Crystal structure and physicochemical properties of doped lanthanum manganites

Substituted lanthanum-strontium manganites La_0.7Sr_0.3Mn_0.9Me_0.1O_{3 ± ??} (Me = Ti, Cr, Fe, and Cu) are obtained by standard ceramic and glycerin-nitrate techniques. High-temperature powder X-ray diffraction is employed to study the crystal structure of La_0.7Sr_0.3Mn_0.9Me_0.1O_{3 ± ??} oxides. It is shown that in the range 298?C1023 K in air, La_0.7Sr_0.3Mn_0.9Me_0.103 ± ?? manganites crystallized in an orthorhombic cell (space group R-3c). The isobaric temperature dependences of unit cell parameters are determined. Thermal expansion coefficients are calculated for La_0.7Sr_0.3Mn_0.9Me_0.10_{3 ± ??} oxides. The conductivity of La_0.7Sr_0.3Mn_0.9Me_0.10_{3 ± ??} is studied as a function of temperature in the range 500 K ?? T ?? 1200 K in air. It is shown that substituting 3d metal for manganese considerably lowers the conductivity of basic La_0.7Sr_0.3Mn_0.9O_{3 ± ??}. The chemical stability of iron-substituted manganite La_0.7Sr_0.3Mn_0.9Fe_0.1O_{3 ± ??} is studied with respect to the electrolyte material.     

Synthesis of rod-shaped La0.7Sr0.3MnO3 by a simple solid state ceramic route

The manganese oxides La_0.7Sr_0.3MnO₃ with rod shape have been synthesized by a simple solid state ceramic method. The morphology and magnetic properties of La_0.7Sr_0.3MnO₃ have been studied by XRD, SEM, TEM and VSM. The diameters of rod-shaped oxide were ranging from 180 nm to1.5 μm and the length was averaging 20 μm. The ratio of length to diameter could reach more than 35:1. The magnetic results showed that the Curie temperature of rod-shaped La_0.7Sr_0.3MnO₃ is far lower than single crystal.     

Correlation between structure and magnetic properties of Cd-substituted La0.7(Ca0.3−xCdx)MnO3 CMR manganites

Structural, electrical and magnetic properties of Cd-doped La_0.7(Ca_0.3−xCd_x)MnO₃ (0?x?0.3) manganites are presented. All compositions were indexed in the orthorhombic (Pnma) space group, except the Cd_0.3 sample, indexed as a combination of trigonal and orthorhombic (Pnma) space groups. Substitution of Ca by Cd has a strong influence on the magnetic and magnetoresistive properties of these compounds, continuously decreasing both the magnetic moment and the Curie temperature (from 3.5 μ_B and 270 K for the x=0 composition to 1.59 μ_B and 90 K for the fully doped x=0.3 one). Samples corresponding to x=0 and 0.1 show a semiconductor-metal transition at temperatures close to the Curie ones. The measured magnetoresistance change is about 49% at 270 K and 95% at 165 K for those samples, respectively. However, the x=0.2 and 0.3 compositions show insulating behaviour in the whole temperature range studied, with values of the magnetoresistance about 85% at 105 K and 74% at 90 K, respectively. The observed weakening of the double-exchange mechanism as the Cd doping level in these samples increases is discussed in terms of structural properties, cationic disorder and Mn³⁺/Mn⁴⁺content ratio.     

Synthesis, structures and magnetic properties of n=3 Ruddlesden-Popper compounds Ca4Mn3−xTaxO10 (0≤x≤0.3)

The structural and magnetic properties of Ta-doped Ca₄Mn_3−xTa_xO₁₀ (0≤x≤0.3) compounds have been investigated. Structural refinement indicates that the Ta doping maintains the orthorhombic layered perovskite structure with space group Pbca as Ca₄Mn₃O₁₀ but induces an increase in both unit cell volume and octahedral distortion. The magnetization measurements reveal that the magnetization first increases and reaches to maximum for the x=0.1 sample and then gradually decreases with the increase of Ta content. There appear short-range ferromagnetic (FM) clusters in all the doped samples, which are caused by the double-exchange interaction between Mn⁴⁺ and Mn³⁺ that is induced by the charge compensation effect. As x is higher than 0.1, the overall results show evidence for the gradual appearance of a cluster glass behavior. When x increases to 0.3, the long-range antiferromagnetic (AFM) ground state is melted into the short-range magnetically ordered regions due to the increase of Ta⁵⁺ and Mn³⁺ at the expense of Mn⁴⁺. The competition between AFM regions and FM clusters makes the short-range magnetic components frustrate when the temperature falls to a frustrating point, and thus cluster glass transition occurs.     

Magnesium substitution in Nd0.7Sr0.3MnO3

Magnesium substitution in Nd_0.7Sr_0.3MnO₃ has been studied by neutron powder diffraction. Polycrystalline samples of nominal compositions Nd_0.7Sr_0.3Mn_1−yMg_yO₃ with y=0.0, 0.1, 0.2 and 0.3 were synthesized by the standard solid-state reaction method. Rietveld refinements of the neutron powder diffraction data showed that all samples had distorted perovskite structure of orthorhombic symmetry. Mg initially preferred to substitute for Nd and only at Mg concentration greater than 0.1, a substantial substitution for Mn occurred. Our study also showed that Mg-substitution did not change the crystal structure of Nd_0.7Sr_0.3MnO₃.     

The Reactivity of Manganites in 18O Isotope Exchange Reactions

The effect of the structural properties and the oxidation state of Mn on the ¹⁸O isotope exchange behaviour of ternary manganites (La_1–xSr_xMnO₃, La_0.5Sr_1.5MnO₄ and SrMnO₃) has been studied. All types of ¹⁸O isotope exchange homomolecular, partially and completely heteromolecular) take place on the very active manganites with perovskite (LaMnO₃ and La_0.7Sr_0.3MnO₃) and perovskite-like (SrMnO₃) structure, but not on the less active K₂NiF₄-structure (La_0.5Sr_1.5MnO₄). The highest ¹⁸O exchange activity is observed for La_0.7Sr_0.3MnO₃, for which the completely heteromolecular ¹⁸O exchange starts to occur at 520 K, already, a T_on which is typical for excellent redox catalysts. The influence of the structural properties on the ¹⁸O exchange and oxygen diffusion behaviour of the manganites is much more pronounced than that of the Mn³⁺/Mn⁴⁺ ratio. The different reduction behaviour of the manganites with perovskite and K₂NiF₄-structure can be explained by means of the bond-valence model.This revised version was published online in November 2005 with corrections to the Cover Date.     

Magnetic behaviour of the orthochromite La0.5Gd0.5CrO3

Magnetic behaviour of the perovskite La_0.5Gd_0.5CrO₃ has been studied. The orthochromite orders canted antiferromagnetically below Neel temperature T_N of ～225 K. Reversal of magnetization is observed in temperature dependence of magnetization measured in field cooled mode under external fields upto 500 Oe. In the field dependence of magnetization below T_N, a small hysteresis is observed with the magnetic anisotropy continuously increasing with lowering of temperature. Estimated values of Cr³⁺ moments, internal field due to sub-lattice of canted ordered Cr³⁺ and the paramagnetic Curie temperature of Gd³⁺ sub-lattice are found to be smaller than reported for GdCrO₃. Compared with Pr substituted analogue La_0.5Pr_0.5CrO₃, Cr³⁺ moment is about the same but the internal field at the Gd³⁺ sub-lattice is much smaller.     

Spin-glass behavior in Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3

With a view to investigate the effect of small size of A-site cations on the magnetic properties of the rare earth cobaltates, Ln_1−xA_xCoO₃ (Ln=rare earth, A=alkaline earth), we have investigated Pr_0.7Ca_0.3CoO₃ and Nd_0.7Ca_0.3CoO₃ in detail. For this purpose, we have carried out low-field DC magnetization and ac susceptibility measurements including a study of magnetic relaxation and memory effects. Both Pr_0.7Ca_0.3CoO₃ and Nd_0.7Ca_0.3CoO₃ show frequency-dependent transitions at 70 and 55 K respectively in the ac susceptibility data, due to the onset of spin-glass like behavior. Their relaxation behavior exhibits aging effects. In addition, memory effects are found in the magnetization behavior. These characteristics establish spin-glass behavior in both these cobaltates, a behavior that is distinctly different from that of La_0.7Ca_0.3CoO₃ and La_0.5Sr_0.5CoO₃ which show well-defined ferromagnetic transitions, albeit without long-range ordering.     

Cobalt spin state above the valence and spin-state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3

(Pr_0.7Sm_0.3)_0.7Ca_0.3CoO₃ belongs to a class of cobalt oxides undergoing a first-order transition (T* ≈90 K) associated to a coupled change in the valence and spin-state degrees of freedom. The Curie–Weiss regime present around room temperature (T >> T*) was analyzed in detail to address the controversial issue of the cobalt spin states above the transition. This magnetic investigation indicates that the Co⁴⁺ are in an intermediate spin-state, while the Co³⁺ are in a mixed state combining low-spin and high-spin states. These results are discussed with respect to the literature on related compounds and recent results of X-ray absorption spectroscopy.     

Effects of magnesium substitution on the magnetic properties of Nd0.7Sr0.3MnO3

Effects of magnesium substitution on the magnetic properties of Nd_0.7Sr_0.3MnO₃ have been investigated by neutron powder diffraction and magnetization measurements on polycrystalline samples of composition Nd_0.7Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3MnO₃, Nd_0.6Mg_0.1Sr_0.3Mn_0.9Mg_0.1O₃, and Nd_0.6Mg_0.1Sr_0.3Mn_0.8Mg_0.2O₃. The pristine compound Nd_0.7Sr_0.3MnO₃ is ferromagnetic with a transition temperature occurring at about 210 K. Increasing the Mg-substitution causes weakened ferromagnetic interaction and a great reduction in the magnetic moment of Mn. The Rietveld analyses of the neutron powder diffraction (NPD) data at 1.5 K for the samples with Mg concentration, y=0.0 and 0.1, show ferromagnetic Mn moments of 3.44(4) and 3.14(4) μ_B, respectively, which order along the [001] direction. Below 20 K the Mn moments of these samples become canted giving an antiferromagnetic component along the [010] direction of about 0.4 μ_B at 1.5 K. The analyses also show ferromagnetic polarization along [001] of the Nd moments below 50 K, with a magnitude of almost 1 μ_B at 1.5 K for both samples. In the samples with Mg substitution of 0.2 and 0.3 only short range magnetic order occurs and the magnitude of the ferromagnetic Mn moments is about 1.6 μ_B at 1.5 K for both samples. Furthermore, the low-temperature NPD patterns show an additional very broad and diffuse feature resulting from short range antiferromagnetic ordering of the Nd moments.     

First principles study of structure and properties of La- and Mn-modified BiFeO3

First principles calculations have been performed to study the effects of the La³⁺ and Mn³⁺ substitutions in the multiferroic BiFeO₃. The real compositions Bi_1−xLa_xFeO₃ and BiFe_1−xMn_xO₃ with x = 0.0, 0.1, 0.2, 0.3 were modeled by substitution of one, two and three Bi³⁺ or Fe³⁺ by La³⁺ or Mn³⁺ in the orthorhombic BiFeO₃ structure, respectively. Density functional theory within the generalized gradient approximation with Hubbard correction of Dudarev (GGA + U) and plane wave pseudo-potential approach has been used to track the changes that occur in the structural parameters, electronic structure, magnetic, optical and polarization properties of the modified BiFeO₃. The substitution of one Bi³⁺ with La³⁺ increases the band gap energy whereas the augmentation of La³⁺ substitutes decreases it. The substitutions of Fe³⁺ with Mn³⁺ do not change the band gap energy. The calculations predicted larger polarization of the modified BiFeO₃, antiferromagnetism for Bi_1−xLa_xFeO₃ and small ferrimagnetism for BiFe_1−xMn_xO₃. Better multiferroic properties are expected for BiFe_1−xMn_xO₃ materials (x = 0.1, 0.2) due to the increasing polarization and ferrimagnetic behavior. The optical properties were estimated by the calculated imaginary and real parts of the dielectric function. The increase of La³⁺ and Mn³⁺ substitutes lead to lower absorption intensity at energy range 2–7 eV.     

Atomic‐Scale Insights into Surface Lattice Oxygen Activation at the Spinel/Perovskite interface of Co3O4/La0.3Sr0.7CoO3

Surface lattice oxygen in transition‐metal oxides plays a vital role in catalytic processes. Mastering activation of surface lattice oxygen and identifying the activation mechanism are crucial for the development and design of advanced catalysts. A strategy is now developed to create a spinel Co₃O₄ /perovskite La_0.3Sr_0.7CoO₃ interface by in situ reconstruction of the surface Sr enrichment region in perovskite LSC to activate surface lattice oxygen. XAS and XPS confirm that the regulated chemical interface optimizes the hybridized orbital between Co 3d and O 2p and triggers more electrons in oxygen site of LSC transferred into lattice of Co₃O₄ , leading to more inactive O^2? transformed into active O^2?x. Furthermore, the activated Co₃O₄/LSC exhibits the best catalytic activities for CO oxidation, oxygen evolution, and oxygen reduction. This work would provide a fundamental understanding to explain the activation mechanism of surface oxygen sites.     

Electrochemical characteristics, thermal and chemical compatibility in the La0.7Sr0.3CoO3 electrode-γ-BIFEVOX electrolyte system

The electrochemical characteristics and compatibility of components of the electrode-electrolyte system, where the electrolyte is chosen to be γ-BIFEVOX compositions crystallizing in a stable tetragonal phase and the cathode material is chosen to be composite electrodes of composition La_0.7Sr_0.3CoO₃ + Bi₄V_1.7Fe_0.3O_{11 ? δ}, were studied.     

Thermochemistry of La0.7Sr0.3Mn1−xFexO3 solid solutions (0<x<1)

The structure, the energetics and the internal redox reactions of La_0.7Sr_0.3Fe_xMn_1−xO₃ have been studied in the complete solid solution range 0.0<x<1.0. High temperature oxide melt drop solution calorimetry was performed to determine the enthalpies of formation from binary oxides and the enthalpy of mixing. There is a noticeable change in the energetics of the solid solution near x=0.7, which is due to the growing concentration of Fe⁴⁺ at higher Fe/(Fe+Mn) ratio. The balance between different valences of the transition metals, Mn and Fe, is the main factor in determining the energetics of the La_0.70Sr_0.30Fe_xMn_1−xO₃ solid solution.     

Influence of silver doping on the electrical and magnetic behavior of La0.7Ca0.3MnO3 manganites

A systematic investigation of structural, magnetic and magnetotransport behavior of La_0.7Ca_0.3?xAg_xMnO₃ manganites has been undertaken. The X-ray diffraction shows a structural transformation from orthorhombic to rhombohedral with increasing Ag concentration. The undoped and 10% Ag substituted samples exhibit double transition in M–T curves. The electrical resistivity in the entire temperature range is fitted to effective medium approximation and phase separation models. The sign of S changes from negative to positive with increase in Ag doping. The low temperature thermopower data has been fitted to an equation containing diffusion, magnon drag and phonon drag terms. The paramagnetic insulting part of the TEP data has been analyzed using small polaron hopping mechanism.     

Neutron diffraction and magnetic study of the Nd0.7Pb0.3Mn1−xFexO3 (0?x?0.1) perovskites

The effect of Fe doping on the ferromagnetic Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075, 0.1) phases has been studied in order to analyze the double-exchange interaction. The structural and magnetic study has been carried out by neutron powder diffraction and susceptibility measurements between 1.7 and 300 K. The substitution of Fe at the Mn site results in reductions in both the Curie temperature T_c and the magnetic moment per Mn ion without appreciable differences in the crystal structures. All the compounds crystallize in Pnma space group. The thermal evolution of the lattice parameters of the Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0.025, 0.05, 0.075) compounds shows discontinuities in volume and lattice parameters close to the magnetic transition temperature. Increasing amounts of Fe³⁺ reduces the double exchange interactions and no magnetic contribution for x=0.1 is observed. The magnetic structures of Nd_0.7Pb_0.3Mn_1−xFe_xO₃ (x=0, 0.025, 0.05, 0.075) compounds show that the Nd and Mn ions are ferromagnetically ordered.     

Redox behavior and transport properties of La0.5−2xCexSr0.5+xFeO3−δ and La0.5−2ySr0.5+2yFe1−yNbyO3−δ perovskites

The effects of doping the mixed-conducting (La,Sr)FeO_3−δ system with Ce and Nb have been examined for the solid-solution series, La_0.5−2xCe_xSr_0.5+xFeO_3−δ (x = 0–0.20) and La_0.5−2ySr_0.5+2yFe_1−yNb_yO_3−δ (y = 0.05–0.10). Mössbauer spectroscopy at 4.1 and 297 K showed that Ce⁴⁺ and Nb⁵⁺ incorporation suppresses delocalization of p-type electronic charge carriers, whilst oxygen nonstoichiometry of the Ce-containing materials increases. Similar behavior was observed for La_0.3Sr_0.7Fe_0.90Nb_0.10O_3−δ at 923–1223 K by coulometric titration and thermogravimetry. High-temperature transport properties were studied with Faradaic efficiency (FE), oxygen-permeation, thermopower and total-conductivity measurements in the oxygen partial pressure range 10⁻⁵–0.5 atm. The hole conductivity is lower for the Ce- and Nb-containing perovskites, primarily as a result of the lower Fe⁴⁺ concentration. Both dopants decrease oxide-ion conductivity but the effect of Nb-doping on ionic transport is moderate and ion-transference numbers are higher with respect to the Nb-free parent phase, 2.2 × 10⁻³ for La_0.3Sr_0.7Fe_0.9Nb_0.1O_3−δ cf. 1.3 × 10⁻³ for La_0.5Sr_0.5FeO_3−δ at 1223 K and atmospheric oxygen pressure. The average thermal expansion coefficients calculated from dilatometric data decrease on doping, varying in the range (19.0–21.2) × 10⁻⁶ K⁻¹ at 780–1080 K.     

High-energy-density LiMn0.7Fe0.3PO4 nanorods synthesized by microwave-assisted solvothermal method

Microwave-assisted solvothermal method has been developed for synthesizing LiMn_0.7Fe_0.3PO₄ cathode materials with an olivine structure. The obtained LiMn_0.7Fe_0.3PO₄ nanorods were characterized by X-ray diffraction, scanning and transmission electron microscope, Brunauer-Emmett-Teller surface area measurements, and electrochemical tests. Electrochemical tests clearly indicate that the as-made LiMn_0.7Fe_0.3PO₄ nanorods exhibit two redox activities of Fe³⁺/Fe²⁺ and Mn³⁺/Mn²⁺ couple at galvanostatic charge-discharge process, which is due to the coexistence of Mn²⁺ with Fe²⁺ at 4c sites. The as-synthesized materials have high energy density, excellent rate capability and cycling stability.     

Comparative study regarding the formation of La1-xSrxCrO3 perovskite using unconventional synthesis methods

The synthesis of strontium-doped lanthanum chromite, La_1−xSr_xCrO₃ (x=0.1 and 0.3), used as an interconnect material for solid oxide fuel cells (SOFC), was investigated using two unconventional synthesis methods: (1) organic precursors’ method based on the thermal conversion of complex combination resulted in the oxidation reaction of 1,2-ethanediol by La³⁺, Sr²⁺ and Cr³⁺ nitrates; (2) combustion synthesis based on the exothermic redox reaction of La³⁺, Sr²⁺ and Cr³⁺ nitrates with urea and glycine as fuels. We also used a mixture of urea and glycine as fuel. The samples were characterized by means of thermal analysis and X-ray diffraction.     

Synthesis and structural characterization of LaxSr1−xMnO2.6+δ (0.1<x<0.4) compounds displaying compressed octahedral coordination of Mn

La_xSr_1−xMnO_2.6+δ (x=0.1-0.4) compounds have been obtained by low-temperature annealing of stoichiometric materials in hydrogen. La_0.1Sr_0.9MnO_2.6+δ (δ=0.15) and La_0.3Sr_0.7MnO_2.6, tetragonal (P4/m), and La_0.2Sr_0.8MnO_2.6, pseudo-tetragonal monoclinic (P2/m), structures are isostructural with oxygen-vacancy-ordered Sr₅Mn₅O₁₃ (, c≈a_P). La_0.4Sr_0.6MnO_2.6 shows cubic perovskite structure with disordered oxygen vacancies. In the vacancy-ordered (La_xSr_1−x)₅Mn₅O₁₃ phases four out of five Mn cations are Mn³⁺ and show a typical Jahn-Teller elongated pyramidal coordination while the fifth one Mn^(4−5x)+, in octahedral environment, shows decreasing formal charge from Mn⁴⁺ (x=0) to Mn^2.5+x=0.3. This unusual selective doping of the octahedral site produces structural strain due to increasing size of the Mn^(4−5x)+ and, in the case of (La_0.2Sr_0.8)₅Mn₅O₁₃, the unusual compressed octahedral arrangement of oxygen atoms around it. The coordination geometry implies that either the d_x²-y² orbital is occupied, which would be a rare example of inverted occupancy of e_g orbitals in manganites, or that disordered Mn³⁺ apically elongated MnO₆ octahedra are present with normal electronic configuration , and the observed bond distances are the average of the long and intermediate in-plane Mn-O bonds. Several structural features favor the second case.