorption of oxygen on the perovskite surface. Thus it is shown that the sum of the enthalpies of the metal-oxygen bonds in the material is closely related to the temperature where reaction switches mechanism. Besides, the differential mean electronegativity of the solids is related both to the reaction rate as well as to the apparent activation energy for nitric oxide reduction.

Relaxor ferroelectric behavior of La substituted BPZT ceramics

Abstract Barium titanate (BT) based compounds have been of great importance in the fabrication of multilayer ceramic capacitors. Several substitutions are used to tailor its structural, electrical and ferroelectric properties. Substituent like zirconium (Zr) for titanium results in decrease in non-centrosymmetric tetragonal to centrosymmetric cubic phase transition temperature (Curie temperature). Whereas substituent like lead (Pb) for barium results in the increase in the Curie temperature. Here we are reporting effect of lanthanum (La) substitution on the properties of Zr and Pb co-substituted BT. The system Ba 0.80− x La x Pb 0.20 Ti 0.90 Zr 0.10 O 3 with x = 0 and 0.01 was selected for study of structural and dielectric properties. Samples were prepared using solid state ceramic route. X-ray diffraction studies (XRD) were used to confirm single phase structure. Dielectric properties were studied as a function of frequency and temperature. Composition with x = 0.01 was found to show relaxor behavior.

Structural and relaxor behavior of (Ba 1− xLa x)(Ti 0.85Sn 0.15)O 3 ceramics obtained by solid state reaction

The effect of lanthanum (La) substitution on dielectric behavior of BaTi 0.85Sn 0.15O 3 ceramics which exhibits a broad maxima near room temperature in its ϵ ′ vs T plots has been studied. (Ba 1− x La x )(Ti 0.85Sn 0.15)O 3 ceramics with x = 0.00 , 0.01 and 0.03 have been prepared by solid state ceramic method. X-ray diffraction patterns indicated that these ceramics presents a cubic structure at room temperature. (Ba 1− x La x )(Ti 0.85Sn 0.15)O 3 ceramics with x = 0.01 and 0.03 exhibit relaxor behavior. Diffuseness of phase transition was determined by full width at half maxima in ϵ ′ vs T plots, increases with increasing La substitution. (Ba 0.97La 0.03)(Ti 0.85Sn 0.15)O 3 falls under capacitors Z5U category. Hysteresis loop measurement shows that coercive field and remanent polarization both decreases with increase of La in the lattice.

Investigations on the properties of pure and rare earth modified bismuth ferrite ceramics

Pure BiFeO 3 (BFO) and La-modified BiFeO 3 (Bi 1− x La x FeO 3 with x = 0.2 and 0.4) ceramic powders were synthesized at relatively low temperature by ferrioxalate precursor method. Pure compositions did not yield phase pure powders and contain secondary phases. At the same time, La-modification at different concentration levels in BFO promoted the formation of perovskite phase with the elimination of secondary phases and phase pure ceramic powders were obtained for the composition Bi 1− x La x FeO 3 with x = 0.4. Further, the effect of lanthanum substitution on the morphology, electrical and magnetic properties was also investigated.

Combustion Synthesis of Lanthanum Substituted LiNiO2 Using Hexamine as a Fuel

Lithium nickelate and its lanthanum substituted compound have been successfully prepared by combustion synthesis process using LiNO3, Ni(NO3)2.6H2O and La(NO3)3.6H2O. Hexamine is used as fuel. The physicochemical properties of the powders were investigated by thermal analysis (TGA/DTA). The crystalline powders were characterized for their phase identification using x‐ray diffraction analysis (XRD). FT‐IR spectroscopy was used to study the local structure of the oxide environment. The morphological features of the powders were characterized by scanning electron microscopy (SEM). DTA analysis reveals the evolution of an exothermic peak at 465 oC indicating the rapid decomposition of the hexamine and dissociation of nitrate salts, forming the final compound lithium nickealte. The XRD pattern reveals the rhombohedral structure of LiNiO2 with trigonal symmetry comprising of two interpenetrating close packed FCC sub‐lattices. The lattice constant values ̒a̓ and ̒c̓ are in good agreement with the reported data. In the FT‐IR spectra, vibrational bands are identified in the range of 400‐800 cm-1 representing the NiO2 layer. LiNiO2 exhibits a very fine crystalline structure with an irregular morphology. The La substituted LiNiO2 powder has shown a smooth‐edged polyhedral structure with an average particle size of 5‐10 μm.

Enhancement of ferromagnetic and dielectric properties in lanthanum doped BiFeO3 by hydrothermal synthesis

Multiferroic Bi1-x1LaxFeO3 (x = 0.0, 0.1, 0.2, and 0.3) micro-particles were successfully synthesized by a hydrothermal technique. All the samples were phase pure crystallizing in a perovskite structure with a space group of R3c. XRD refinement revealed that the lattice parameters increased along with increase of La content, while the Fe–O octahedra became more distorted. It was found that the morphologies of as-obtained micron-sized particles turned from spheroidal to octahedral according to different doping levels. The dielectric constant of Bi1-x1LaxFeO3 sample increased after La doping, and reached the largest value for the sample of x = 0.2, both in low and high frequency range at room temperature. All the as-prepared samples exhibited magnetic moments starting above room temperature. It was found that the magnetic moment was significantly enhanced from 0.264 emu/g of BiFeO3 to 0.658 emu/g of Bi0.9La0.1FeO3 in a field of 3 T at 77 K. Both enhancements of ferromagnetic and dielectric properties possibly attribute to the changes of lattice parameters and Fe–O bond lengths caused by lanthanum substitution.

Microwave absorption properties of La-substituted M-type strontium ferrites

M-type strontium ferrites with the substitution of Sr2+ by La3+ were prepared by a sol–gel process. The crystalline structure, sublattice structure and microwave absorption properties of Sr1−xLaxFe12O19 (x = 0.05, 0.1, 0.15, 0.2) were studied by X-ray diffraction, Mössbauer spectroscopy and vector network analyzer. The results showed that the single lattice structure of strontium ferrite can be obtained when the substitution amount x < 0.15, with some Fe3+ changes to Fe2+ on 2a site. The minimum reflection loss of the substituted ferrite (x = 0.15) reaches −41.7 dB with the −10 dB bandwidth over 4 GHz at the thickness of 2.3 mm. The microwave absorption performance was remarkably improved due to the grain dimension reduction and the valence change of Fe ions induced by the substitution of lanthanum.