Citrate-gel synthesis and characterization of yttrium-doped multiferroic BiFeO<Subscript>3</Subscript>

Perovskite Bi_1−x Y _x FeO₃ (0.0 ≤ x ≤ 0.1) oxides were prepared by a citrate-gel method. The crystal structure examined by X-ray powder diffraction indicates that the samples were single-phase and crystallize in a rhombohedral (space group, R-3c no. 161) structure. The structural phase transition from rhombohedral to orthorhombic phase was observed at x = 0.10. Increase in magnetization was observed as a result of Y doping. The optical band-gap of (Bi, Y)FeO₃ materials were determined. The observed increase in magnetization and low band-gap of (Bi, Y)FeO₃ ceramics position them for potential magenotoelectric and photocatalytic applications, respectively.     

Effect of strontium deficiency on the critical behavior at paramagnetic to ferromagnetic phase transition in La0.57Nd0.1Sr0.33MnO3 manganite oxide

The samples of manganese perovskite La_0.57Nd_0.1Sr_0.33−x_xMnO₃ (0.00 ≤ x ≤ 0.05) ( is the strontium deficiency) are prepared by solid-state methods, and all of them have a rhombohedral perovskite structure, revealed by X-ray diffraction. The critical properties of the samples around the paramagnetic–ferromagnetic phase transition were investigated through various techniques such as modified Arrott plot (MAP), Kouvel–Fisher (KF) method and critical isotherm (CI) analysis based on the data of static magnetic measurements recorded around the Curie temperature T_C. The experimental results have revealed that the samples exhibited the second-order magnetic phase transition and the critical exponents of β and γ for La_0.57Nd_0.1Sr_0.33MnO₃ are close to those found out by the 3D-Heisenberg model. Furthermore, the estimated critical exponents of La_0.57Nd_0.1Sr_0.33−x_xMnO₃ (x = 0.025 and 0.05) are consistent with the prediction of the 3D-Ising model. We deduced, following the Harris criterion, that the disorder in our case is relevant which can be the cause of the change in the universality class and we noted that the critical exponents β are almost similar to the value of the mean-field theory which can be explained by the existence of a long-range dipole–dipole interaction.     

Magnetic properties of quasi-two-dimensional La1−xSr1+xMnO4 and the evolution of itinerant electron ferromagnetism in the SrO · (La1−xSrxMnO3)n system

Quasi-two-dimensional oxides of the La_1−xSr_1+xMnO₄ system, possessing the K₂NiF₄ structure, show no evidence for ferromagnetic ordering in contrast to the corresponding three-dimensional La_1−xSr_xMnO₃ perovskites. Instead, there is an increasing tendency toward antiferromagnetic ordering with increasingx in La_1−xSr_1+xMnO₄. Furthermore, these oxides are relatively high-resistivity materials over the entire compositional range. Substitution of Ba for Sr in La_0.5Sr_1.5MnO₄ decreases the ferromagnetic interaction. Increasing the number of perovskite layers in SrO · (La_1−xSr_xMnO₃)_n causes an increase in electrical conductivity as well as ferromagnetic interaction. The oxide becomes a highly conducting ferromagnet whenn ≥ 2.     

Synthesis,Crystal Structure and Reducibility of K2NiF4 Type Oxides Sm2—xSrxCuO4

K2NiF4 type compounds Sm2-xSrxCuO4(0≤x≤1.2)were prepared.Rietveld refinement of powder diffraction data shows that the crystal system of these compounds belongs to T′ type tetragonal structure of space group I4/mmm,and the addition of Sr causes a dramatic shift of the O(I) ions along the c axis from Sm(Sr) toward Cu while scarcely affecting the Cu-O(Ⅱ）bond in basal CuO4 phane.From XRD data,it can be seen that when 0.0≤x≤0.6,Sm2-xSrxCuO4 belongs to a singlephase K2NiF4 type structure,while at x≥0.7,both the free SrO phase and the CuO phase exist in these compounds.From the results of XPS and rietveld refinement,it can be seen that after replacing Sm^3 with Sr^2 of lower valence,the valence of Cu and Sm does not change apparently,and thus some defect must be formed at the oxygen ion positions and/or position A on A2BO4 to keep charge balance and to stabilize the structure.     

Phase equilibria and crystal structures of complex oxides in systems La-M-Fe-O (M = Ca or Sr)

Phase equilibria in systems La-M-Fe-O (M = Ca or Sr) at 1100° in air were studied. The homogeneity ranges and structures of solid solutions La_{1 ? x} M _x FeO_{3 ? δ} (0 ≤ x ≤ 0.3 for M = Ca and 0 ≤ x ≤ 0.8 for M = Sr), Sr_{2 ? y} La _y FeO_{4 ? δ} (0.8 ≤ y ≤ 1.0), and Sr_{3 ? z} La _z Fe₂O_{7 ? δ} (0 ≤ z ≤ 0.2) were determined using X-ray powder diffraction. The structural parameters of complex oxides were refined using the full-profile Rietveld technique. Correlations between the unit cell parameters and the compositions of solid solutions were derived. Isobaric/isothermal phase diagrams were constructed for systems La-M-Fe-O (M = Ca or Sr) at 1100°C in air.     

Effect of the A Cation Size on the Structural, Magnetic, and Electrical Properties of Perovskites (La1−xNdx)0.7Sr0.3r003nMnO3

An intense effort has recently been devoted to studying the interplay between structure, magnetism, and transport in manganese perovskite Ln_1−xA_xMnO₃ (Ln=La, Pr, Nd, Sm; A=Ca, Ba, Sr). As a function of temperature, applied magnetic field, doping, A-site ionic radius 〈r_A〉, and A-site size disorder, this system displays a rich phase diagram for both magnetotransport and structural properties. We have investigated the structural, magnetic, and transport properties of (La_1−xNd_x)_0.7Sr_0.3MnO₃. The crystal structure was examined by X-ray powder diffraction which indicated that all the samples were single phase and revealed a transition from rhombohedral to orthorhombic structure with increasing x. The magnetization and resistivity investigation shows that for all values of x, (La_1−xNd_x)_0.7Sr_0.3MnO₃ are ferromagnetic-metallic at low temperatures and paramagnetic-semiconductor above the Curie temperature T_c.     

X-ray spectroscopy of lanthanum manganites: Nature of doping holes,correlation effects,and orbital ordering

The Mn3s X-ray photoelectron spectra of manganites were studied. It was shown that for the formal valence of manganese from 3+ to 3.3+, the doping holes are O2p in character; as the valence of manganese increases further, the Mn3d states acquire holes. For La_0.7Sr_0.3MnO₃, the Mn3p-3d resonance spectra provided information about the occupied and unoccupied Mn3d states, and the correlation energy U = 6.7 eV was determined experimentally. An analysis of X-ray dichroism on the L absorption spectra of three-dimensional La_7/8Sr_1/8MnO₃ showed that the cooperative Jahn Teller distortion of the orthorhombic phase at 240 K was related to (x ² ? z ²)/(y ² ? z ²) type orbital ordering.     

Phase equilibria in the system LaMnO<Subscript>3</Subscript>-SrMnO<Subscript>3</Subscript>-SrCrO<Subscript>4</Subscript>-LaCrO<Subscript>3</Subscript>

A continuous solid solution LaMn_1?y Cr _y O₃ with an orthorhombic structure is found to exist in the range of 0.0 ≤ y ≤ 1.0. An orthorhombic solid solution La_1?x Sr _x CrO₃ exists in the range of 0.0 ≤ x ≤ 0.1. The stability boundaries are determined for the perovskite phase La_1?x Sr _x Mn_1?y Cr _y O₃. An isobaric-isothermal section LaMnO₃-SrMnO₃-SrCrO₄-LaCrO₃ of the system La₂O₃-SrO-Mn₃O₄-Cr₂O₃ in air at 1100°C is designed.     

Structural and optical absorption studies of barium substituted strontium ferrite powder

Attempt has been made to synthesize Ba_xSr_1−xFeO_3−ξ (x = 0–1.0) ferrite powder by decomposition of sol–gel derived oxalate at 800–1000 °C for 5–10 h to study the effect of barium insertion with regard to phase(s), stability, optical behavior, oxidation states of iron, and oxygen deficiency. It is shown that these ferrites possess a perovskite-type cubic phase (a = 3.877–4.020Å, Z = 1, space group Pm3m) for 0.1 ≤ x ≤ 0.94, a mixture of 82% rhombohedral (a_R = 5.666Å and α_R = 59.761°, Z = 2, space group R3c) and 18% hexagonal phases for x = 0.96 and a pure hexagonal (a = 5.689Å, c = 13.944Å, Z = 6, space group P6₃/mmc) phase for x = 1. Barium substitution in SrFeO_3−ξ system leads to lattice expansion, weakening of the metal-oxygen bond, reduction of tetravalent iron ions (as evident from Mossbauer analysis), and decrease of oxygen content. The optical absorption peaks observed in the range 3.17–4.11 eV are attributed to charge transfer transitions from O²⁻ (2p) to Fe (3d) band. The values of optical energy band gap of Ba_xSr_1−xFeO_3−ξ are found to be ∼5.48 and ∼4.04 for x = 0.1 and 1.0, respectively. A stable perovskite-type cubic phase in Ba_xSr_1−xFeO_3−ξ system with significant anion deficiency (ξ = 0.26–0.32) may possibly act as an oxygen permeable membrane.     

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.     

La1-xCaxMnO3纳米颗粒的制备及氧还原催化性能

本文采用溶胶凝胶法制备了一系列不同Ca含量的钙钛矿型氧化物La_1-xCa_xMnO₃(x=0~0.4)纳米颗粒, X射线粉末衍射及精修、扫描电镜表征显示其相纯度和结晶度高, 颗粒平均粒径约40 nm。在0.1 mol·L^-1 KOH水溶液中进行的氧还原电催化性能测试显示, La_0.7Ca_0.3MnO₃样品催化活性最高, 表观电子转移数接近4, 还原电流密度与Pt/C催化剂相当, 而催化稳定性优于Pt/C。进一步研究了La_1-xCa_xMnO₃样品中Mn价态、晶胞参数的改变对氧还原催化活性的影响, 结果表明当x=0.3时, 催化剂中Mn处于混合价态, Mn-O键长适中, 最有利于电催化反应。     

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.     

Hole and Electron Doping of the 4d Transition-Metal Oxyhydride LaSr3NiRuO4H4

Hole or electron doping of phases prepared by topochemical reactions (e.g. anion deintercalation or anion-exchange) is extremely challenging as these low-temperature conversion reactions are typically very sensitive to the electron counts of precursor phases. Herein we report the successful hole and electron doping of the transition-metal oxyhydride LaSr₃NiRuO₄H₄ by first preparing precursors in the range La_xSr_4−xNiRuO₈ 0.5<x<1.4 and then converting into the corresponding La_xSr_4−xNiRuO₄H₄ phases. This is particularly noteworthy as the (Ni/Ru)H₂ sheets in the La_xSr_4−xNiRuO₄H₄ phases are structurally analogous to the CuO₂ sheets in cuprate superconductors and hole doping (Ni^1+/2+, Ru²⁺) or electron doping (Ni²⁺, Ru^1+/2+) yields materials with partial occupancy in Ni/Ru –H 1s bands which are analogous to the partially occupied Cu –O 2p bands present in the CuO₂ sheets of doped superconducting cuprates.     

La1-xCaxMnO3纳米颗粒的制备及氧还原催化性能

本文采用溶胶凝胶法制备了一系列不同Ca含量的钙钛矿型氧化物La1-xCaxMnO3(x=0~0.4)纳米颗粒,X射线粉末衍射及精修、扫描电镜表征显示其相纯度和结晶度高,颗粒平均粒径约40 nm。在0.1 mol.L-1KOH水溶液中进行的氧还原电催化性能测试显示,La0.7Ca0.3MnO3样品催化活性最高,表观电子转移数接近4,还原电流密度与Pt/C催化剂相当,而催化稳定性优于Pt/C。进一步研究了La1-xCaxMnO3样品中Mn价态、晶胞参数的改变对氧还原催化活性的影响,结果表明当x=0.3时,催化剂中Mn处于混合价态,Mn-O键长适中,最有利于电催化反应。     

XAFS characterization of La1−xSrxMnO3±δ catalysts prepared by Pechini’s method

XAFS experiments at the Mn and Sr K-edges were carried out in order to investigate the short-range arrangement of Mn and Sr sites on La_1−xSr_xMnO_3±δ highly doped perovskites (x = 0, 0.2, 0.4 and 0.6). The Mn K-edge EXAFS spectra show a static Jahn–Teller distortion of the MnO₆ for x = 0 and 0.2, which is drastically reduced as x increases. The distortion of perovskite, characterized by the Mn–O–Mn tilt angle, progressively decreases with increasing Sr contents. Sr K-edge results indicated a decrease on the Sr–Mn coordination number upon Sr doping. Based on this and TPD results, a charge compensating mechanism is proposed suggesting a partial Mn oxidation and formation of Mn defect vacancies due to the introduction of Sr.     

Some Factors Influencing Metal-Insulator Transitions

Three problems are studied. (i) The optimal shape of the electronic densities of states of metal and gapped insulator related by a geometrical distortion, for a temperature induced transition between them. (ii) The dependence of valence band width on cation identity and the temperature of the metal Mott-Hubbard insulator transition in the series RENiO₃ (RE = Rare Earth). (iii) Electronic insights into the loss of a Jahn-Teller distortion at Mn during the insulator-metal transition in La_1?xSr_xMnO₃ as x increases but not at Cu for La_2?xSr_xCuO₄.     

Electron doping effects on Sr2FeReO6

We substitute La for Sr in the Sr₂FeReO₆ double perovskite to see how electron doping affects its properties. Sr_2−xLa_xFeReO₆ compounds can be prepared up to x = 0.5. Beyond this value, a secondary phase is formed. The replacement of Sr by La leads to an increase of the unit cell volume and to a rise of the Curie temperature. However, both the saturated magnetic moment and the room-temperature magnetoresistance decrease with increasing the La content. The substituted compounds are magnetically hard and show maxima in the electrical resistivity at field values close to the coercive field. Our structural study reveals a low content of anti-site defects for all samples studied. The average Fe–O bond lengths remain almost constant upon doping whereas the Re–O distances increase as x increases. This suggests that electrons mainly go to the Re orbitals.     

The effect of a cation radii on structural, magnetic and electrical properties of doped manganites La0.6−xPrxSr0.4MnO3

Structural, magnetic and transport properties of La_0.6−xPr_xSr_0.4MnO₃ with x=0.0, 0.03, 0.06, 0.18, 0.3, 0.42, 0.54 and 0.6 are studied. The system exhibits a rhombohedrally distorted perovskite structure for x?0.3. A rhombohedral-orthorhombic (Pnma) structure transition is detected in the doping range from x=0.42 to 0.6. The structure refinement by Rietveld analysis of the X-ray powder diffraction data shows that the average distance Mn-O increases in the rhombohedral phases and decreases in the orthorhombic phases. Results show that the Curie temperature decreases from 374 to 310 K when 〈r_A〉 varies from 1.254 to 1.231 Å. Electrical measurements show that all samples exhibit a metallic to semiconducting transition with increasing temperature. Meanwhile, the size of the resistivity ρ increases near T_C. This phenomenon is interpreted as a gradual bending of the Mn-O-Mn bond angle, with decreasing 〈r_A〉, which causes the narrowing of the electronic bandwidth and the effect of the A-site variance σ².