Anelastic relaxation of Pr1/2Ca1/2MnO3 at low and medium frequencies in the 350–500 K range

In previous works (Jornadas SAM 2000; J. Magn. Magn. Mater. 226–230 (2001) 988) the low-frequency anelastic effects of two manganite perovskites of La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃ were studied at 1 Hz and temperatures above those of the metal–insulator transition. An important internal friction peak (P1) appears at 414 and 419 K, respectively, with a modulus variation and no appreciable contribution to electrical resistivity. Another compound Pr_1/2Ca_1/2MnO₃ of the same family has been studied by anelastic mechanical spectroscopy in order to add new data which could elucidate the (P1) peak mechanism. The measurements were made in a sub-resonant forced pendulum (0.1–20 Hz) and in a vibrating reed equipment (kHz range). A stable (P1) peak has been detected at 410 K (1 Hz), with a corresponding modulus step. The appearance of this relaxation peak in the Pr_1/2Ca_1/2MnO₃ compound, and its high activation energy value, similar to the other compounds measured, indicates that the microscopic origin of this relaxation does not correspond to an oxygen jump (characteristic activation energy0.8–0.9 eV).     

Structural and vibrational properties of dilute GaNxAs1−x(P1−x)

We report a comprehensive analyzes of the Fourier transform infrared (FTIR) absorption and Raman scattering data on the structural and vibrational properties of dilute ternary GaAs_1−xN_x,[GaP_1−xN_x] (x<0.03) alloys grown on GaAs [GaP] by metal organic chemical vapor deposition (MOCVD) and solid source molecular beam epitaxy (MBE). By using realistic total energy and lattice dynamical calculations, the origin of experimentally observed N-induced vibrational features are characterized. Useful information is obtained about the structural stability, vibrational frequencies, lattice relaxations and compositional disorder in GaNAs (GaNP) alloys. At lower composition (x<0.015) most of the N atoms occupy the As [P] sublattice {N_As[N_P]}—they prefer moving out of their substitutional sites to more energetically favorable locations at higher x. Our results for the N-isotopic shifts of local mode frequencies compare favorably well with the existing FTIR data.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

Properties of nonstoichiometric Pr0.6−xSr0.4MnO3 as the cathodes of SOFCs

A series of samples Pr_0.6−xSr_0.4MnO₃ (x=0, 0.01, 0.05, 0.1, 0.15, 0.2) were synthesized by a solid state reaction method. Pr deficiency at the A site has a great effect on the properties of Pr_0.6−xSr_0.4MnO₃ as the cathode of SOFCs (solid oxide fuel cells). Compared to the commonly used La_0.6Sr_0.4MnO₃ and La_0.55Sr_0.4MnO₃ cathode, Pr_0.6−xSr_0.4MnO₃ is better in the properties of conductivity, overpotential and impedance. In all the samples, the one with x=0.05, Pr_0.55Sr_0.4MnO₃, revealed the best performance in the measured temperature range.     

Oxygen tracer diffusion in La1 − xSrxCoO3

Tracer diffusion of ¹⁸O in dense, polycrystalline La_1−xSr_xCoO₃ for x = 0.1 has been measured in the temperature range 400 to 600 °C and at 500 °C for x = 0.2 at an oxygen partial pressure of 1 × 10⁵ Pa. Depth profiles were obtained by secondary ion mass spectrometry. The diffusion coefficient for La_0.9Sr_0.1CoO₃ is given by D = (17–247) exp[(−232 ± 8 kJ/mole)/RT] cm²/s. This value is several orders of magnitude lower than D extrapolated from the results for x = 0.2 measured in the 700–900 °C temperature range. One possible explanation for the discrepancy is that the two measurements reflect different diffusion paths. As expected, La_0.8Sr_0.2CoO₃ exhibits a higher diffusivity at 500 °C than does La_0.9Sr_0.1CoO₃.     

Ca1−xYxMnO3 manganites: synthesis and ESR characterization

Ceramic samples of Ca_1−xY_xMnO₃ were synthesized by a liquid-mix method obtaining single phase materials, for 0.1x1, with orthorhombic structure. The cell volume increases with x indicating that changes in the Mn electronic state overcomes the progressive diminishing of the (Ca, Y) cationic radius, r_Ca>r_Y. We observed a continuous broadening of the electronic spin resonance line width with x. This is explained in terms of the increasing orthorhombic distortions. We have measured the DC magnetization for the x=1 compound YMnO₃. Our results are compatible with a G-type antiferromagnet with T_N=44 K.     

Magnetic polarons in Ca1−xYxMnO3

Experimental evidence shows that in the magnetoresistive manganite Ca_1−xY_xMnO₃, ferromagnetic (FM) polarons arises in an antiferromagnetic (AF) background, as a result of the doping with yttrium. This hypothesis is supported in this work by classical Monte Carlo calculations performed on a model where FM double exchange and AF superexchange compete.     

Crossover of critical behavior in La0.7Ca0.3Mn1−xTixO3

Critical behavior of Ti-doped La_0.7Ca_0.3MnO₃ ceramics are studied using magnetization methods. The results show that the paramagnetic–ferromagnetic transition is first order for the undoped sample, then for low-doping samples the mean-field model is suitable, and finally a 3D Heisenberg model is satisfied for higher doping samples. These findings demonstrate that the critical behavior of the magnetic transition for manganites is sensitive to Mn-site doping.     

Structural characteristics and morphology of SmxCe1−xO2−x/2 thin films

Effect of the deposition temperature (200 and 500 °C) and composition of Sm_xCe_1−xO_2−x/2 (x = 0, 10.9–15.9 mol%) thin films prepared by electron beam physical vapor deposition (EB-PVD) and Ar⁺ ion beam assisted deposition (IBAD) combined with EB-PVD on structural characteristics and morphology/microstructure was investigated. The X-ray photoelectron spectroscopy (XPS) of the surface and electron probe microanalysis (EPMA) of the bulk of the film revealed the dominant occurrence of Ce⁴⁺ oxidation state, suggesting the presence of CeO₂ phase, which was confirmed by X-ray diffraction (XRD). The Ce³⁺ oxidation states corresponding to Ce₂O₃ phase were in minority. The XRD and scanning electron microscopy (SEM) showed the polycrystalline columnar structure and a rooftop morphology of the surface. Effects of the preparation conditions (temperature, composition, IBAD) on the lattice parameter, grain size, perfection of the columnar growth and its impact on the surface morphology are analyzed and discussed.     

Phonon Raman scattering in Nd1+xBa2−xCu3O7−δ and Pr1+xBa2−xCu3O7−δ single crystals

The polarized Raman spectra of Nd_1+xBa_2−xCu₃O_7−δ (−0.023≤x≤0.107) and Pr_1+xBa_2−xCu₃O_7−δ (0.01≤x≤0.15) single crystals have been investigated. It was found that the Cu(2) A_g mode softens by 6 cm⁻¹ in Nd 1:2:3 and 4 cm⁻¹ in Pr 1:2:3 as x increases. These frequency shifts cannot be explained by the change in the relevant bond lengths due to Nd(Pr)-substitution for Ba. The variations with x of the two low frequency modes may be affected by change of their hybridization and/or change of their force constants. The linewidths of Ba mode in Pr 1:2:3 are broader than those in Y 1:2:3. This result suggests that the Pr substitution on Ba sites occurred even in a very small value of x. In x(yy) geometry the relative intensity of the Ba and O(4) modes in Nd 1:2:3 is greater than those in Pr 1:2:3. The difference between Nd 1:2:3 and Pr 1:2:3 in the relative intensity of the Ba and O(4) modes may be produced by the chains.     

Magnetocaloric and magnetoresistance properties of La2/3Sr1/3Mn1−xCoxO3 compounds

Structural, magnetic, magnetoresistance and magnetocaloric studies on La_2/3Sr_1/3Mn_1−xCo_xO₃ compounds were reported. The samples were prepared by the conventional ceramic method. X-ray analysis showed the presence of one phase only, in all studied samples. From electrical resistance measurements it was found that the samples show large negative magnetoresistance behavior. The magnetic measurements were performed in a large temperature range, 4.2–750 K and external magnetic fields up to 5 T. The adiabatic magnetic entropy changes, |ΔS|, were determined from magnetization data. Large magnetocaloric effect (MCE) has been obtained in all studied samples.     

High-resolution X-ray diffraction analysis of SnTe/Sn1−xEuxTe superlattices grown on (1 1 1) BaF2 substrates

A set of SnTe/Sn_1−xEu_xTe superlattice (SL) samples with increasing nominal Eu content x up to 0.28 was successfully grown on (1 1 1) BaF₂ substrates by molecular beam epitaxy. A complete structural characterization was performed by triple-axis X-ray diffractometry and reciprocal space mapping. The X-ray results showed that, despite the phase separation that normally occurs for unstrained Sn_1−xEu_xTe layers with x0.02, an SL stack with homogeneous individual layers can be formed for SL samples with a nominal Eu content up to 0.16. No SL satellite peak structure could be identified for samples with x values higher than 0.24. The structural parameters of the individual layers that compose the SL were determined using a best-fit simulation procedure which compared the calculated X-ray spectra to the measured (2 2 2) ω/2Θ scans. The strain information used in the simulation was obtained from the reciprocal space maps measured around the (2 2 4) lattice point.     

The effect of Ru doping on the alkali-doped manganite Pr0.8Na0.2MnO3

Polycrystalline manganites Pr_0.8Na_0.2MnO₃ doped by ruthenium (0.0≤x_Ru≤0.2) were prepared by the sol-gel process. The magnetic field induced metamagnetism was observed to occur with a large resistivity drop at 3 K for x_Ru≤0.02 samples, which can be sorted into the kind of CMR phenomenon. It was found that the 0.01 Ru doping increased T_C and decreased the metamagnetic critical field than that of the undoped sample. The doped manganites show a quick increase in their magnetic moments as x_Ru increases from 0.01 to 0.04, but the larger fraction of Ru doping (0.04≤x_Ru≤0.2) reduces their M and T_C. All the five doped samples have larger magnetic moments than that of the host sample. Ru doping of x_Ru>0.01 results in a rapid disappearance for the observation of long range spin and charge ordering in the samples’ M-T curves, which is characteristic of the undoped sample. It was found that larger low-temperature MR favored decrease in the metamagnetic critical field. Finally, the phase diagram of Ru doping vs. transition temperatures is presented to summarize the experiments.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.     

Impacts of compressive strain on phase diagram of epitaxial Pr0.5Sr0.5MnO3 films grown on LaAlO3 (0 0 1)

Series Pr_0.5Sr_0.5MnO₃ (PSMO) films of thickness ranging from 20 to 400 nm were epitaxially grown on (0 0 1)-oriented LaAlO₃ using pulsed laser deposition method. The biaxial compressive strain effect on phase transition of the films was systematically investigated by both electrical and magnetic measurements. The 60 nm film shows a ferromagnetic metal to antiferromagnetic insulator (FMM-AFI) transition at a temperature of ∼190 K. Such a FMM-AFI transition is depressed as the films become thicker, and finally disappears in the strain-relaxed situation. On the other hand, the Curie temperature is remarkably enhanced (∼50 °C) when the film thickness increases from 60 to 400 nm. These results may yield the possibility to modulate the phase transitions by varying the structural strain.     

Structural and compositional analysis of InBixAsySb(1−x−y) films grown on GaAs(0 0 1) substrates by liquid phase epitaxy

The growth of epitaxial InBi_xAs_ySb_(1−x−y) layers on highly lattice mis-matched semi-insulating GaAs substrates has been successfully achieved via the traditional liquid phase epitaxy. Orientation and single crystalline nature of the film have been confirmed by X-ray diffraction. Scanning electron micrograph shows abrupt interface at micrometer resolution. Surface composition of Bi(x) and As(y) in the InBi_xAs_ySb_(1−x−y) film was measured using energy dispersive X-ray analysis and found to be 2.5 and 10.5 at.%, respectively, and was further confirmed with X-ray photoelectron spectroscopy. Variation of the composition with depth of the film was studied by removing the layers with low current (20 μA) Ar⁺ etching. It was observed that with successive Ar⁺ etching, In/Sb ratio remained the same, while the As/Sb and Bi/Sb ratios changed slightly with etching time. However after about 5 min etching the As/Sb and Bi/Sb ratios reached constant values. The room temperature band gap of InBi_0.025As_0.105Sb_0.870 was found to be in the range of 0.113–0.120 eV. The measured values of mobility and carrier density at room temperature are 3.1×10⁴ cm² V⁻¹ s⁻¹ and 8.07×10¹⁶ cm⁻³, respectively.     

Structure, magnetic and transport properties of La1−xBixMnO3

We have investigated the structural, magnetic and transport properties of La_1−xBi_xMnO₃ samples. As the Bi content increases, a structural transition from rhombohedral to pseudocubic and a magnetic phase transition from ferromagnetic ordering to cluster glass are identified. Metal–insulator (MI) transitions and large magnetoresistance (MR) effects are observed at low Bi doping levels, while insulating behavior of resistivity is found in the whole measured temperature range at high-doping levels. Two distinct ferromagnetic insulating (FI) states are found at low temperatures in this system. One can be suppressed and the other can be enhanced by applying magnetic fields. Possible reasons for the observed structural, magnetic phase transitions and changes of resistivity behavior with Bi doping are discussed.     

Electrochemical behaviour of oxygen at nickel nest cathodic material with catalyst La1−xSrxCoO3

The reduction of oxygen in the three phases gas–liquid–solid using a nickel nest as electrode with a La_1−xSr_xCoO₃ (x=0, 0.3, 0.5, 0.7, 1) series of combined oxides as a catalyst in different electrolytes of acidic H₃PO₄ or alkaline KOH, NaOH or LiOH solution at a fixed optimum oxygen flow-rate at room temperature was studied by the electrochemical method. The electrochemical parameters, such as electron transfer coefficient and exchange current density, were also determined. The La_1−xSr_xCoO₃ series of catalysts was synthesized by a solid-state reaction. XRD was used to confirm the structure of the catalyst. BET, EDS and resistivity measurements were used to investigated the electrochemical behaviour of the cathodic reduction of oxygen in the presence of the catalyst. SEM was also used to inspect the change before and after the reaction with the catalyst. The electron transfer coefficients, β, from the experiments with the various catalysts in the different alkaline electrolytes were determined as follows:

β(in KOH)>β(in NaOH)>β(in LiOH).

It was discovered that La_0.7Sr_0.3CoO₃ as a catalyst has a higher catalytic activity, a higher specific surface area, lower electrical resistivity, better stability and less agglomeration. Therefore, the above catalyst is the best catalyst for oxygen reduction of those studied.     

Crystal structure, electrical and magnetic properties of La1 − xSrxCoO3 − y

The crystal structure and properties of La_{1 − x}Sr_xCoO_{3 − y} with strontium contents ranging from x = 0.1 to x = 0.7 have been studied. The lattice parameters were measured as a function of temperature (4.2–400 K) and the crystal structure was found to change from rhombohedral (at low temperatures and values of x) to cubic. While LaCoO₃ is paramagnetic the oxides in the composition range 0.2 < x < 0.6 are soft ferromagnets. The strontium additions are compensated by the formation of Co⁴⁺ (cobalt ions with one positive effective charge, Co_Co^.) and oxygen vacancies (V_o^..). From the results it is concluded that the relative importance of oxygen vacancies increases with increasing temperature and decreasing oxygen activity. As a result the concentration of electronic charge carriers — and the resultant electrical conductivity — decrease with increasing temperature. The defect structure is discussed and it is concluded that defect associations — probably between oxygen vacancies and strontium ions — and formation of microdomains of perovskite-related phases are important aspects of the overall structure of these perovskite phases.