Phase Of La0 Research Articles

Nature of stripes in the generalized t–J model applied to the cuprate superconductors

A recent study of the transport properties on the stripe phase in La1.875Ba01.25CuO4 by Li et al (2007 Phys. Rev. Lett. 99 067001) found two-dimensional (2D) superconductivity over a wide temperature range including a Berezinski–Kosterlitz–Thouless transition at a temperature T=16 K, with 3D superconducting (SC) ordering only at T=4 K. These results contradict the long standing belief that the onset of superconductivity is suppressed by stripe ordering and suggest coexistence of stripe and SC phases. The lack of 3D SC order above T=4 K requires an antiphase ordering in the SC state to suppress the interlayer Josephson coupling as proposed by Berg et al (2007 Phys. Rev. Lett. 99 127003). Here, we use a renormalized mean field theory for a generalized t–J model to examine in detail the energetics of the spin and charge stripe ordered SC states including possible antiphase domains in the SC order. We find that the energies of these modulated states are very close to each other and that the anisotropy present in the low temperature tetragonal crystal structure favors stripe resonating valence bond states. The stripe antiphase SC states are found to have energies very close to, but always above, the ground state energy, which suggests additional physical effects are responsible for their stability.

Effect of substituting Al for Co on the hydrogen-storage performance of La0.7Mg0.3Ni2.6AlxCo0.5−x (x=0.0–0.3) alloys

La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys were prepared by induction melting, and the effects of partially substituting Al for Co on the structure and hydrogen-storage properties of the alloys were investigated systematically. It is found that La(Ni, Co, Al)5 phase with hexagonal CaCu5-type structure, LaNi3 phase with PuNi3 structure and MgNi2 phase exist as the main phases in La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys, and the cell volume of the La(Ni, Co, Al)5 phase increases with the amount of Al added. The results show that the substitution of Al for Co can reduce the plateau pressure and the hysteresis between hydrogen absorption and desorption, and improve the hydrogen-absorption capacity and thermal stability of the hydride. Moreover, the addition of Al can delay the oxidation of the surface layer of the alloy electrodes in electrolyte, slow down the capacity degradation and prolong the cycling lifetime, and enhance the electrocatalytic activity of the hydrogen-storage electrodes for hydrogen oxidation.

Interpretation of Thermal Conductivity in the Ferromagnetic Metallic Phase of La0.83Sr0.17MnO3 Manganites: Scattering of Phonons and Magnons

The lattice contribution to the thermal conductivity (κph) is theoretically analyzed within the framework of Kubo model in La0.83Sr0.17MnO3 manganites. The theory is formulated when thermal conduction is limited by the scattering of phonons from defects, grain boundaries, charge carriers, and phonons. The lattice thermal conductivity dominates in La-Sr-MnO manganites and is artifact of strong phonon-impurity and -phonon scattering mechanism in the ferromagnetic metallic state. The electronic contribution to the thermal conductivity (κe) is estimated following Wiedemann-Franz law. This estimate sets an upper bound on κe, and in the vicinity of Curie temperature (Tc) is about 1% of total heat transfer of manganites. Another important contribution in the metallic phase should come from spin waves (κm). It is noticed that κm increases with a T2 dependence on the temperature. These channels for heat transfer are algebraically added and κtot develops a broad peak at about 120 K, before falling off at lower temperatures. The behaviour of the thermal conductivity in manganites is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between electron, magnon, and phonon contributions. The numerical analysis of heat transfer in the ferromagnetic metallic phase of manganites shows similar results as those revealed from experiments.

Neutron-Scattering Study of Impurity Effect on Stripe Correlations in La-Based 214 High-Tc Cuprate

We present results of neutron-scattering study of impurity effects on the static stripe correlations in La-based 214 high-Tc cuprate. We found that the superlattice peaks from the charge-density-wave (CDW) order was induced by doping magnetic Fe ions in the low-temperature-orthorhombic (LTO) phase of La2−xSrxCuO4 (LSCO). The spin-density-wave (SDW) order is also stabilized by Fe-doping, suggesting the appearance of stripe ordered state in the LTO phase, as is seen it the low-temperature-tetragonal (LTT) phase of La1.875Ba0.125Cu4 (LBCO). Therefore, the stripe order can be potentially realized in the La-214 system irrespective of crystal structure. On the other hand, the onset temperature for the appearance of SDW and CDW orders is lowered by Zn-doping in La1.875Ba0.125CuO4 system. This result suggests that the existing well-stabilized stripe order is degraded by substituting non-magnetic Zn ions.

Magnetocaloric effect and Griffiths-like phase in La0.67Sr0.33MnO3 nanoparticles

The effect of grain size on electrical and magnetic properties of La0.67Sr0.33MnO3 nanoparticles with average grain size 32–85 nm has been investigated. The metal-insulator transition temperature TP gradually decreases with decreasing grain size, while the ferromagnetic-paramagnetic transition temperature TC remains almost constant. For the 32 nm sample, the larger effective magnetic moments and the deviation of the inverse susceptibility from the Curie–Weiss law are observed, indicating the possible existence of a Griffiths-like cluster phase. The ferromagnetic transition of the samples is further investigated by measuring magnetocaloric effect (MCE). The presence of short-range magnetic order greatly depresses the magnetic entropy of the paramagnetic phase. Moreover, the analysis of the MCE using Landau theory of phase transition confirms the importance of magnetoelastic coupling and electron interaction in magnetocaloric properties of manganites.

Mechanochemical synthesis of nonstoichiometric nanocrystals La1 − y Ca y F3 − y with a tysonite structure and nanoceramic materials from CaF2 and LaF3 crystals

The nonstoichiometric phases La1 − yCayF3 − y (y = 0.15, 0.20) with a tysonite (LaF3) structure have been prepared for the first time by the mechanochemical synthesis from CaF2 and LaF3 crystals. The average size of coherent scattering regions is approximately equal to 10–30 nm. It has been shown that the compositions of the phases prepared by the mechanochemical synthesis are inconsistent with the phase diagram of the CaF2-LaF3 system. The “mechanohydrolysis” of the La1 − yCayF3 − y phase has been observed for the first time. Under these conditions, the La1 − yCayF3 − y phase partially transforms into lanthanum calcium oxyfluoride for a milling time of 180 min with intermediate sampling. The La1 − yCayF3 − y nanoceramic materials have been prepared from a powder of the mechanochemical synthesis product by pressing under a pressure of (2–6) × 108 Pa at room temperature. The electrical conductivity of the synthesized materials at a temperature of 200°C is equal to 4.9(6) × 10−4 S/cm, and the activation energy of electrical conduction is 0.46(2) eV. These data for the nanoceramic materials coincide with those obtained for migration of fluorine vacancies in single-crystal tysonite fluoride materials.

Ordering and twinning in the nonstoichiometric phase La1 − y Ca y F3 − y

The nonstoichiometric phase La1 − y Ca y F3 − y with a LaF3-related structure has been studied by electron diffraction. In addition to diffraction features corresponding to twinning on the (001) and {1\( \bar 2 \)0} planes, typical of LaF3, evidence has been found for twinning on the {101} and {102} planes. This behavior is accounted for by the formation of ordering domains with an ordering direction inclined to mirror planes. It seems likely that ordering domains determine many of the properties of nonstoichiometric phases.

Evidence for Griffiths phase in La0.4Ca0.6MnO3 film with strip-domain inclusions

The microstructure and the magnetic properties of La0.4Ca0.6MnO3 film, prepared by rf magnetron sputtering on a LaAlO3 substrate, have been investigated. The electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c for the orthorhombic symmetry. The magnetic measurements show that in addition to the expected antiferromagnetic transition at TN≃120 K with decreasing temperature, the film manifests the Griffiths phase behavior in a wide temperature range.

Diagonal charge-density-wave state in the insulating spin-glass phase ofLa2−xSrxCuO4

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Raman scattering studies of the temperature- and field-induced melting of charge order inLaxPryCa1−x−yMnO3

We present Raman scattering studies of the structural and magnetic phases that accompany temperature- and field-dependent melting of charge- and orbital-order (COO) in La0.5Ca0.5MnO3 and La0.25Pr0.375Ca0.375MnO3. Our results show that thermal and field-induced COO melting in La0.5Ca0.5MnO3 exhibits three stages in a heterogeneous melting process associated with a structural change: a long-range, strongly JT distorted/COO regime; a coexistence regime; and weakly JT distorted/PM or FM phase. We provide a complete structural phase diagram of La0.5Ca0.5MnO3 for the temperature and field ranges 6<=T<=170 K and 0<=H<=9 T. We also investigate thermal and field-induced melting in La0.25Pr0.375Ca0.375MnO3 to elucidate the role of disorder in melting of COO. We find that while thermal melting of COO in La0.25Pr0.375Ca0.375MnO3 is quite similar to that in La0.5Ca0.5MnO3, the field-induced transition from the COO phase to the weakly JT-distorted/FM phase in La0.25Pr0.375Ca0.375MnO3 is very abrupt, and occurs at significantly lower fields (H~2 T at T~0 K) than in La0.5Ca0.5MnO3 (H~30 T at T=0 K). Moreover, the critical field H_c increases with increasing temperature in La0.25Pr0.375Ca0.375MnO3 in contrast to La0.5Ca0.5MnO3. To explain these differences, we propose that field-induced melting of COO in La0.25Pr0.375Ca0.375MnO3 is best described as the field-induced percolation of FM domains, and we suggest that Griffiths phase physics may be an appropriate theoretical model for describing the unusual temperature- and field- dependent transitions observed in La0.25Pr0.375Ca0.375MnO3.

Delamination and magnetism softening of polycrystalline La0.9Ba0.1MnO3

Two ferromagnetic transitions have been found by temperature dependence ac susceptibility experiments on as-prepared bulk polycrystalline La0.9Ba0.1MnO3. Investigation indicates that the higher temperature transition (TCh) can be attributed to the oxygen excessive phase La0.9Ba0.1MnO3+δ at the surface, while the lower temperature transition (TCl) can be attributed to the interior stoichiometric phase La0.9Ba0.1MnO3 of the samples. Softening of ferromagnetic system near TCl has been observed in La0.9Ba0.1MnO3, which can be explained by the freezing of misorder of metal ion at higher temperature.

Preparation and Thermophysical Properties of (La&lt;sub&gt;0.4&lt;/sub&gt;Sm&lt;sub&gt;0.5&lt;/sub&gt;Yb&lt;sub&gt;0.1&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;(Zr&lt;sub&gt;0.7&lt;/sub&gt;Ce&lt;sub&gt;0.3&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt; Ceramic for Thermal Barrier Coatings

A complex rare-earth zirconate (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 powder for thermal barrier coatings (TBCs) was synthesized by coprecipitation method. The phase composition, microstructure and the thermophysical properties were investigated. XRD results revealed that single phase (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 with pyrochlore structure was prepared and the SEM result showed that the microstructure of the product was dense and no other phases existed among the particles. With the temperature increasing, the thermal expansion coefficient (CTE) of the ceramic increased, while the thermal conductivity decreased. The results indicated that CTE of the ceramic was slightly higher than that of La2Zr2O7 and the thermal conductivity of the ceramic was lower than that of La2Zr2O7. These results imply that (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.3)2O7 can be explored as the candidate material for the ceramic layer in TBCs system.

Variable range hopping in the spin-glass phase ofLa2−xSrxCuO4

The results of the in-plane transport measurements of strongly underdopedLa2−xSrxCuO4 films in magnetic fields up to 14 T and at temperatures down to 1.6 K are presented.While at high temperatures the resistivity of the films is metallic-like, thelow-T transport is governed by the variable range hopping (VRH) mechanism. Careful analysisshows that the temperature dependence of the pre-exponential factor in Mott’s VRH lawcannot be neglected and the zero-field conductivity is best described by the formulaσ = σ0T−2wexp[−(T0/T)w]. In a magneticfield parallel to CuO2 planes the VRH law is retained, with both parameters,σ0 and T0, decreasing by about 10–20% at 14 T. This effect may be related to the decrease of thetunnelling barrier between different antiferromagnetic clusters in the presence of amagnetic field.

Magneto-transport and magneto-dielectric effects in Bi-based perovskite manganites

The study of the substitution of manganese by cobalt in the perovskite (LaBi)MnO3 in view of discovering magneto-dielectric properties in manganites, has allowed the perovskite La1.2Bi0.8Mn1.2Co0.8O6.02 to be selected. This phase is indeed ferromagnetic, like the parent phase La1.2Bi0.8Mn2O6.20, yet with a much higher TC (181 K instead of 103 K). Moreover, it is a better insulator than the other members with an activation energy of 86 meV. Importantly, it exhibits a significant magneto-dielectric effect of around 0.25% at 80 K. Finally, it is also shown to exhibit magnetoresistance (MR) properties at low temperature, with MR values up to −19% at 125 K. These properties are explained on the basis of Mn4+/Co2+ ferromagnetic interactions, electronic phase separation and spin–lattice interactions.

Phase equilibria in the system LaMnO3-SrMnO3-SrCrO4-LaCrO3

A continuous solid solution LaMn1−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 La1−x Sr x CrO3 exists in the range of 0.0 ≤ x ≤ 0.1. The stability boundaries are determined for the perovskite phase La1−x Sr x Mn1−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.

Order and Disorder in Cluster Phases – The Case of La≈0.7(Al≈0.1I≈0.9)

AbstractThe structure of the new partially ordered cluster phase La0.69(1)(Al0.12(1)I0.88(1)) was determined by a combined methodical approach of electron microscopy techniques and X‐ray diffraction. The structure is derived from the Gd3CCl3‐type and ca. 50 % of the cluster positions in this structure type are occupied. The majority of the clusters are condensed via common edges, however, the degree of cluster condensation is low. The arrangement of neighboring clusters is comparable to chemically related, but strongly disordered cluster phases. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2007)

Observation of a Griffiths-like phase in bilayered manganites

The authors report the observation of a Griffiths-like phase in La2−2xSr1+2xMn2O7 (x=0.30, 0.33, and 0.40) and (La0.8Eu0.2)2−2xSr1+2xMn2O7 (x=0.33) single crystals by means of electron spin resonance, magnetic susceptibility, and magnetoresistance measurements. In the temperature region TC&amp;lt;T&amp;lt;350K, the resonance signal consists of a ferromagnetic resonance line and a paramagnetic resonance line, which suggests that the system is not in a pure paramagnetic phase. The ferromagnetic resonance signal disappears and the magnetic susceptibility starts to deviate from the Curie-Weiss law at the same temperature T*≈350K, independent of doping level and anisotropy. These results indicate that a Griffiths-like phase exists at TC&amp;lt;T&amp;lt;T* in bilayered manganites, which is due to intrinsic inhomogeneity caused by quenched disorder.

Synthesis and Electrical Properties of La-Pr-Mn-O Thin Films and Heterostructures

In this paper we report the results of synthesis and study of both ceramic samples and thin films of electronically doped La0.7Pr0.3MnO3 and related heterostructures composed of La0.7Pr0.3MnO3 and p-type La0.67Ca0.33MnO3. The ceramic La0.7Pr0.3MnO3 samples were prepared by a conventional solid state reaction technique. Single phase La0.7Pr0.3MnO3 thin films and La0.7Pr0.3MnO3/La0.67Ca0.33MnO3 heterostructures were grown on lattice-matched perovskite NdGaO3 substrates by pulsed laser deposition. Electron doping was indicated both for ceramic La0.7Pr0.3MnO3 samples and thin films from thermopower data. Both ceramic samples and thin films of La0.7Pr0.3MnO3 demonstrated resistivity of about 10 mΩ cm at 300 K and semiconductor-like resistance vs. temperature behavior with cooling down to 78 K. Meanwhile, the resistance of the La0.7Pr0.3MnO3/La0.67Ca0.33MnO3 interface showed an anomalous peak at 185 K. A series of post-deposition annealing experiments demonstrated a crucial role of annealing temperature and ambience on both electrical and magnetic properties of La0.7Pr0.3MnO3 material and the heterostructures.

Neutron Powder Diffraction Study of a Phase Transition in La0.68(Ti0.95Al0.05)O3

Crystal structures and structural changes of the compound La0.68(Ti0.95Al0.05)O3 have been studied using neutron powder diffraction data and the Rietveld method in the temperature range from 25° to 592°C. The Rietveld profile‐fitting analyses of the neutron data and the synchrotron diffraction profile revealed that the crystal symmetry of the low‐temperature phase of La0.68(Ti0.95Al0.05)O3 is orthorhombic Cmmm (2ap× 2ap× 2ap; p: pseudo‐cubic perovskite). The unit‐cell and structural parameters were successfully refined with the orthorhombic Cmmm for the intensity data measured at 25°, 182°, and 286°C, and with the tetragonal P4/mmm (ap×ap× 2ap) for intensity data obtained at 388° and 592°C. The P4/mmm‐to‐Cmmm phase transition was found to be induced by tilting of the (TiAl)O6 octahedron. The tilt angle decreased with increasing temperature, reaching 0° at the Cmmm–P4/mmm transition temperature.

Potential New Solid Oxide Fuel Cell (SOFC) Anode Materials in the La‐Ca‐Cr‐Ti‐Ru‐O System

AbstractSix single phase La1–xCaxCr1–yTiy–γRuγO3–δ (x = 0.1, 0.2, y = 1.0, 0.8 and γ = 0.0, 0.1, 0.2) compositions are prepared by solid state reaction. Oxygen non‐stoichiometry, electrical conductivity, catalytic activity, and electrochemical efficiency of these compositions is investigated over a wide temperature and oxygen partial pressure range. The compositions with the highest electrical conductivity (50 S cm–1) at reducing conditions and highest catalytic activity for the oxidation of hydrocarbons are found. A combination of highly conductive La1–xCaxTiO3–δ in a reducing atmosphere and an intermediate functional film of catalytically active La0.1Ca0.9Ti1–xRuxO3–δ for the oxidation of hydrocarbons could potentially be very useful in a multilayer SOFC anode.