Effect Of A-site Substitution Research Articles

Effect of A-site substitution by Ca or Sr on the structure and electrochemical performance of LaMnO3 perovskite

The effect of A-site substitution by Ca or Sr on the crystal structure and electrochemical performance of LaMnO3 (LMO) perovskite have been investigated. La0.85Ca0.15MnO3 (LCM) keeps the same orthorhombic structure with the parent LMO composition, while La0·85Sr0·15MnO3 (LSM) transforms to rhombohedral structure. The A-site substituted compositions with more oxygen vacancies are more beneficial to the oxygen intercalation than the parent LMO sample according to the modified anion-intercalation mechanism, and the electrochemical performance is significantly optimized. LCM and LSM electrodes exhibit a specific capacitance of 140.5 mF cm−2 and 129.0 mF cm−2 at 0.5 mA cm−2 in an aqueous electrolyte, respectively, which are four times larger than that of LMO. LCM electrode yields the highest capacitance behavior because of the finer morphology, lower ion diffusion resistance, higher concentration of oxygen vacancy and fuller utilization of the perovskite bulk structure. In addition, LCM//LCM symmetric supercapacitor exhibits an energy density of 29.7 μWh.cm−2 at a powder density of 500 μW cm−2. Our work indicates that A-site substituted compositions by Ca or Sr exhibit great potential in the supercapacitor applications, and further optimization is expected.

Study of A-site disorder dependent structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3

This paper reports the studies on the effect of A-site substitution by strontium on the structural properties and magnetic ordering in polycrystalline perovskite Sm0.5Ca0.5−xSrxMnO3 (x = 0.1, 0.2 and 0.3). The investigated samples are prepared by conventional solid-state reaction technique. XRD analysis at room temperature has confirmed orthorhombic structure of the sample with space group Pnma. The dependence of structural parameter, Curie temperature and coercivity on Sr doping content has been thoroughly investigated. It is observed that substitution of Sr2+ for Ca2+ increases lattice parameter, tolerance factor and the Curie temperature. However, the coercivity (Hc) decreases with increasing Sr content while the charge ordering process is weakened with increasing Sr content. Field cooled (FC) and zero-field cooled (ZFC) dc magnetizations measurements at low field and low temperature indicate that there is a spin-glass (SG) like state occurred. Temperature dependent ac susceptibility at different frequency indicates a spin-glass-like transition of the sample.

Effect of A-site substitution on the simultaneous catalytic removal of NOx and soot by LaMnO3 perovskites

The simultaneous catalytic removal of NOx and soot by La1−xMxMnO3 (M = K, Sr, Pr; x = 0, 0.1, 0.2, 0.3) perovskites was investigated.

A-site substitution effect on crystal structure and properties of Nd1−xAxMn0.5Fe0.5O3−δ (A[dbnd]Ca, Sr, Ba; x = 0, 0.25)

The effect of A-site substitution by alkaline earth metals (ABa, Sr, Ca) on the crystal structure and properties of the Nd1−xAxMn0.5Fe0.5O3−δ (x = 0, 0.25) perovskite-like oxides were studied in the temperature range of 25–1000 °C in air. The Nd1−xAxMn0.5Fe0.5O3−δ samples were synthesized by the citrate-nitrate combustion technique. X-ray powder diffraction (XRPD) patterns were refined by the Rietveld method using orthorhombic unit cell for NdMn0.5Fe0.5O3−δ and Nd0.75Ba0.25Mn0.5Fe0.5O3−δ (sp. gr. Pnma); and monoclinic unit cell for Nd0.75Ca0.25Mn0.5Fe0.5O3−δ and Nd0.75Sr0.25Mn0.5Fe0.5O3−δ (sp. gr. P21/n). The refinement results for the single-phase samples revealed that the unit cell volume decreased while decreasing the size of A-dopant in Nd1−xAxMn0.5Fe0.5O3−δ. Oxygen content and oxidation state of Mn in Nd1−xAxMn0.5Fe0.5O3−δ increased in the row: Nd0.75Ca0.25Mn0.5Fe0.5O2.81, Nd0.75Sr0.25Mn0.5Fe0.5O2.85, Nd0.75Ba0.25Mn0.5Fe0.5O2.93 at all temperature studied. Oxygen non-stoichiometry of the Nd1−xAxMn0.5Fe0.5O3−δ samples was almost independent of temperature even at high temperatures except for NdMn0.5Fe0.5O3−δ. The thermal expansion coefficient (TEC) for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ was found to be 10.1 × 10−6 K−1 in the range of 25–800 °C. Temperature dependencies of total conductivity possessed semiconductor-type behavior and were interpreted within the small polaron hopping mechanism. The maximum value of conductivity (25 S/cm) was obtained for Nd0.75Ba0.25Mn0.5Fe0.5O3−δ at 1000 °C in air. The Seebeck coefficient values decreased with temperature from positive to negative values indicating that both electrons and electron holes may contribute to the electric conductivity of the oxides.

The effect of ion doping at different sites on the structure, defects and multiferroic properties of BiFeO3 ceramics

The polycrystalline samples of undoped BiFeO3, Ce doped BiFeO3 (Bi0.9Ce0.1FeO3) and Zr dpoped BiFeO3 (BiFe0.9Zr0.1O3) were prepared by rapid liquid phase sintering technique. The effects of A-site substitution with Ce and B-site substitution with Zr on the structure, defects and multiferroic properties of BiFeO3 ceramics were comparative investigated. Undoped and doped BiFeO3 samples are well crystallized in perovskite structure, whilst Ce and Zr doping results in slight distortion in the lattice structure. With Ce and Zr doping, the average grain sizes decrease. Positron annihilation lifetime measurements reveal that there are vacancy-type defects in the three samples, and Bi0.9Ce0.1FeO3 sample has the maximum open volume and concentration of the vacancy defect among those three samples. Both Ce and Zr doping can improve the leakage current, ferroelectric and magnetic properties, whereas Zr doping exhibited better ability. The better electric properties in BiFe0.9Zr0.1O3 may be due to the effective reduction of oxygen vacancies, Fe2+ and impurity phases by Zr doping. The cause for better magnetic properties in BiFe0.9Zr0.1O3 sample may be that the insertion of Fe vacancies and nonmagnetically active Zr4+ ions in antiferromagnetic Fe sublattice can effectively suppress the spiral spin modulation structure.

Exchange couplings calculation in A2CrMoO6(A= Sr, Ca) Double Perovskites

Exchange couplings in A2CrMoO6 (A = Sr, Ca) Double Perovskites have been calculated for the first time by using a Monte Carlo Simulation and a relation between internal energy per site and magnetization per site. This work shows how effects of A-site substitution could be captured by an overall scaling factor 'alpha', which allows to deduce a numerical estimation of exchange couplings in question. Thermal dependence of magnetization per site, magnetic susceptibility, internal energy per site and specific heat have been investigated. Super Exchange coupling has been found equal to −7,488meV in the presence of Sr and −1,977meV in the presence of Ca. Double Exchange couplings corresponding respectively to Cr and Mo sublattices have been found equal to 17.473meV and 5.611meV in the presence of Sr and 4.614meV and 1.481meV in the presence of Ca. Internal energy per site for both materials at low temperatures has been given.

Low-temperature NO decomposition through microwave catalysis on BaMnO3-based catalysts under excess oxygen: Effect of A-site substitution by Ca, K and La

The low-temperature NO decomposition with high efficiency remains a grand challenge. Herein, we report an attractive approach for the low-temperature NO decomposition by microwave catalysis over BaMnO3 and substituted Ba0.8A0.2MnO3 (ACa, K, La) catalysts. Importantly, impressively high NO conversion and N2 selectivity with 99.9% and 99.9%, respectively, were achieved over Ba0.8K0.2MnO3 catalyst at 250°C. Comparatively, under identical conditions in the conventional reaction mode, the highest NO conversion and N2 yield are respectively only 45.4% and 28.7% for Ba0.8K0.2MnO3 at 650°C. Unexpectedly, the effect of Ca, K and La substitution is different in these two reaction modes. The catalysts were characterized by H2-TPR, O2-TPD, and microwave absorbing properties to illustrate possible reasons causing such obvious differences in catalytic performance. The apparent activation energies for BaMnO3, Ba0.8Ca0.2MnO3, Ba0.8K0.2MnO3 and Ba0.8La0.2MnO3 catalysts under microwave irradiation drop down to as low as 33.4, 41.6, 14.7 and 18.2kJ/mol, separately, suggesting a significant microwave catalytic effect.

Effect of A-site substitution in perovskites: Catalytic properties of PrMnO3 and Ba/K/Ce substituted PrMnO3 for CO and PM oxidation

ABO3 type perovskite based catalysts with PrMnO3 composition and its A site substituted variants Pr0.8A0.2MnO3+δ/−δ (A=Ba or K or Ce) have been synthesized by sol–gel method. A systematic study has been carried out to understand the change in physicochemical properties, catalytic properties and related mechanistic aspects of the A site substituted ABO3 type perovskite materials with the help of XRD, BET-SA, SEM, XPS and H2-TPR techniques. These studies have inferred the altered oxidation states of B site cation Mn, as well as change in overall redox properties of PrMnO3, as a function of A site substitution. On the other hand, to study the impact on catalytic properties, these catalysts have been evaluated for two different reactions namely, CO and PM oxidation. The substitution/promotion of alkali metal (K) at A site of the PrMnO3 decreases its catalytic activity towards CO oxidation. However, Ba and Ce substitution in PrMnO3 enhanced the CO oxidation activity. The catalytic activity for PM oxidation was prominently enhanced with the substitution of K at the A site of PrMnO3 as well as with that of Ba and Ce substitutions. XRD, BET-SA, XPS and TPR data enabled to correlate the material properties with the catalytic activity, pointing out the role of substitution element with different valence state. The structure–property correlation for these two different reactions therefore, brings out certain findings about the role of substituted cation in PrMnO3 perovskite type catalysts.

The effect of A-site substitution by Sr, Mg and Ce on the catalytic performance of LaMnO3 catalysts for the oxidation of vinyl chloride emission

Catalytic oxidation of vinyl chloride (VC) emission was carried out over LaMnO3 and La0.8A0.2MnO3 (A=Sr, Mg and Ce) perovskite oxides synthesized via co-precipitation method. Numerous characterization techniques were performed to investigate the relationship between the catalytic performance and its physicochemical properties. It was found that the partial substitution of lanthanum by cerium and magnesium had a positive effect on the catalytic performance for VC oxidation, whereas strontium involved a negative effect. Under the reaction conditions (VC concentration=1000ppm, GHSV=15,000h−1), the overall catalyst ranking in terms of the catalytic activity from the best to the worst performance was La0.8Ce0.2MnO3>La0.8Mg0.2MnO3>LaMnO3>La0.8Sr0.2MnO3 with regard to the temperature of T50 and T90. The Ce-doped perovskite catalyst showed the optimum catalytic performance due to its higher specific surface area and its ability to promote the low-temperature reducibility. Moreover, as the active species, the increased surface adsorbed oxygen was also responsible for the enhancement of the catalytic performance.

Self-propagating High-temperature Synthesis of La1−xSrxMnyFe1−yO3 Perovskites for Soot Combustion Catalyst

AbstractThe La1−xSrxMnyFe1−yO3 samples of perovskite-type oxides were synthesized by Self-propagating High-temperature Synthesis (SHS) method. In this study, first, we synthesized B-site substituted perovskite-type oxides, LaMnxFe1−xO3. The SHS method successfully synthesized LaMnxFe1−xO3. The most active composition for soot combustion among synthesized samples of LaMnxFe1−xO3 was x = 0.4, LaMn0.4Fe0.6O3, from model-soot combustion experiment with TG/DTA analysis. We then examined the effect of A-site substitution by Sr on LaMn0.4Fe0.6O3 perovskite. The SHS of La1−xSrxMn0.4Fe0.6O3 was successfully ignited, propagated and completed. XRD peaks of perovskite-type oxide phase was detected in all the synthesized La1−xSrxMn0.4Fe0.6O3 but unreacted material, SrCO3, was also detected in the sample above x = 0.6 from XRD analysis. The soot combustion activity of Sr-doped LaMn0.4Fe0.6O3 was not enhanced according to the impurity in the samples. The most active composition among synthesized samples of La1−xSrxMnyFe1−yO3 was LaMn0.4Fe0.6O3 which exhibited lower activation energy than conventional Pt/Al2O3 catalyst, that is, LaMn0.4Fe0.6O3 synthesized by SHS method had superior soot combustion activity despite its relatively small specific area.

Influence of Ce3+ on dielectric properties of tungsten bronze structured PBN ceramics

The effect of A-site substitution of Ce3+ in tungsten bronze structured PBN ceramics with the general formula, Pb(x − 3y/2)CeyBa(1 − x)Nb2O6 and the stoichiometric chemical formula, Pb(0.65 − 3y/2)CeyBa0.35Nb2O6, where y = 0, 2, 4, 6, 8 and 10 mol% ceramic compositions synthesized through solid state reaction method are reported. The X-ray diffraction studies exhibited the presence of an orthorhombic phase, and its intensity increased with the increasing Ce3+ content up to y = 6 mol% or A3 composition. The lattice parameters, unit cell volume and density as a function of Ce3+ concentration are discussed. It is observed that increasing Ce3+ content in A-site influenced the dielectric properties. The optimum dielectric properties of room temperature dielectric constant (eRT) and dielectric maximum (\( \varepsilon {T} _{{\text{c}}} \)) are observed in y = 6 mol% or A3 composition while Curie temperature (Tc) and dielectric loss (tan δ) constantly decreased from undoped to y = 10 mol%, and thus A3 composition or 6 mol% Ce modified tungsten bronze structured-PBN could be suitable for capacitor applications.

Effect of A-Site Substitution of Calcium on Zr-Rich Lead Zirconate Titanate

Effect of calcium substitution on the A-site of Zr-rich lead zirconate titanate Pb(Zr 0.94 Ti 0.06 )O 3 and Pb(Zr 0.92 Ti 0.08 )O 3 is studied. It is found that the rhombohedral structure of Zr-rich lead zirconate titanate changes to tetragonal and finally to cubic with increasing amount of calcium substitution. Both the dielectric constant and the Curie temperature decrease with the substitution of calcium on the A-site of the ABO 3 perovskite structure. A pyroelectric current peak is observed which is resulted from the phase transition from the low temperature rhombohedral ferroelectric phase F R ( LT ) to the high temperature F R ( HT ) one. Small amount of calcium substitution increases the peak value and also shifts the peak position to higher temperatures. However, further increase in the calcium content lowers the peak value and finally smears out the peak.

Raman scattering on the B-site order controlled by A-site substitution in relaxor Perovskite ferroelectrics

The control of the B-site order of Pb(Mg1/3Nb2/3)O3 was studied by the partial substitution at the A site by both donor La3+/Bi3+ and acceptor K+1. Different A-site cations and their concentrations were chosen to modify the chemical order at the B site. The Raman scattering results over a wide temperature range (−190–300 °C) confirm that the first order scattering is dominant in phonon bands resulting from both ordered regions and disordered matrix. The degree of the B-site order is estimated by the full width at half maximum of the phonon bands associated with the ordered regions, specifically, the highest frequency breath-type A1g mode of the BO6. It is concluded that the order is enhanced with the donor La3+/Bi3+ doping, while with the acceptor K+1 doping, the order is suppressed. The effect of A-site substitution on the B-site order has thus been elucidated.

Effect of A-Site Substitution and Firing Temperature on Microstructure and Electrical Properties of BaTiO3 Semiconducting Ceramics Fired by Reduction-Reoxidation Method

The effects of A-site substitution and firing temperature on the microstructure and electrical properties of BaTiO3 semiconducting ceramics fired by the reduction-reoxidation method were experimentally investigated. Ba ions were substituted with Ca or (Sr0.5Pb0.5). Various microstructures were obtained by varying the firing temperature in reducing atmosphere. Using these samples, the recovery of the characteristics of positive temperature coefficient of resistivity (PTCR) was evaluated. In particular, the minimum grain size required for the reoxidation of only grain boundaries during annealing in oxygen atmosphere in the cooling process was investigated.

Effect of A Site Substitution on Order-Disorder Transition in Pb(Fe<sub>1/2</sub>Nb<sub>1/2</sub>)O<sub>3</sub>(PFN) and Pb(Sc<sub>1/2</sub>Ta<sub>1/2</sub>)O<sub>3</sub> (PST)

The effect of A site substitution of Pb(Fe1/2Nb1/2)O3 (PFN) and Pb(Sc1/2Ta1/2)O3 (PST) with Na or La on the order-disorder transition and dielectric characteristics was examined. It was clarified that the degree of ordering at the B site did not change by the substitution though it has been reported that in case of Pb(Mg1/3Nb2/3)O3 (PMN), the degree of ordering can be controlled by the A site substitution. The reason may be due to the difference in cation size ratio of B site, that is, Mg:Nb=1:2 for PMN and Fe:Nb and Sc:Ta=1:1 for PFN and PST. By partial substitution of A site by La or Na, the dielectric characteristics of PFN and PST drastically changed from normal ferroelectric to “relaxor ferroelectric”. This indicates that “relaxor ferroelectrics” can not be necessarily derived from the order-disorder state at the B site sublattice as understood so far. The “relaxor ferroelectric” property of Na or La doped PST and PFN would be classified into the same group of (Pb, La) (Zr, Ti)O3 (PLZT) and La doped (Sr, Ba)Nb2O6 (SBN).