Nominal Composition La0 Research Articles

Cationic substitution engineering in GdInO3 at A-site: Insights into phase evolution and search for compositionally tailored relaxors

The present work reports detailed structural and electrical studies on A-site substituted LaxGd1-xInO3 (0.0 ≤x ≤ 1.0) system with structurally different end members in search for tailored lead-free relaxors. The motivation is the possibility to manipulate the crystal structure to maneuver electrical properties in hexagonal GdInO3, a candidate for geometric ferroelectricity. The hybrid synthesis protocol involving gel-combustion led to stabilisation of metastable polymorphs in some nominal compositions due to kinetic stabilisation. Non-preference of La3+ for 7- fold coordinated A-site in hexagonal structure manifested in wide-ranging orthorhombic phase field prevailing even in Gd3+-rich region, also supported by theoretical studies. P-E studies on Gd3+-rich compositions exhibited hysteresis loop which suddenly became narrow for the nominal composition La0.4Gd0.6InO3 with very low remanent polarisation. This along with broad maximum exhibited in plot of dielectric constant vs temperature that shifts to higher T upon increasing frequency, for La0.4Gd0.6InO3, supports presence of relaxor-type behavior. Signature of hexagonal-type vibrations in otherwise bulk orthorhombic (by XRD) La0.4Gd0.6InO3 and anomalous trend in La/Gd vibrational mode along with narrowing down of Raman modes have been cited as plausible structural reasons for the relaxor behaviour. This has significance in context of structure-electrical property relationship leading to development of potential lead-free relaxors.

Critical Behavior in Ga-Doped Manganites La0.65Bi0.05Sr0.3Mn1 – xGaxO3 (x = 0 and 0.06)

In this paper, we present a detailed study of the critical behavior around paramagnetic–ferromagnetic phase transition in the nominal composition La0.65Bi0.05Sr0.3Mn1 – xGaxO3 (x = 0 and 0.06). Our powder specimens were synthesized using the conventional solid-state reaction method and analyzed by X-ray diffraction and magnetic measurements. X-ray diffraction analysis at room temperature shows that our specimens La0.65Bi0.05Sr0.3Mn1 – xGaxO3 (x = 0 and 0.06) adopt a rhombohedral structure with R-3C space group. Based on Banerjee criterion, the positive slopes of the Arrott plot confirm the second order magnetic phase transition of our samples. The critical exponent values are calculated by distinct techniques like modified Arrott plot (MAP), Kouvel–Fisher method (KF) and critical isotherm (CI). The determined critical exponents values are close to the 3D-Ising model. Furthermore, the substitution of Mn by Ga leads to the decrease of the critical exponent β and at the same time to the reduction of the Curie temperature TC. The validity of the determined exponent values is authenticated using the scaling hypothesis, in which the M(T, e) curves integrate in two universal curves below and above TC.

Structure, magnetism and electrical transport of sol-gel derived La0.30Pr0.30Ca0.40MnO3: Elucidating consequences of size effect

The phase separation (PS) tendencies in low bandwidth manganites like La1−x−yPrxCayMnO3 have not been probed in terms of size effects. The present paper attempts to decipher the nature of competing phase tendencies in terms of the particle size in archetypical phase separated compound with nominal composition La0.30Pr0.30Ca0.40MnO3 (LPCMO). Differently sized materials have been synthesized by sintering the sol-gel derived powders at TS = 600 °C, 900 °C, 1100 °C and 1300 °C. Phase pureness is confirmed by the Rietveld refinement of the powder X-ray diffraction (XRD) data, which authenticates orthorhombic structure with Pnma space group and thermally sensitive lattice constants. The crystallite (particle) size increases from ~19 nm (54 nm) to ~31 nm (457 nm) as TS goes up from 600 °C to 1300 °C and all the three lattice constants increase with particle size. X-ray photoemission spectroscopy (Mn2p and Mn2s core-levels) substantiates the coexistence of Mn3+ and Mn4+ in the desired ratio. The magnetization measured (H@100 Oe) as a function of temperature exhibits size dependent paramagnetic (PM) to ferromagnetic (FM) transition and also brings out that (i) the hysteresis between the field cooled cooling (FCC) and field cooled warming (FCW) magnetization first increases with increasing crystallite /particle size and then decreases, (ii) the divergence between the zero field cooled (ZFC) and FCW magnetization decreases with increasing crystallite/particle size and (iii) sharp increase of the magnetic moment beyond particle size of ≈90 nm (crystallite size ≈ 23 nm). At lower temperatures a glassy behaviour is visible at higher crystallite size/Particle size. The isothermal magnetization loops also confirm the increase in the FM component at higher crystallite/particle size. A thermally hysteretic insulator-metal transition (IMT) appears at particle size >90 nm (Crystallite size > 23 nm) and the hysteresis decreases as the particle size increases. The PM state in all the samples exhibits variable range hopping conduction with size dependent activation. Our study unravels that the phase separation (PS) tendencies are dominant only in the vicinity of particle size ~100 nm.

Study of magnetic structure and electrical-transport properties of La1−yBayMn1−xFexO3 (y = 0.15, 0.40) perovskite manganites

An O 2p itinerant-electron model for magnetic oxides, which is similar to the O 2p hole-carrier model proposed by Alexandrov et al. [Phys. Rev. Lett. 96 (2006) 117003], was used to explain the magnetic and electrical-transport properties of two series of samples with the nominal composition La1−yBayMn1−xFexO3 (0.00 ≤ x ≤ 0.20; y = 0.15, 0.40). The samples were prepared using the sol–gel method, and X-ray diffraction patterns showed that all samples were single-phase ABO3 perovskite structures. The magnetic and electrical-transport properties of the samples were characterized. Interesting phenomena were observed for the magnetic moment, μobs, of the samples at 10 K: μobs of the samples with y = 0.15 decreased approximately linearly when the Fe-doping level was increased from zero to x = 0.2. However, there was a characteristic Fe-doping level, xC = 0.10, for the samples with y = 0.40. When x ≤ xC, μobs decreased very slowly; when x > xC, μobs decreased rapidly. These interesting phenomena suggest that the samples had a canted magnetic structure, but the cant angle of the samples with y = 0.15 was smaller than that of the samples with y = 0.40. These magnetic structures were further confirmed by the electrical-transport properties of the samples.

Preparation, Structure and Physicochemical Properties of La0.95Bi0.05Mn1-yCuyO3+δ (у=0.1-0.4) and Composites with Bi2O3-Based Electrolytes

Samples of La0.95Bi0.05Mn1-yCuyO3+δ (у=0.1-0.4) were prepared by solid-state synthesis. Additionally, the sample with nominal composition La0.95Bi0.05Mn0.7Cu0.3O3+δ was obtained using citrate-nitrate method. It was determined by X-ray diffraction analysis that the compounds have rhombohedral (space group R-3c) or orthorhombic (space group Pbnm) structure, depending on the composition. Single-phase compounds are synthesized at у=0.1; 0.2. The investigation using scanning electron microscope showed that the grain sizes for the samples sintered using different techniques are close to each other because of the high calcination temperature. For the sample with orthorhombic structure the phase transition into rhombohedral one was found around 390 °С by means of dilatometry. Thermal expansion coefficient of the sample La0.95Bi0.05Mn0.7Cu0.3O3+δ is equal to 6·10-6 K-1 (T 390 °C) and 15·10-6 K-1 (T 390 °C). The composite materials of substituted lanthanum manganites with Bi4V1.7Fe0.3O11-δ and Bi7Nb1.8Zr0.2O15.5-δ solid electrolytes were obtained at 650 °C. The electrical conductivity values for the latter one are by three orders of magnitude higher than for pure bismuth niobate.

Enhanced magnetocaloric effect with the high tunability of bismuth in La0.8Na0.2Mn1−xBixO3 (0 ≤ x ≤ 0.06) perovskite manganites

In this paper, we have reported in details the effect of bismuth doping on the structural, magnetic and magnetocaloric properties of the nominal composition La0.8Na0.2Mn1−xBixO3 (0 ≤ x ≤ 0.06). All the compounds were synthesized by using the conventional solid-state reaction with a final sintering temperature of 1400 °C. For the undoped compound, the X-ray refinement shows the coexistence of two phases: a major phase crystallizing in the rhombohedral structure with R-3C space group and a minor one corresponding to Mn3O4, which adopts the tetragonal symmetry with I41/amd space group. However, the doped compounds (x = 0.03 and x = 0.06) are only indexed in the rhombohedral structure with R-3C space group without the presence of any minor phase.M (T) and M (H) investigations show a single ferromagnetic-paramagnetic transition with increasing temperature. The Curie temperature values decrease gradually with Bismuth doping, 330 K for x = 0, 320 K for x = 0.03, and 310 K for x = 0.06.Moreover, a wide variation in the magnetic entropy change with respect to temperature has been noticed and it indicates a significant magnetocaloric effect. The maximums of the magnetic entropy change −ΔSM at μ0H = 5 T are 4.73 J/Kg K, 4.77 J/Kg K and 5.20 J/Kg K for x = 0, 0.03 and 0.06 at 335 K, 325 K and 315 K, respectively. The RCP values of our current compounds are 241 J Kg−1, 218 J Kg−1 and 229 J Kg−1 with negligible hysteresis losses, thus offering the opportunity to use them in the magnetic refrigeration. The master curve investigations display the collapse of the entire curves in the same plot to convey the second order transition of the compounds.

Excitation and photoluminescence spectra of single- and non-single-phased phosphors based on LaInO3 doped with Dy3+, Ho3+ activators and Sb3+ probable sensitizer

Single-phased La0.95Ln0.05InO3 (Ln – Dy, Ho), La0.90Dy0.05Ho0.05InO3, LaInO3 ceramic samples as well as the La0.95Ln0.05In0.98Sb0.02O3 (Ln – Dy, Ho), La0.90Dy0.05Ho0.05In0.98Sb0.02O3, LaIn0.98Sb0.02O3 samples with additional impurity LaSbO3 phase were prepared by solid-state reactions method. Their excitation and photoluminescence (PL) spectra were measured at room temperature. It was established that PL bands intensity of spectra obtained for samples of nominal composition La0.95Dy0.05In0.98Sb0.02O3, La0.95Ho0.05In0.98Sb0.02O3, La0.90Dy0.05Ho0.05In0.98Sb0.02O3 is much higher than that of single-phase La0.95Dy0.05InO3, La0.95Ho0.05InO3, La0.90Dy0.05Ho0.05InO3 solid solutions. It may be probably explained by sensitizing effect of Sb3+ ions on Dy3+, Ho3+ ions photoluminescence. Although it can’t be excluded that the reason could be higher PL of Dy3+ and Ho3+ ions in impurity matrix than in LaInO3-based matrix. It was observed in the PL spectrum of La0.90Dy0.05Ho0.05In0.98Sb0.02O3 sample that under the excitation by λex = 320nm an intense simultaneous emission of blue, green and yellow light took place.

Polycrystalline La0.8Sr0.2GaO3 perovskite synthesized by sol–gel process along with temperature dependent photoluminescence

This work covers for the first time the protocol of the preparation of nominal composition La0.8Sr0.2GaO3 by the sol–gel method using the citric acid. The structural and the optical properties were investigated, by X-ray diffraction (XRD), Fourier Transform infrared Spectroscopy (FT-IR) and Photoluminescence Spectroscopy (PL) techniques, respectively. The Rietveld refinement, from the X-ray diffraction patterns, showed the presence of La0.63Sr0.21Ga0.95O2.68 perovskite as a major phase which crystallizes in an Hexagonal structure and the appearance of minor impurity phase identified as Gallium Lanthanum Oxide (La4Ga2O9) belonging to a Monoclinic structure. Two peaks indicating blue photoluminescence emission were observed at room temperature. One emission was related to secondary phase and the other was assigned rather to the oxygen deficiency in perovskite structure, leading to the formation of intermediary energy levels in the band gap.

Influence of thermal treatment on the ionic valence and the magnetic structure of perovskite manganites La0.95Sr0.05MnO3

In traditional views, the magnetic ordering of oxides may be explained using magnetic superexchange (SE) or double exchange (DE) interaction models. Both models are based on an assumption that the valences of all oxygen ions be -2. For example, both La and Mn in LaMnO3 are assumed to be trivalent, in which antiferromagnetic spin structure is explained using the SE interaction between Mn3+ cations mediated by oxygen anions. In La1-xSrxMnO3, there exists a part of Mn4+ cations with the content ratio of Mn4+/Mn3+ being x/(1-x), in which spin structure and electronic transport properties are explained by DE interaction. However, there is a part of monovalent oxygen ions existing in oxides. Cohen [Nature 358 136] has calculated the densities of states for valence electrons in the perovskite oxide BaTiO3 using density functional theory. Results indicate that the average valence of Ba is +2, being the same as that in the traditional one, but the average valences of Ti and O are +2.89 and -1.63 respectively, agreeing with the results obtained using ionicity investigation [Rev. Mod. Phys. 42 317] and X-ray photoelectron spectra (XPS) analysis, but different from the conventional results +4 and -2. In this paper, three samples with the nominal composition La0.95Sr0.05MnO3 are prepared by different thermal-treatments. Likewise, there are only Mn2+ and Mn3+ cations, but no Mn4+ cations in La0.95Sr0.05MnO3, a result obtained by XPS analysis, and the average valence of Mn in La0.95Sr0.05MnO3 samples increases with increaseing thermal-treatment. Although the crystal structures of the samples are the same, the magnetic moments per formula are obviously different. This magnetic structure cannot be explained using the conventional SE and DE interaction models. Using the O 2p itinerant electron model for spinel ferrites proposed recently by our group, we can explain this magnetic structure. The variation trend of the average valences of Mn cations calculated using the magnetic moments per formula of the samples at 10 K, is in accordance with the experiment results of XPS. The O 2p itinerant electron model is based on an assumption that there is a part of monovalent oxygen ions in the oxides, which is the fundamental difference from SE and DE interaction models.

Enhanced thermionic emission properties in textured two-phase LaB6–BaB6 system prepared by spark plasma sintering

Structure, crystallographic orientation, and thermionic emission properties of the two-phase LaB6–BaB6 system fabricated by a combined process of evaporation–condensation and spark plasma sintering (SPS) were investigated systematically. The powder X-ray diffraction (XRD) reveals that the obtained nominal composition (La0.6Ba0.4)B6 compound exhibits coexistence of two isostructural phases of LaB6 and BaB6. The electron backscatter diffraction (EBSD) confirms that the surface of the (La0.6Ba0.4)B6 forms a clear (001) texture, which favors reduction in work function. Thermionic emission properties were measured in the temperature range between 1500 and 1873K. The results indicated that the (La0.6Ba0.4)B6 emitter displayed a low work function of 1.95eV, and a high zero field emission current density of 10Acm−2 at 1773K. Compared to the reported LaB6 and (La0.6Sm0.4)B6 prepared for identical synthesis conditions, (La0.6Ba0.4)B6 is shown to have the most excellent thermionic emission performance and could be promising candidate for cathode material applications. It was revealed that the addition of BaB6 to LaB6 plays an important role for the reduction of work function and enhancement of electron emission properties.

Effects of Bismuth Doping on the Physical Properties of La0.6−x Bi x Sr0.4CoO3 (0≤x≤0.15) Cobaltites

The structural and magnetic properties of cobaltites with the nominal composition La0.6−x Bi x Sr0.4CoO3 (0≤x≤0.15) have been investigated. X-ray diffraction analysis using Rietveld refinement show that all our samples crystallize in the rhombohedral structure. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT exhibits thermomagnetic irreversibility signature of short range ferromagnetic interactions in our samples. The ferromagnetic–paramagnetic transition temperature decreases with increasing Bi amount. In the paramagnetic phase, all our synthesized samples obey to the Curie–Weiss law. The Curie–Weiss analysis suggests that the spin state of Co3+ is IS while Co4+ ions are in IS for x=0 and LS for x≥0.05. The Bi doping also leads to a weakening in the magnetic moment. The magnetic entropy change exhibits a maximum value which decreases from 2.28 J/kg K for x=0 to 1.24 J/kg K for x=0.15 upon a magnetic field change of 5 T.

Behavior of Mn2+ in perovskite manganites with nominal composition La0.6−x Nd x Sr0.1MnO3

The fact that there are Mn2+ at the A sites in the ABO3 perovskite phase of manganites with the nominal composition La0.6−xNdxSr0.1-MnO3 showed by detailed experimental study and theoretical calculations. The magnetic moments of these Mn2+ are antiparallel to those of the Mn ions at the B sites. The content of the Mn2+ increases as the average ionic radius, 〈rA〉, of the ions at A sites decreases, resulting in the experimentally observed phenomenon that the content of the Mn3O4 phase in the manganites decreases with decreasing 〈rA〉.

Investigation on the maximum content of vacancy at the B sites in La0.75Sr0.25Mn1−xO3 perovskite manganites

The effects of the vacancy at the B sites on the structural and magnetic properties of perovskite manganites with a nominal composition La0.75Sr0.25Mn1−xO3 (0.0 ≤ x ≤ 0.13), prepared by the sol-gel method, were investigated. X-ray diffraction patterns indicate that there is a ABO3 perovskite structure single phase in the samples with x ≤ 0.08; however, there are second and third phases containing La, except ABO3 perovskite phase, in the samples with x ≥ 0.10. Magnetic measurement indicates that the sample with x = 0.08 has the lowest Curie temperature. By a quantitative phase analysis, we believe that there is a maximum value of vacancy content at the B sites in ABO3 perovskite manganites.

Investigation on cation distribution and magnetic properties in self-doped manganites La0.6−xSr0.4MnO3−δ

Self-doped manganites with nominal composition La0.6−xSr0.4MnO3−δ (0≤x≤0.175) have been prepared by the sol–gel method. The X-ray diffraction (XRD) patterns and magnetic measurements indicate that the samples have two phases with the ABO3 perovskite structure being the dominant phase and Mn3O4 being the minor phase when the doping level x≥0.05. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions in the perovskite phases were estimated, in which there are Mn2+ ions and no vacancies at A sites. The ion contents have been corrected by Rietveld fitting of the powder samples' X-ray diffraction data. The change tendency of the Curie temperature TC vs. the Mn4+ ion content ratio at the B sites of ABO3 perovskite phase is in accord with the experimental result of the samples La1−xSrxMnO3.

Structural and chemical analyses of the new ternary La5MgNi24 phase synthesized by Spark Plasma Sintering and used as negative electrode material for Ni-MH batteries

In the present work, starting from the nominal composition La0.85Mg0.15Ni3.8, samples have been synthesized by SPS (Spark Plasma Sintering) technique at different temperature from 810°C to 900°C. The crystallographic structures as well as the phase compositions have been studied by X-ray diffraction (XRD) and Electron-Probe Micro Analysis (EPMA). A new ternary La5MgNi24 phase with stacking structure (space group R-3m) has been identified and its structure determined by XRD analysis. This (1:4) phase forms at higher temperature than the (5:19) and (2:7) ones. In the present work, the phases (2:7); (5:19); (1:4) and (1:5) coexist for all samples. The substitution of La by Mg only occurs in the layer corresponding to the Laves phase for all the stacking structure phases. The substitution rate (La/Mg ratio in the Laves layer) is equal to half and does not change with the SPS temperature treatment. The hydrogen storage capacity and the electrochemical capacity are not too much influenced by the SPS temperature. In contrast, the cycling stability shows better resistance to corrosion for the samples containing larger amount of (5:19) phase.

Structural and magnetic properties of self-doped perovskite manganites La0.8-xSr0.2MnO3−δ

Self-doped perovskite manganites with nominal composition La0.8−xSr0.2MnO3−δ (0 ≤ x ≤ 0.20) have been prepared by the sol-gel method. The highest heat treatment temperature used was 1073 K. The x-ray diffraction (XRD) patterns indicate that the samples had a single phase with the ABO3 perovskite structure when the doping content was x ≤ 0.10 and that when the doping content was x ≥ 0.15 the samples had two phases with the ABO3 perovskite structure being the dominant phase and Mn3O4 being the minor phase. On the basis of the thermal equilibrium theory of crystal defects, the contents of various ions were estimated for the perovskite phases in which there are Mn2+ ions and no vacancies at the A sites. The ion contents have been corrected by Rietveld fitting of the powder sample x-ray diffraction data. Magnetic measurements indicated that the evolution of the Curie temperature (TC) vs. the Mn4+ ion content ratio at the B sites of the ABO3 structure is in accord with experimental results for La1−xSrxMnO3 samples.

Elastic and magnetic properties of the La0.6Pr0.1Ca0.3MnO3 single crystal

The temperature dependences of the velocity of longitudinal sound, internal friction, and magnetization of the single crystal with the nominal composition La0.6Pr0.1Ca0.3MnO3 have been measured. It has been found that the substitution of praseodymium for lanthanum in La0.7Ca0.3MnO3 leads to a decrease in the velocity of sound and to an increase in the spontaneous magnetization. The method of determining the Curie temperature distribution function during a first-order transition has been proposed. It has been shown that, in the crystal under study, this function is asymmetric.

Roles of Te and Mn in the two phases of manganite with nominal composition La0.6Sr0.1Te x MnO3

Powder samples with nominal composition La0.6Sr0.1TexMnO3 (x = 0.00, 0.05, 0.10, 0.15, 0.20) were prepared using the sol-gel method with thermal treatment up to 1473 K. On the basis of powder X-ray diffraction (XRD), thermogravimetric and magnetic measurements, it was found that almost all of the Te and a few of the Mn ions were lost from the samples when they were calcined at 1473 K. The reason for the Te loss and a quantitative phase analysis for the samples calcined at 1473 K are discussed in detail.

Tailoring of LaxSr1‐xCoyFe1‐yO3‐δ Nanostructure by Pulsed Laser Deposition

AbstractPulsed Laser Deposition (PLD) was used to prepare thin films with the nominal composition La0.58Sr0.4Co0.2Fe0.8O3‐δ (LSCF). The thin film microstructure was investigated as a function of PLD deposition parameters such as: substrate temperature, ambient gas pressure, target‐to‐substrate distance, laser fluence and frequency. It was found that the ambient gas pressure and the substrate temperature are the key PLD process parameters determining the thin film micro‐ and nanostructure. A map of the LSCF film nanostructures is presented as a function of substrate temperature (25–700 °C) and oxygen background pressure (0.013–0.4 mbar), with film structures ranging from fully dense to highly porous. Fully crystalline, dense, and crack‐free LSCF films with a thickness of 300 nm were obtained at an oxygen pressure lower than 0.13 mbar at a temperature of 600 °C. The obtained knowledge on the structure allows for tailoring of perovskite thin film nanostructure, e.g., for solid oxide fuel cell cathodes. A simple geometrical model is proposed, allowing estimation of the catalytic active surface area of the prepared thin films. It is shown that voids at columnar grain boundaries can result in an increase of the surface area by approximately 25 times, when compared to dense flat films.

Growth and magnetic properties of bulk electron doped La0.7Ce0.3MnO3 manganites

AbstractWe report on the growth of bulk samples (composite fibers) of the nominal composition La0.7Ce0.3MnO3 (LCMO) manganites (with Curie temperature TC,max = 300 K) by using the laser heated pedestal growth (LHPG) technique. Samples composition was verified through scanning electron microscopy with energy dispersive X‐ray (EDX) analysis. The magnetic behavior of the fibers was probed by magnetization measurements. In addition to a weak ferromagnetic transition at TC0 = 45 K (due to MnO2 impurities), two more transitions with TC1 = 126 K and TC2 = 180 K were identified and linked to regions with cerium concentration of x = 0.063 and 0.13, respectively.