Neutron diffraction studies of the magnetic properties of Pr0.5Sr0.5Co1 ? x Mn x O3 solid solutions

The crystal structure and magnetic properties of a system of Pr_0.5Sr_0.5Co_{1 ? x} Mn _x O₃ solid solutions were studied by neutron diffraction and magnetization measurements. It is shown that, at a low manganese concentration, the structure can be described by the I/2a monoclinic space group; with increasing substitution level x the structure becomes orthorhombic. For x > 0.9 the crystal structure is tetragonal at high temperatures and the symmetry is lowered to orthorhombic with lowering the temperature. The substitution of cobalt for manganese leads to the destruction of long-range ferromagnetic order near x ?? 0.25. A transition from the high-temperature ferromagnetic phase to the A-type low-temperature antiferromagnetic phase is observed at x ?? 0.93 in the temperature range 110?C160 K.     

Successive structural phase transitions in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>CoO<Subscript>3</Subscript> in the range 10–1120 K

The crystal and magnetic structures of the Pr_0.5Sr_0.5CoO₃ metallic ferromagnet have been studied using neutron diffraction and synchrotron radiation. Successive structural transitions with the reduction of the crystal symmetry from cubic (space group Pm3m) to rhombohedral (\(R\bar 3c\), ～800 K), orthorhombic (Imma, ～300 K) and, then, to triclinic at ～120 K are detected during cooling from 1120 K. The transition from the orthorhombic system to a phase with a lower symmetry is characterized by a sharp change in the anisotropy of the unit cell, which indicates the partial ordering of the e _g orbitals of cobalt. The accompanying change in the interatomic distances and valence angles give rise to an anomaly in the temperature dependence of the magnetic susceptibility at T ≈ 120 K. The ordered magnetic moment μ_Co ≈ 2μ_B corresponds to the assumption of the intermediate spin state of Co³⁺ ions and the mixture of low- and intermediate-spin states of Co⁴⁺ ions.     

Crystal structure and the magnetic state of Pr0.5Sr0.5Co0.5Fe0.5O3

The magnetic properties and crystal structure of the Pr_0.5Sr_0.5Co_0.5Fe_0.5O₃ compound are studied by neutron and x-ray diffractions using synchrotron radiation. These measurements show that this compound is a dielectric spin glass with a magnetic moment freezing temperature of about 70 K. As temperature decreases in the range 30–95 K, a structure phase transition of the first order occurs with an increase in the symmetry from orthorhombic (space group Imma) to tetragonal (space group I4/mcm). It is assumed that the transition is caused by a change in the 4f electron configuration of the Pr³⁺ ions.     

Crystal electric field effects in Pr0.5Sr0.5CoO3

The energy structure and temperature evolution of the magnetic excitation spectra of Pr_0.5Sr_0.5CoO₃ are studied by inelastic neutron scattering. The measurements are performed on a polycrystalline sample of Pr_0.5Sr_0.5CoO₃ and its non-magnetic analogue La_0.5Sr_0.5CoO₃ on the high intensity time-of-flight spectrometer IN4 (ILL, Grenoble) in the temperature range 10 K < T < 300 K. The crystal electric field parameters in Pr_0.5Sr_0.5CoO₃ are calculated and the splitting scheme of the 4f ground multiplet of Pr³⁺ ions is determined based on the experimental data.     

Structure and magnetization studies of Nd0.5−xPrxSr0.5MnO3 system

The structural and magnetic properties of Nd_0.5−xPr_xSr_0.5MnO₃ (x=0, 0.1, 0.2, 0.3, 0.4 and 0.5) system have been investigated. With the substitution of Pr in Nd_0.5Sr_0.5MnO₃, it shows a gradual structure transformation from the Imma orthorhombic symmetry to the tetragonal I4/mcm phase, and the crystallographic transition remains incomplete, even in Pr_0.5Sr_0.5MnO₃. A large bifurcation between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility has been observed below Curie temperature (T_C), which is characteristic of coexistence of ferromagnetism (FM) and antiferromagnetism (AFM) at low temperature region. The magnetization of Pr_0.5Sr_0.5MnO₃ is larger than that of Nd_0.5Sr_0.5MnO₃, while Nd_0.5Sr_0.5MnO₃ with more CE-type AFM shows larger magnetization than Nd_0.3Pr_0.2Sr_0.5MnO_3, which mixed with CE-type (majority) and A-type (minority) AFM at low temperature, indicating that the magnetization of Nd_0.5−xPr_xSr_0.5MnO₃ system is affected by A-site disorder combined with orbital ordering of A-type AFM and CE-type AFM.     

Magnetic phases in Pr0.5Sr0.5Mn1 ? xCoxO3 (x ≤ 0.5) solid solutions

The magnetic and magnetotransport properties of Pr_0.5Sr_0.5Mn_{1 − x} Co _x O₃ (x ≤ 0.5) solid solutions have been investigated using neutron diffraction methods. The magnetization and electrical conductivity have been measured in magnetic fields up to 140 kOe. It has been established that, during cooling in the temperature range from 160 to 110 K, the compounds of compositions with a cobalt content x ≤ 0.07 undergo a structural phase transition from the high-temperature ferromagnetic phase to the antiferromagnetic phase. A further substitution of cobalt for manganese leads to a stabilization of the inhomogeneous dielectric ferromagnetic state, whereas a state of the cluster spin-glass type has been revealed in compositions with x = 0.15 and 0.20. At x ≥ 0.25, a new magnetic phase with a Curie temperature up to 210 K is formed as a result of the magnetic interaction between manganese and cobalt ions. A magnetic phase diagram of the system under investigation has been constructed.     

Effect of Mo doping on magnetic and transport properties of La0.5Sr0.5CoO3

The substitutional effect of Mo on the magnetic and transport properties of double exchange ferromagnets, La_0.5Sr_0.5Co_1−x Mo_xO₃ (0?x?0.2) has been investigated. Substitution of 10% Mo at the Co-site of La_0.5Sr_0.5CoO₃ decreases the Curie temperature by ∼60 K than that of the parent compound and the long-range ferromagnetic ordering disappears for x?0.2. The Mo-doped samples, however, undergo a transition from the parent metallic state to the insulating state below T_c. The insulating state is found to obey Mott's variable range hopping of conduction. The effect of Mo substitution is attributed to the factors namely, (i) the dilution of magnetic Co sublattice, (ii) the reduction of Co⁴⁺/Co³⁺ ratio resulting in a reduced carrier concentration and (iii) disruption of the intermediate spin structure of Co, namely Co³⁺: t_2g⁵e_g¹.     

Short- and long-range order in La1−xSrxCoO3 and La1−xBaxCoO3

The short- and long-range order correlations of the crystal structure in the distorted perovskites La_1−xSr_xCoO₃ and La_1−xBa_xCoO₃ (0.0?x?0.5) have been studied by the neutron powder diffraction (NPD) and the Co K-edge X-ray absorption spectroscopy (XAS) measurements. The results of XAS and NPD indicate a local distortion around the Co³⁺ ions in LaCoO₃ at room temperature. The substitution of the La³⁺ ions by the Sr²⁺(Ba²⁺) ions leads to a gradual increase of the Co-O-Co angle and is accompanied by an increase of the mean square relative displacement (MSRD) of the Co-O bond. These results correlate with an increase of the oxygen amplitude vibration in the direction perpendicular to the Co-O bond. The possible explanation of the observed changes of the crystal and electronic structures in the above-mentioned cobaltites is discussed.     

Phase transformations and magnetotransport properties of the Pr0.5Sr0.5Co1 ? xMnxO3 system

The structural, magnetic, and magnetotransport properties of Pr_0.5Sr_0.5Co_{1 − x} Mn _x O₃ (x < 0.65) perovskites are studied by magnetization and electrical conductivity measurements in magnetic fields up to 14 T and by neutron diffraction. In the manganese concentration range x < 0.5 and T = 300 K, the crystal structure is described by monoclinic space group I2/a; at x > 0.5, it is described by orthorhombic space group Imma. When the temperature decreases, a structural transformation without changing the symmetry takes place in all compounds. This transformation is caused by an active role of the inner shells of the praseodymium ion in chemical bond formation. The substitution of manganese for cobalt breaks a long-range ferromagnetic order near x ≈ 0.25, and a metal-dielectric transition occurs at x ≈ 0.15. The negative magnetoresistance is found to be maximal near a critical manganese concentration, where a long-range magnetic order is broken; it reaches 95% in a field of 14 T at T = 10 K for x = 0.2. An unusual dielectric magnetic state with a small spontaneous magnetic moment and a sharp transition into a paramagnetic state at T > 200 K is revealed in the concentration range 0.30 ≤ x ≤ 0.65 in spite of the absence of coherent magnetic neutron scattering. A model is proposed to explain the behavior of the magnetic properties in this phase.     

X-ray photoelectron spectroscopy analysis of (Ln1−xSrx)CoO3−δ (Ln: Pr,Nd and Sm)

The chemical states of the surface of (Ln_0.5Sr_0.5)CoO_3?δ (Ln (lanthanides) = Pr, Nd and Sm) used for cathode materials of intermediate temperature operating solid oxide fuel cells (IT-SOFCs) were investigated by X-ray photoelectron spectroscopy (XPS). Oxygen peaks comprised of lower binding energy (LBE) and higher binding energy (HBE) peaks from (Ln_0.5Sr_0.5)CoO_3?δ and Pr_0.3Sr_0.7CoO_3?δ (PSC37) showed that some merged oxygen peak behavior is a function of the Sr and lanthanide concentrations. By investigating the oxygen peaks, it was determined that more oxygen vacancies were generated on the surface of the cathodes when the lanthanides and Sr were substituted into perovskite oxides. When comparing the binding energies (BEs) of PSC37 with Pr_0.5Sr_0.5CoO_3?δ (PSC55), the LBE and HBE of the Sr peaks both increased when Sr was substituted at the A-site of a perovskite. Surface analysis of the Co peak on the surface of the cathode materials showed that the Co exists mainly as Co³⁺ and partially oxidized to Co⁴⁺ on the cathode materials. The partial existence of Co⁴⁺ can provide some polaron hopping providing electronic conduction for the solid oxide fuel cell.     

Magnesium,a promising dopant for the improvement of the CMR properties of perovskite manganites

The study of the substitution of magnesium for manganese in the type I Pr_0.7(Ca,Sr) _0.3MnO₃ and type II Pr_0.5Sr_0.5MnO₃ manganites has been performed. Remarkable colossal magneto resistance (CMR) properties have been evidenced for the manganites Pr_0.7Ca_0.2Sr_0.1 Mn_{1 x}Mg_xO₃, with x ≤ 0.02, for which R_OT/R_7T resistance ratio values ranging from 10⁴ to 4.10⁵ at 105 K and 70 K respectively were obtained in a magnetic field of 7 T. The study of the type II phases Pr_0.5Sr_0.5M_{1 x}Mg_xO₃, shows their similarity with the trivalent metal doped manganites Pr_0.5Mn_1?x M_xO₃ with M = Al, Ga, In, in contrast to the tetravalent metal doped manganites with M = Ti, Sn. The latter properties are interpreted in terms of two factors, the molar ratio Mn(III): Mn(IV), and the size of the doping cation.     

Phase transformations in Pr1–x Sr x CoO3

The elastic properties and crystal structure of the Pr_1?x Sr _x CoO₃ system are studied. Two types of crystal structure transitions are found. For the composition x = 0.5, the monoclinic phase transforms to a rhombohedral one in the high-temperature transition (T ≈ 310 K), while the unit cell symmetry remains monoclinic though the unit cell parameters change drastically in the low-temperature transformation (T ≈ 110 K). It is suggested that the high-temperature transition is caused by the dimensional effect, while the low-temperature transition is associated with the presence of praseodymium ions actively involved in chemical bonding.     

Charge correlations in cobaltates La2–xSrx CoO4

We report on an elastic neutron scattering study of the charge correlations in La_2–xSr_x CoO₄ with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La_2–xSr_x CoO₄.

Nano‐phase separation in La_2–xSr_x CoO₄ (schematically). Red areas: undoped La₂CoO₄ islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co³⁺ ions, magnetic Co²⁺ ions and oxygen ions, respectively; green arrows indicate Co²⁺ spins [1, 2].     

Magnetic ordering and magnetoresistive effect in La1−x Srx(Mn1−y Mey)O3 perovskites (Me = Nb, Mg)

Perovskites of composition La_1?x Sr_x(Mn_1?x/2Nb _x/2)O₃ and La_0.49Sr_0.51(Mn_1?y Nb_y)O₃ have been synthesized and investigated. The substitution of nonmagnetic niobium ions for manganese was shown to lead to a transition from the metallic into the insulating state due to a decrease in the number of dissimilar (different-valence) manganese atoms in the lattice. In spite of the high resistivity, the niobium-containing perovskites exhibit a large magnetoresistive effect and ferromagnetic ordering. Small additions of Nb⁵⁺ to La_0.49Sr_0.51MnO₃ stimulate the transition from the antiferromagnetic into the ferromagnetic state, whereas the substitution of Mg²⁺ for Mn stabilizes the antiferromagnetic state. It is supposed that the ferromagnetism in the insulating perovskites at hand is due to the positive superexchange of the Mn³⁺-O-Mn³⁺ type, and the magnetoresistive effect owes to the intergranular transfer of spin-polarized charge carriers and the suppression of magnetic nonuniformities by an applied magnetic field near T _C.     

Sr doping effects on the transport and magnetic properties of GdBaCo2O5+δ

Studies on some new members of the cobalt perovskite Gd_1−xSr_xBaCo₂O_5+δ with low strontium concentrations (0<x<0.1) have been carried out with the aim of investigating possible metallization of the GdBaCo₂O_5+δ system by hole doping. Low temperature electrical resistivity, magnetic susceptibility and thermopower of the above system have been studied. The pristine compound with x=0 and δ∼0.5 exhibits a semiconductor-like behavior and two magnetic transitions below room temperature. Upon Sr²⁺ substitution, there is a fall in resistivity by 2-3 orders of magnitude at low temperature and also a dramatic reduction in the ferromagnetic to antiferromagnetic transition temperature. These changes can be explained on the basis of hole doping (and increase in the Co⁴⁺content). Evidence for an increase in Co⁴⁺ with Sr²⁺ substitution is provided by iodometric analysis.     

Valence changes of manganese and praseodymium in Pr1−xSrxMn1−yInyO3−δ perovskites upon cation substitution as determined with XANES and ELNES

Systematic valence changes in Pr_1−xSr_xMn_1−yIn_yO_3−δ upon cation substitution with Sr²⁺ and In³⁺ have been found using Mn K-edge and Pr L-edge X-ray absorption, and Mn L_II,III and Pr M_IV,V electron energy-loss spectroscopy. The average valence of the praseodymium ions is close to +3.0 and virtually constant over the sample set when the samples also contained manganese ions. Pr_0.5Sr_0.5InO_3−δ showed a distinct increase in the praseodymium valence state. In contrast, the average valence of the manganese ions changed from the trivalent state to intermediate values between +3.0 and +4.0 and approached the tetravalent state depending on the level of substitution. The knowledge of the valence is required to understand the conduction mechanisms in the material due to the small polaron hopping (electronic conductivity) and motion of oxygen ions along the vacancies (ionic conductivity). Addition of strontium and indium led to the formation of oxygen vacancies. A previously assumed intermediate valence of praseodymium as causal factor for the higher oxygen catalytic activity cannot be confirmed with room temperature measurements.     

Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

The valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ has been investigated by an analysis of the Co-L₃ X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L₃ XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr_1−xCa_xCoO_3−δ and Pr_1−xSr_xCoO_3−δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.     

Performance and stability of niobium-substituted Ba0.5Sr0.5Co0.8Fe0.2O3 − δ membranes

The phase stability, thermal expansion, electrical conductivity, and oxygen permeation of perovskite-type oxides Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_1 − xNb_xO_3 − δ (x = 0 − 0.2) have been investigated. Room-temperature X-ray diffraction of as-prepared powders indicates that in the investigated compositional range solid solutions are formed. Long-term annealing experiments both in flowing air and nitrogen, at 750 °C, demonstrate that the phase instability observed in parent Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3 − δ (BSCF) is suppressed already at the minimum substitution of 5 mol% of niobium for (Co, Fe). Both electrical conductivity and thermal expansion are found to decrease with increasing niobium concentration, which behaviors can be explained by defect chemical considerations, taking into account charge compensation mechanisms by doping BSCF with Nb⁵⁺ donor cations. The oxygen permeation flux of 10 mol% Nb-substituted BSCF, in the range 800-900 °C, is reduced by 10% relative to that found for parent BSCF. Switching from helium to a CO₂-containing purge gas results in a severe reduction or cessation of the oxygen flux. Options are discussed to avoid undesired formation of surface carbonates.     

Microstructure of LaCoO3 prepared by freeze-drying of metal-citrate precursors revealed by EPR

Two methods for the preparation of LaCoO₃ samples were used: thermal decomposition of La-Co citrate precursors obtained by freeze-drying of the corresponding solutions and by a solid state reaction. Microstructural characterization was made by electron paramagnetic resonance spectroscopy (EPR). For assignment of the EPR signals, La_1−xSr_xCoO₃ samples were used as EPR references. The LaCoO₃ oxides prepared from citrates and by a solid state reaction were shown to differ in respect of the mean oxidation state of the cobalt ions, the specific surface area and the particle morphology. EPR spectroscopy reveals for ex-citrate LaCoO₃ ferromagnetic Co³⁺ and Co⁴⁺ coupled ions. For LaCoO₃ samples obtained by a solid state reaction, EPR permits detecting Co₃O₄ impurities only.