Type La0 Research Articles

Mechanochemically Synthesized Solid Solutions La1−xCexFeO3+x/2 for Activation of Peroxydisulfate in Catalytical Reaction for Tetracycline Degradation

The synthesis of orthoferrites of the type La1−xCexFeO3+x/2, x = 0.00, 0.01, 0.03, 0.05, and 0.07, by applying a simple and effective mechanochemical transformation from the constituent oxides is presented. Physicochemical methods such as powder X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–Vis spectroscopy, and Brunauer–Emmett–Teller (BET) adsorption were applied to gain information about the effect of Ce4+ content on the structural, textural, and optical properties of the samples. The catalytic activity of the samples for water decontamination was determined in a photo-Fenton-like activation of persulfate for removal of tetracycline hydrochloride as model pollutant. The presence of persulfate, PDS, considerably increased the removal efficiency under visible light illumination.

Activity and Thermal Aging Stability of La1-xSrxMnO3 (x = 0.0, 0.3, 0.5, 0.7) and Ir/La1-xSrxMnO3 Catalysts for CO Oxidation with Excess O2.

The catalytic oxidation of CO is probably the most investigated reaction in the literature, for decades, because of its extended environmental and fundamental importance. In this paper, the oxidation of CO on La1-xSrxMnO3 perovskites (LSMx), either unloaded or loaded with dispersed Ir nanoparticles (Ir/LSMx), was studied in the temperature range 100-450 °C under excess O2 conditions (1% CO + 5% O2). The perovskites, of the type La1-xSrxMnO3 (x = 0.0, 0.3, 0.5 and 0.7), were prepared by the coprecipitation method. The physicochemical and structural properties of both the LSMx and the homologous Ir/LSMx catalysts were evaluated by various techniques (XRD, N2 sorption-desorption by BET-BJH, H2-TPR and H2-Chem), in order to better understand the structure-activity-stability correlations. The effect of preoxidation/prereduction/aging of the catalysts on their activity and stability was also investigated. Results revealed that both LSMx and Ir/LSMx are effective for CO oxidation, with the latter being superior to the former. In both series of materials, increasing the substitution of La by Sr in the composition of the perovskite resulted to a gradual suppression of their CO oxidation activity when these were prereduced; the opposite was true for preoxidized samples. Inverse hysteresis phenomena in activity were observed during heating/cooling cycles on the prereduced Ir/LSMx catalysts with the loop amplitude narrowing with increasing Sr-content in LSMx. Oxidative thermal sintering experiments at high temperatures revealed excellent antisintering behavior of Ir nanoparticles supported on LSMx, resulting from perovskite's favorable antisintering properties of high oxygen storage capacity and surface oxygen vacancies.

Lanthanum Strontium Cobalt Ferrite (LSCF) perovskites by Sol-Gel Method for Potential Application in Solid Oxide Fuel Cells (SOFC)

Fuel cells are one of the most efficient and effective solutions to environmental problems and high energy demand. They are devices which change chemical energy into electrochemical energy, allowing much higher efficiency than conventional thermomechanical conversion methods. Lanthanum Strontium Cobalt Ferrite (LSCF) perovskites have been widely studied for application as cathodes in solid oxide fuel cells (SOFC) due to their high electrical conductivity, high thermal and chemical stability, low difference in thermal expansion coefficient, and physico-chemical compatibility with the other cells components. The aim of this work was to synthesize perovskitas type La0.7Sr0.3Co0.5Fe0.5O3 by sol-gel method and evaluate the potential for application as a cathode for fuel cell. The results obtained by X-ray powder diffraction (XRD) indicate that the sol-gel method calcined at 900ºC obtained an amount of the perovskite phase above 95%. The Field Emission Gun-Scanning Electron Microscope (FEG-SEM) images of LSCF film produced with 4 layers showed a better quality. Thus, the results obtained by XRD and FEG-SEM, indicate that the sol-gel method calcined at 900ºC has a potential application as cathode in solid oxide fuel cells.

139La-NMR Study of Spin Dynamics Coupled with Hole Mobility in T*-type La0.86Eu0.86Sr0.28CuO4−δ

In T*-type cuprate oxides with five-oxygen coordination, the relationship between spin correlations and doped carriers has not been well understood. To clarify this relationship, and the magnetic and superconducting (SC) properties of T*-type cuprate oxides, we performed 139La-nuclear magnetic resonance (NMR) and electrical resistivity measurements on as-sintered (AS) and oxidation-annealed (OA) polycrystalline T*-type La0.86Eu0.86S0.28CuO4 (LESCO) to investigate its magnetic and SC properties. Upon cooling, the NMR spectrum of AS LESCO broadened below 3 K, where the nuclear spin–lattice relaxation rate 1/T1 with respect to the temperature exhibited a maximum, indicating the appearance of static magnetism. The temperature dependence of 1/T1 between 3 and 20 K was similar to that of resistivity, which displays semiconducting behavior. Furthermore, it was found that the energy scales of the transport gap and spin dynamics were comparable. These results suggest a close connection between the mobility of the doped carriers and low-energy spin dynamics, as reported for lightly doped T-type La2−xSrxCuO4. In the OA SC sample, we confirmed the absence of a magnetic order and the linear relation between 1/T1 and T above 10 K. These results suggest that the magnetic state coupled with holes markedly changes to the weakly correlated metallic state by oxidation annealing in the T*-type LESCO with x = 0.28.

Electrochemical properties of AB5 type La0.8Ce0.2Ni4Co0.5Mn0.3Al0.2 alloy for unified metal hydride fuel cell

Metal hydride materials are widely applied for electrochemical energy storage. AB5 type alloys are the main anode material for Ni-MH batteries. Reversible hydrogen uptake and release provides good opportunities to use metal hydrides for alkaline fuel cell applications. Metal hydrides have good activation performance, high hydrogen sorption/desorption rates, catalytic activity in alkaline solution. Current work presents the first experimental results of AB5 type metal hydride electrodes, measured in a battery mode. The electrochemical properties of the prepared electrodes, such as activation performance, maximum discharge capacity, and high rate dischargeability are investigated in detail.

(Invited) Incorporation of Bulk Proton Carriers in Manganese Perovskites Triggered By Charge Disproportionation between Metal and Oxygen Atoms

Protonic ceramic cells (PCCs), including fuel cells and electrolysis cells, are based on protonic conducting Ba(Zr, Ce)O3 electrolytes, and have more advantages at operating temperatures below 600 °C than solid oxide cells (SOCs) based on a oxide ion conducting Y-ZrO2 because the activation energy required for proton conduction (0.3–0.6 eV) is lower than that required for oxide ion conduction (0.8 eV), which is key to lowering fuel cell costs and extending lifetimes. One major issue of protonic cells is a lack of suitable air electrodes that promote the association of protons and oxide ions via oxygen evolution reaction (OER) / oxygen reduction reaction (ORR). Cobaltite-based cathodes, such as (Sm,Sr)CoO3, (Ba, Sr)(Co,Fe)O3, and (La,Sr)(Co,Fe)O3 are typical oxide ion conductors; there is a mismatch of main ionic carriers between the air electrodes and the electrolyte, which limits the efficient area for air electrodes’ reaction to the electrode–electrolyte–gas triple phase boundaries (TPB). Therefore, it is urgent task to develop electrode materials that exhibit H+/e-/O2- triple conduction to extend the efficient reaction zones beyond the TPB and thus reduce the air electrodes’ overpotentials and improve electrochemical performances for PCCs. The transition metal oxides which is capable of hydration at relatively high temperatures must be promising candidate. Nevertheless, robust design concepts for MPECs, namely, transition metal oxides realizing enhanced bulk hydration ability at intermediate temperatures are still missing. Therefore, it is of both fundamental and technological importance to come up with a key ‘descriptor’ to enhance the proton uptakes of electron-conducting metal oxides. Although La1-x Sr x MnO3 (LSM) is a well-known cathode material for SOFCs and recognized as a good oxide ion electron mixed conductor at elevated temperature, it has not been subject to a proton conducting material. Herein, we report pronounced hydration ability of cubic perovskite type La0.7Sr0.3MnO3- δ in air conditions attributed to the association of water and oxygen vacancies mediated by Mn oxidation, and subsequent proton attachment to oxygen neighbors in exchange for oxygen hole carriers [1,2]. Hence La0.7Sr0.3MnO3- δ is capable of up-taking bulk proton carriers in wet air with the proton contents equaling to that of well-known protonic electrolyte material Ba(Zr,Y)O3 at the intermediate temperature (IT) region.

Morphological and electrical properties of La0.8Ca0.1Pb0.1FeO3 perovskite nanopowder for NH3 and CO gas detection

Nanocrystalline gas sensitive material based on Perovskite type La0.8Ca0.1Pb0.1FeO3 (LCPFO) sample has been prepared by the sol-gel method with the citric acid route. The gas sensitivities under different temperatures of the (LCPFO) compound have been investigated toward the two NH3 and CO reducing gases. The synthesized compound has been analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS) and atomic force microscopy (AFM). It was found that the compound exhibits an orthorhombic symmetry with the Pnma space group. The effect of the experiment environment (under vacuum and air) on the conductance and activation energy Ea of the LCPFO compound has been evaluated. The Ea values have been found to be in the range of 0.43 eV < Ea < 0.53 eV and to be located between 0.42 and 0.62 eV under vacuum and air, respectively. Furthermore, the lowest detectable concentrations (10 and 40 ppm) of NH3 and CO, respectively, have been achieved.

Magnetic Properties of La0.7Ba0.3Mn1−xAlxO3 (0 ≤ × ≤ 0.15) Perovskites

The effects of aluminum substitution on the magnetic properties of perovskite type La0.7Ba0.3Mn1−xAlxO3 (0.05 ≤ × ≤ 0.15) compounds have been studied. The phase purity of the samples has been determined with X-ray powder diffraction. Comparison among various structural parameters has been performed to understand the effect of Al3+ substitution at Mn3+ site. The DC magnetization study includes the thermo-magnetic measurements in zero field cooled (ZFC) mode within the temperature range of 100 to 300 K and the isothermal M-H curve at 100 K. It has been observed that incorporation of Al3+ ion at the Mn3+ site significantly affects the exchange interaction processes and hence results in suppressed magnetic moments. It has been observed that the uniform distribution of Al3+ at Mn3+ sites results in weakening of long-range ferromagnetic (FM) ordering in the parent compound. The inclusion of 10–15% of Al3+ significantly affects the double-exchange (DE)-interactions, thereby promotes a non-magnetic phase along with separated short-range FM clusters. The presence of non-magnetic phase and non-spontaneous magnetization among FM clusters has been observed for samples with 10–15% of aluminum substitution.

In My Element: Oxygen

The In My Element series celebrates the personal accounts from Chemistry – A European Journal Editorial Board members for the 2019 International Year of the Periodic Table. In this contribution, C. N. R. Rao gives his story on oxygen. My love for oxygen can be traced to the very beginning of my research career. This is not because oxygen was discovered by the Father of Chemistry, Lavoisier, and it is essential for life as O2 and as H2O, but because it forms a variety of metal oxides with interesting structures and a wide range of properties1 (Table 1). Oxygen can exist as O21−, O22− and O2− in solids and as O1− on certain surfaces. Oxygen stoichiometry is an important aspect of oxides, oxygen deficiency often giving rise to new types of ordered structures (TinO2n−1 Magneli phases, Pr2O2n−2). Metallic: ReO3, NbO, LaNiO3 Dielectric: BaTiO3 Antiferromagnetic: NiO, LaCrO3 Ferromagnetic: CrO2, La0.5Sr0.5CoO3, SrRuO3 Ferroelectric: BaTiO3, KNbO3 Ferroelastic: Gd2(MoO4)3 Superconducting: BaPb0.75Bi0.25O3, YBa2Cu3O7 Insulator-Metal Transition: V2O3, VO2, La1−xSrxCoO3 Colossal Magnetoresistance (CMR): La0.7Ca0.3MnO3 Multiferroic: BiFeO3, YMnO3, TbMnO3 I have worked on metal oxides for a long time,2 and my very first research paper in materials chemistry in the late 1950′s was on the transformation of the anatase form of TiO2 to the rutile form. I also studied the so-called nonstoichiometric oxides of rare earths of the type Pr6O11, Pr7O12 and Tb4O7. Soon I realized how perovskite oxides (ABO3) exhibit a variety of phenomena and properties such as ferroelectricity and magnetism, and how the pervoskite motif is present in several families of oxides. I have been working on perovskite oxides including two-dimensional oxides for many years. I worked on La2CuO4, as early as 1971, mother of cuprate superconductors. My group was one of the first to characterize YBa2Cu3O7, the first liquid nitrogen superconductor, which also has the perovskite motif. The number of cuprate families exhibiting high temperature superconductivity is impressive.3 The discovery of colossal magnetoresistance in rare earth manganites of the type La1−xCaxMnO3 which show both orbital and charge-ordering led to a variety of investigations and to the observation of the phenomenon of electronic phase separation.4 We observed major effects of phase separation in Nd0.5Sr0.5MnO3, which is ferromagnetic at ordinary temperatures, and occurs in three phases (FM+2AFMs) at liquid helium temperatures. I started working on multiferroics in the 1990’s, the well-known examples being BiFeO3 and YMnO3. Cation substitution in oxides is commonly carried out to cause changes in structures and properties. An impressive example is that of Sr substituted LaCoO3 where a paramagnetic insulator becomes a ferromagnetic metal on substitution of Sr in place of La. Cation substitution generally affects the unoccupied states and anion substitution, specially aliovalent anions on the other hand, cause drastic changes in the valence bands. In recent months, I have been working on oxide analogues wherein oxygen is substituted by nitrogen and fluorine, resulting is novel electronic and catalytic properties.5 For example, Zn2NF and TiNF are isoelectronic with ZnO and TiO2 respectively, but exhibit entirely different electronic structures and properties. Position Linus Pauling Research Professor E-mail cnrrao@jncasr.ac.in Homepage http://www.jncasr.ac.in/cnrrao Education B.Sc (Mysore, 1951), M.Sc (Banaras, 1953) Ph.D (1957, Prof R.L. Livingston, Purdue, “Molecular structure investigations by gas phase electron diffraction)” D.Sc (1960) Mysore “Studies of Structural Chemistry and Nitrogen Chemistry” Post-doctoral fellowship (1958-1959; Berkeley, Prof K.S. Pitzer) Awards Dan David Prize for Science in the Future Dimension (2005) Ernesto Illy Trieste Science Prize (2011) for materials research The Royal Medal (The Queens Medal) of the Royal Society, London, U.K (2009) The August-Wilhelm-von-Hoffmann Medal for outstanding contributions to chemistry by the German Chemical Society (2010) Von Hippel Award for materials research by Materials Research Society, US (2017) Chevalier de la Légion D′Honneur by the President of the French Republic (2005) Order of the Rising Sun, Gold and Silver Star by the Emperor of Japan (2015) Bharat Ratna (Jewel of India), Highest Civilian Award of India (2014) Research interests Chemistry of Materials, in particular, oxide materials, 2D materials, solar water-splitting Hobbies and interests Listening to classical music and cooking

Fabrication and Characterization of Manganite Based p–n Junction

In this communication, we report the process to fabricate high quality LaMnO3 / La0.7Ca0.3MnO3 / LaAlO3 heterostructure consists of oxygen deficient n–type semiconducting LaMnO3–δ 50nm layer attached with p–type La0.7Ca0.3MnO3 100nm manganite layer. Heterostructure was grown using easy, simple, vacuum free, cost effective and environmentally friendly chemical solution deposition (CSD) method. Manganite based p–n junction was characterized by current–voltage (I–V) characteristics and electric field dependent junction resistance measurements at different temperatures. I–V curves have been understood for the charge transport mechanisms across the junction in the context of thermionic emission mechanism while complex electroresistance (ER) behavior has been discussed on the basis of role of depletion region and charge injection processes across the junction.

The Misfit Dislocation Core Phase in Complex Oxide Heteroepitaxy

AbstractMisfit dislocations form self‐organized nanoscale linear defects exhibiting their own distinct structural, chemical, and physical properties which, particularly in complex oxides, hold a strong potential for the development of nanodevices. However, the transformation of such defects from passive into potentially active functional elements necessitates a deep understanding of the particular mechanisms governing their formation. Here, different atomic resolution imaging and spectroscopic techniques are combined to determine the complex structure of misfit dislocations in the perovskite type La0.67Sr0.33MnO3/LaAlO3 heteroepitaxial system. It is found that while the position of the film–substrate interface is blurred by cation intermixing, oxygen vacancies selectively accumulate at the tensile region of the dislocation strain field. Such accumulation of vacancies is accompanied by the reduction of manganese cations in the same area, inducing chemical expansion effects, which partly accommodate the dislocation strain. The formation of oxygen vacancies is only partially electrically compensated and results in a positive net charge q ≈ +0.3 ± 0.1 localized in the tensile region of the dislocation, while the compressive region remains neutral. The results highlight a prototypical core model for perovskite‐based heteroepitaxial systems and offer insights for predictive manipulation of misfit dislocation properties.

Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions (ORER) in Alkaline Electrolyte

Oxygen reduction and evolution reactions (ORER) are vital for electrochemical conversion and storage in many electrochemical systems such as metal-air hybrid systems, conventional and regenerative fuel cells, photolysis and electrolysis (water and chlor-alkali). The breakage of oxygen bonds during reduction as well as formation of oxygen via oxygen evolution coupled in one and the same electrode, however shows sluggish kinetics for discharge and charge reactions with high overpotentials. The platinum group metals (PMG) such as Pt, RuO2 and IrO2 are considered to be the state-of-the-art electrocatalysts for both the ORR and OER (1-3). Although very low PMG loadings are currently used in the electrochemical reactions, the poor stability, high cost and rarity of these metals are of considerable challenge in the long run for high scale application and commercialization. Interest in rechargeable metal-air batteries in alkaline electrolyte is on the upsurge due to their high theoretical specific energy density and increased cycle life as well as abundance and low cost materials for both cathode and anode (4, 5). Among the class of promising electrocatalysts for the cathode, perovskites with ABO3 structure have shown bifunctionality, availability and stable performances in aqueous alkaline electrolytes. By fine-tuning the cations either by total or partial substitution of the site A having rare earth metals with alkali earth metal and B of the transition metals (Mn, Fe, Co, Ni, Al), the oxide ion mobility, electronic conductivity and oxygen vacancies can be modified in order to accommodate surface and catalytic properties on par or even better than the PMG (6,7). In this work, we report synthesis and various physicochemical characterizations of the perovskite type La0.1Ca0.9MnO3 both with and without co-mixing of nano Co3O4. Electrochemical activities for the ORR in a conventional three-electrode set-up were carried out by linear sweep voltammetry using an RDE at a sweep rate of 2 mV s-1, rotation rates between 500 and 2500 rpm and at the potential range of +0.1 to -0.9 V by oxygen saturation in 1M KOH. Furthermore, cyclic voltammetry measurements at a scan rate of 0.5 mV s-1 for composite electrodes in the potential region of +0.55 to -0.3 V and were obtained at 50 cycles in 6M KOH. The electrode kinetics, intrinsic activity and stability for the ORER were assessed for respective electrocatalysts and their mixtures.

Mixed valence Mn,La,Sr-oxide based magnetic nanoparticles coated with silica nanoparticles for use in an electrochemical immunosensor for IgG

We describe the preparation and properties of mixed valence manganite nanoparticles (NPs) of the type La0.67Sr0.33MnO3 for use in electrochemical immunoassays. The NPs were synthesized using the reverse micelle method and their surface was then functionalized with silica nanoparticles. The resulting NPs (La0.67Sr0.33MnO3@SiO2) were characterized by X-ray diffraction, scanning electron microscopy and physical property measurements. Their typical diameter is ~200 nm, and the magnetic saturation occurs at 75 emu⋅g‾1, and both coercive force and hysteresis loop remain at almost zero. The silica NPs immobilized on the La0.67Sr0.33MnO3 NPs have a diameter of ~20 nm. The La0.67Sr0.33MnO3@SiO2 NPs were used to develop an electrochemical immunoassay for human IgG. Antibody (anti-human IgG) was immobilized on the silica NPs on the magnetic core, and this process was monitored by cyclic voltammetry and square-wave voltammetry. The formation of the immunocomplex (Ab/Ag) on the surface of the electrode was monitored using hexacyanoferrate redox system as the redox marker. The assay has a linear working range up to an IgG concentration of 5 ng⋅mL‾1, and the detection limit is 0.6 ng⋅mL‾1.

Effect of Sn substitution for Co on microstructure and electrochemical performance of AB5 type La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0–0.5) alloys

The effects of substitution of Sn for Co on the microstructure, hydrogen storage and electrochemical discharge capacity of La0.7Mg0.3Al0.3Mn0.4Co0.5–xSnxNi3.8 (x=0, 0.1, 0.2, 0.3 and 0.5) alloys were investigated using X-ray diffraction (XRD), pressure composition isotherm (PCT) and electrochemical discharge cycle. XRD, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests showed that all of alloys are mainly composed of LaNi5 and MgNi2 phases, but when increasing the content of Sn in alloys, the LaNiSn phase appears and microstructure is refined. The PCT showed that increasing substitution of Sn for Co results in decrease of the maximum hydrogen storage capacity from 1.48% (x=0) to 0.85% (x=0.5). The electrochemical tests indicated that the maximum discharge capacity decreases from 337.1 mA·h/g (x=0) to 239.8 mA·h/g (x=0.5); however, the discharge capacity retention at the 100th cycle increases from 70.2% (x=0) to 78.0% (x=0.5).

Fabrication of a Microtubular La0.6Sr0.4Ti0.2Fe0.8O3−δMembrane by Electrophoretic Deposition for Hydrogen Production

Microtubular type La0.6Sr0.4Ti0.2Fe0.8O3−δ(LSTF) membranes were prepared by electrophoretic deposition (EPD). The oxygen permeation and hydrogen production behavior of the membranes were investigated under various conditions. LSTF green layer was successfully coated onto a carbon rod and, after heat treatment at 1400°C in air, a dense LSTF tubular membrane with a thickness of 250 mm can be obtained. The oxygen permeation and hydrogen production rate were enhanced by CH4in the permeate side, and the hydrogen production rate by water splitting was 0.22 mL/min·cm2at 1000°C. It is believed that hydrogen production via water splitting using these tubular LSTF membranes is possible.

Non-thermal plasma assisted LKMO catalyst approach to NOx removal in diesel engine emissions

Perovskite type La0·8K0·2MnO3 (LKMO) was prepared by citric acid sol–gel method. X-ray diffraction, SEM and Brunauer–Emmett–Teller were used to characterise the products. The non-thermal plasma assisted catalyst (NPAC) system was established by combining the non-thermal plasma (NTP) reactor with LKMO catalyst. Following up on the bench test, NOx reductions in diesel emissions were investigated with the NPAC system. Results showed that NOx conversion reaches its highest level at 300°C with LKMO catalyst and at 200°C with the NPAC system. Improved catalytic properties of LKMO can be explained by the combination of the porous structure and the active species generated by NTP.

Coexistence of sign reversal of both magnetization and exchange bias field in the core-shell type La0.2Ce0.8CrO3 nanoparticles

We report an extraordinary coexistence of sign reversal of both magnetization and exchange bias field in the La0.2Ce0.8CrO3 nanoparticles. From the high resolution transmission electron microscopy image, and field dependence of thermoremanent and isothermoremanent magnetization measurements, the nanoparticles are found to be of core-shell nature. The core-shell configuration with an antiferromagnetic core of the Cr3+ and Ce3+ spins and a disordered shell with the uncompensated spins, explains the sign reversal of both magnetization and exchange bias field. The present study shows an excellent way of tuning the sign of both magnetization and exchange bias field in a single magnetic system.

Structural and magnetocaloric properties of La1−yNayMnO3 compounds prepared by microwave processing

Polycrystalline samples of sodium doped lanthanum manganites of the type La1−yNayMnO3 (0 < y ⩽ 0.15) having nanometric crystallite size have been prepared by microwave processing. The Curie temperature TC of the prepared pellets is strongly dependent on the sodium content in the compound and ranges between 196 and 312 K. A detailed structural analysis has been performed on the prepared samples using the Rietveld FULLPROF program and important structural parameters are estimated. Close to TC, a significant change in magnetic entropy has been observed in all the studied samples. The maximum entropy change at 0.8 T field (µ0H) for the La0.90Na0.10MnO3 compound shows an enhancement by ∼158% over that measured for a sample with Na content of 0.05 and is ∼67% of that known for the prototype magnetic refrigerant (pure Gd). The maximum adiabatic temperature change (ΔTad) is estimated to be ∼2 K. The temperature dependence of the magnetic entropy change for La1−yNayMnO3 compounds, measured under various magnetic fields, could be explained fairly well by including the contributions from the magnetoelastic and electron interaction in the expression for the Landau theory of phase transition.

Effect of sintering temperature on electrical transport in La0.67 Ca0.33 MnO3/BaTiO3composites

It is shown that the electrical transport in ferromagnet–ferroelectric–type La0.67Ca0.33MnO3/BaTiO3 composites fabricated by the sol–gel method strongly depends on sintering temperature (Ts). For the Ts > 1000 °C samples, the insulator–metal transition temperature (TIM) decreases with increasing BaTiO3 content and the magnetoresistance (MR) is enhanced at low temperature (T < TIM). For the Ts = 1000 °C samples, the TIM value initially increases with an increase of BaTiO3 content and thereafter it decreases as BaTiO3 content further increases. For the Ts = 900 °C samples, the TIM value increases throughout the increase in BaTiO3 content and MR is enhanced over the whole temperature range studied. The experimental results have been discussed in terms of the magnetoelectric coupling effect and the interfacial diffusion reaction, especially at grain boundaries.

Interfacial capacitance in epitaxial heterostructures La0.67Ca0.33MnO3/SrTiO3/La0.67Ca0.33MnO

Epitaxial trilayer heterostructures of the type La0.67Ca0.33MnO3/SrTiO3/La0.67Ca0.33MnO3 were grown by laser ablation on (001)[(LaAlO3)0.3+(Sr2AlTaO6)0.7] substrates. The real part of the dielectric permittivity e and the loss factor tan δ of a 1100-nm-thick SrTiO3 interlayer were studied in the temperature interval T=4.2–300 K in a nonbiased state and at a bias voltage of ±2.5 V applied to the manganite electrodes. Using the temperature dependence e(T) measured for the SrTiO3 layer grown between the manganite electrodes, we have estimated the capacitance of La0.67Ca0.33MnO3/SrTiO3 interfaces (C1≈2 μF/cm2) related to the electric field penetrating from the interlayer into La0.67Ca0.33MnO3.