Showing papers on "Perovskite (structure) published in 2007"

Superconducting Interfaces Between Insulating Oxides

Abstract: At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Effect of oxygen vacancies in the SrTiO 3 substrate on the electrical properties of the LaAlO 3 ∕ SrTiO 3 interface

Abstract: We experimentally investigated optical, electrical, and microstructural properties of heterointerfaces between two thin-film perovskite insulating materials, SrTiO3 (STO) and LaAlO3 (LAO), deposited at different oxygen pressure conditions. Cathode and photoluminescence experiments show that oxygen vacancies are formed in the bulk STO substrate during the growth of LAO films, resulting in high electrical conductivity and mobility values. In both high and low oxygen pressure interfaces, the electrical Hall mobilities follow a similar power-law dependence as observed in oxygen reduced STO bulk samples. The results are confirmed on a microscopic level by local strain fields at the interface reaching 10 nm into the STO substrate.

Rapid oxygen ion diffusion and surface exchange kinetics in PrBaCo2O5+x with a perovskite related structure and ordered A cations

Abstract: As part of an investigation of new cathode materials for intermediate temperature solid oxide fuel cells, we have investigated particular perovskite oxides with ordered A cations which, in turn, localize the oxygen vacancies into layers. The oxygen exchange kinetics of polycrystalline samples of the oxygen-deficient double perovskite PrBaCo2O5+x (PBCO) have been determined by electrical conductivity relaxation (ECR) and by oxygen-isotope exchange and depth profiling (IEDP). The ECR and IEDP measurements reveal that PBCO has high electronic conductivity and rapid oxygen ion diffusion and surface exchange kinetics. Both techniques demonstrate that the oxygen kinetics in this structure type are significantly faster than in corresponding disordered perovskites.

Evidence for an oxygen diffusion model for the electric pulse induced resistance change effect in transition-metal oxides.

Abstract: Electric-pulse induced resistance hysteresis switching loops for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ perovskite oxide films were found to exhibit an additional sharp ``shuttle tail'' peak around the negative pulse maximum for films deposited in an oxygen-deficient ambient. The resistance relaxation in time of this ``shuttle tail'' peak as well as resistance relaxation in the transition regions of the resistance hysteresis loop show evidence of oxygen diffusion under electric pulsing, and support a proposed oxygen diffusion model with oxygen vacancy pileup at the metal electrode interface region as the active process for the nonvolatile resistance switching effect in transition-metal oxides.

Infinite-layer iron oxide with a square-planar coordination

Abstract: Conventional high-temperature reactions limit the control of coordination polyhedra in transition-metal oxides to those obtainable within the bounds of known coordination geometries for a given transition metal. For example, iron atoms are almost exclusively coordinated by three-dimensional polyhedra such as tetrahedra and octahedra. However, recent works have shown that binary metal hydrides act as reducing agents at low temperatures, allowing access to unprecedented structures. Here we show the reaction of a perovskite SrFeO3 with CaH2 to yield SrFeO2, a new compound bearing a square-planar oxygen coordination around Fe2+. SrFeO2 is isostructural with 'infinite layer' cupric oxides, and exhibits a magnetic order far above room temperature in spite of the two-dimensional structure, indicating strong in-layer magnetic interactions due to strong Fe d to O p hybridization. Surprisingly, SrFeO2 remains free from the structural instability that might well be expected at low temperatures owing to twofold orbital degeneracy in the Fe2+ ground state with D(4h) point symmetry. The reduction and the oxidation between SrFeO2 and SrFeO3 proceed via the brownmillerite-type intermediate SrFeO2.5, and start at the relatively low temperature of approximately 400 K, making the material appealing for a variety of applications, including oxygen ion conduction, oxygen gas absorption and catalysis.

Perovskite oxides for semiconductor-based gas sensors

Abstract: The oxygen partial pressure dependence of the point defect concentration, and thus conductivity, in oxide semiconductors allows for their use in high-temperature gas sensors. In addition to responding to oxygen partial pressure, the resistance of oxide semiconductors can be affected by other gases, such as carbon monoxide, hydrocarbons and ethanol, which creates opportunities for developing new sensors, but also leads to interference problems. The most common oxide used in such sensors is tin oxide, although other simple oxides, and some mixed oxides, are also used. The focus of this paper is on the use of perovskite oxides in semiconductor-based gas sensors. The perovskite structure, with two differently-sized cations, is amenable to a variety of dopant additions. This flexibility allows for control of the transport and catalytic properties, which are important for improving sensor performance.

Phase structures and electrical properties of new lead-free (Na0.5K0.5)NbO3–(Bi0.5Na0.5)TiO3 ceramics

Abstract: Highly dense (1−x)(Na0.5K0.5)NbO3–x(Bi0.5Na0.5)TiO3 (NKN-BST) solid solution piezoelectric ceramics have been fabricated by ordinary sintering. All compositions show pure perovskite structures, showing room-temperature symmetries of orthorhombic at x⩽0.02, of tetragonal at 0.03⩽x⩽0.09, of cubic at 0.09 0.20. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal ferroelectric phases was identified in the composition range of 0.02

Oxide spintronics

Abstract: Concomitant with the development of metal-based spintronics in the late 1980's and 1990's, important advances were made on the growth of high-quality oxide thin films and heterostructures While this was at first motivated by the discovery of high-temperature superconductivity in perovskite Cu oxides, this technological breakthrough was soon applied to other transition metal oxides, and notably mixed-valence manganites The discovery of colossal magnetoresistance in manganite films triggered an intense research activity on these materials, but the first notable impact of magnetic oxides in the field of spintronics was the use of such manganites as electrodes in magnetic tunnel junctions, yielding tunnel magnetoresistance ratios one order of magnitude larger than what had been obtained with transition metal electrodes Since then, the research on oxide spintronics has been intense with the latest developments focused on diluted magnetic oxides and more recently on multiferroics In this paper, we will review the most important results on oxide spintronics, emphasizing materials physics as well as spin-dependent transport phenomena, and finally give some perspectives on how the flurry of new magnetic oxides could be useful for next-generation spintronics devices

Electronic structure, chemical bonding and elastic properties of the first thorium‐containing nitride perovskite TaThN3

Abstract: The full-potential linearized augmented plane wave method with the generalized gradient approximation for the exchange and correlation potential (LAPW-GGA) is used to understand the electronic and elastic properties of the first thorium-containing nitride perovskite TaThN3. Total and partial density of states, charge distributions as well as the elastic constants, bulk modulus, compressibility, shear modulus, Young modulus and Poisson ratio are obtained for the first time and analyzed in comparison with cubic ThN. The chemical bonding in TaThN3 is a combination of ionic Th–N and of mixed covalent–ionic Ta–N bonds. The cubic TaThN3 is semiconducting with the direct gap at about 0.65 eV. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduced Band Gap Hybrid Perovskites Resulting from Combined Hydrogen and Halogen Bonding at the Organic−Inorganic Interface

Abstract: Undistorted perovskite layers are formed in PbI4-based hybrid perovskites incorporating X−(CH2)2−NH3+ (X = Br, Cl) cations. The outstanding structural feature of these compounds is the result of halogen and hydrogen bonding at the organic−inorganic interface, leading to the absence of hydrogen bonds between equatorial iodine atoms of the perovskite layer and ammonium parts, these last being located out of perovskite layers. As a consequence, these red salts display a reduced band gap (2.2 eV) which is assigned to a more disperse HOMO band compared to other salts such as (I−(CH2)2−NH3)2PbI4.

Structural, photocatalytic, and photophysical properties of perovskite MSnO3 (M = Ca, Sr, and Ba) photocatalysts

Abstract: The photophysical properties of MSnO3 (M = Ca, Sr, and Ba) including optical absorption, photoluminescence, and energy band structure including band edge positions were investigated experimentally and theoretically in association with their photocatalytic properties. Photocatalytic reactions for H2 and O2 evolution in the case of sacrificial reagents were performed under ultraviolet (UV) light irradiation. The order of the activities of H2 evolution was CaSnO3 > SrSnO3 > BaSnO3, agreeing not only with that of the conduction-band edges (or band gaps) but also with that of the transferred excitation energy, while that of O2 evolution was CaSnO3 < SrSnO3 < BaSnO3, consistent with that of the angle of the Sn–O–Sn bonds as well as the delocalization of excited energy. When loaded with RuO2 cocatalyst, both CaSnO3 and SrSnO3 can efficiently split pure water into hydrogen and oxygen in a stoichiometric ratio under UV light irradiation. In addition, RuO2-loaded SrSnO3 showed higher water splitting activity than RuO2-loaded CaSnO3 did. This is attributed to the suitable conduction and valence band edges and to high mobility of the photogenerated charge carriers caused by the proper distortion of SnO6 connection in SrSnO3. The RuO2-loaded BaSnO3 photocatalyst cannot split pure water, which might be because of a high concentration of defect centers such as Sn2+ ions and the probability of radiative recombination in BaSnO3.

Structural and electrical properties of Ca2+-modified PZT electroceramics

Abstract: A perovskite ceramic oxide having the general formula Pb0.97Ca0.03Zr0.05Ti0.95O3 (PCZT) was prepared using precursor solution method. The compound belongs to the tetragonal system. TEM micrographs show that the particles are more or less spherical in shape. Complex impedance analysis indicates the presence of grains separated by grain boundaries, which are also evident from the SEM micrographs. The sample impedance behavior has been explained in terms of the bricklayer model. The relaxation process taking place is short range. The conductivity variation shows a typical Arrhenius-type behavior having linear dependence on logarithm of conductivity.

Sr2CrOsO6 : End point of a spin-polarized metal-insulator transition by 5d band filling

Abstract: In the search for new spintronic materials with high spin polarization at room temperature, we have synthesized an osmium-based double perovskite with a Curie temperature of 725 K. Our combined exp ...

Modern Physics of Ferroelectrics: Essential Background

Abstract: Principles of ferroelectricity and information about ferroelectric materials and their applications are reviewed. The characterization of ferroelectric behavior through measurement of electrical hysteresis is discussed in detail. The main families of ferroelectric oxides, including perovskite compounds and solid solutions, lithium niobate, layered oxides, magnetic ferroelectric oxides, and electronic ferroelectrics are presented and their crystal structures and polarizations given. The effects of pressure and epitaxial strain on perovskites are described. Recent advances in the understanding of ferroelectricity in thin films, superlattices and nanostructures are mentioned. Finally, an overview of applications of feroelectric materials, both established applications and those under development, is included.

Ceria-based oxides as supports for LaCoO3 perovskite; catalysts for total oxidation of VOC

Abstract: Supported LaCoO3 perovskites with 10 and 20 wt.% loading were obtained by wet impregnation of different Ce1−xZrxO2 (x = 0–0.3) supports with a solution prepared from La and Co nitrates, and citric acid. Supports were also prepared using the “citrate method”. All materials were calcined at 700 °C for 6 h and investigated by N2 adsorption at −196 °C, XRD and XPS. XRD patterns and XPS measurements evidenced the formation of a pure perovskite phase, preferentially accumulated at the outer surface. These materials were comparatively tested in benzene and toluene total oxidation in the temperature range 100–500 °C. All catalysts showed a lower T50 than the corresponding Ce1−xZrxO2 supports. Twenty weight percent LaCoO3 catalysts presented lower T50 than bulk LaCoO3. In terms of reaction rates per mass unit of perovskite calculated at 300 °C, two facts should be noted (i) the activity order is more than 10 times higher for toluene and (ii) the reverse variation with the loading as a function of the reactant, a better activity being observed for low loadings in the case of benzene. For the same loading, the support composition influences drastically the oxidative abilities of LaCoO3 by the surface area and the oxygen mobility.

Property-processing relationship in lead-free (K, Na, Li) NbO3-solid solution system

Abstract: There has been a significant driving force to eliminate the utilization, recycling, and disposal of ferroelectric ceramics with high content of toxic element (Pb). Recently, the ternary system of KNN-LT-LS has proven to be an outstanding lead-free piezoceramic with properties almost comparable to their lead-based counterpart, PZT. This study reports the effect of various processing conditions on the electromechanical properties of (K0.44Na0.52Li0.04)(Nb0.84Ta0.10Sb0.06)O3 system. This includes powder processing, humidity, and exposure to oxygen rich environment during sintering. The Perovskite and Mixed-Oxide methods are used to prepare the stoichiometric powders. It will be shown that both processing methods are notably sensitive to the moisture of as received raw materials and the humidity of environment. Optimum results are obtained when the raw materials undergo a pre-heat treatment followed by formulating the desired composition in an inert atmosphere. The highest electromechanical properties are achieved when the ceramics are completely exposed to oxygen with a high flow rate. Sintered at 1150 °C for 1 h with an oxygen flow rate of 180 cm3/min, the KNN-LT-LS ceramics prepared by Perovskite and Mixed-Oxide routes have d 33 ≥ 300 pC/N, $$\varepsilon ^{T}_{{33}} = 1865$$ , tan δ = 0.02, k 33 = 0.65.

Reduced leakage current in La and Ni codoped BiFeO3 thin films

Abstract: La (2.5%) and Ni (2.5%) codoped BiFeO3 (BLFNO) thin films were formed by chemical solution deposition on Pt∕Ti∕SiO2∕Si(100) structures. BiFeO3 (BFO), La(5%)-doped BFO, and Ni(5%)-doped BFO films were studied for comparison. X-ray diffraction analysis showed that the crystal structure of all films was single perovskite phase, however, structural modifications were observed in La-doped films. The leakage current density at 500kV∕cm was reduced by approximately three orders of magnitude by codoping La and Ni atoms, compared with BFO films. In BLFNO films, well saturated remanent polarization of 70μC∕cm2 was obtained at room temperature at 10kHz with the reduced coercive field by approximately 30%.

Microstructure and electrical properties of MnO-Doped (Na0.5Bi0.5)0.92Ba0.08TiO3 lead-free piezoceramics

Abstract: Microstructure and electrical properties of manganese oxide (MnO)-doped (Na0.5Bi0.5)0.92Ba0.08TiO3 (NBBT) piezoceramics were investigated in this work. X-ray diffraction analysis shows that the suitable substitution of Mn ion into the B site induces the lattice distortion of perovskite NBBT: the solution limit is at 0.3 wt% MnO. Besides, it is observed that the sintering properties can be improved by adding a small amount of MnO, thus increasing the grain size and the relative density. Further, the temperature dependence of the dielectric permittivity of NBBT ceramics indicates that the MnO addition reconstructs the disorder array destroyed by joining BaTiO3 in the Na0.5Bi0.5TiO3 system due to the sizable radius of the B-site cations. Combining these effects of MnO addition, the optimal electrical properties were acquired for NBBT ceramic with addition of 0.30 wt% MnO. The excellent electrical properties of MnO-doped NBBT ceramics indicate its promising application in large displacement actuators.

LaCoO3: Effect of synthesis conditions on properties and reactivity

Abstract: Two nanostructured bulk LaCoO3 powders were prepared by co-precipitation and with the citrate gel method and compared with the two nanocomposites obtained depositing, by wet impregnation, cobalt oxide on the La2O3 surface. All the prepared samples were characterized by means of X-ray photoelectron (XP) and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermal analysis. XRD results suggest that only the citrate gel method allows to obtain a single phase lanthanum cobaltite powder having a rhombohedral perovskite structure. Moreover, SEM images confirm the wide scale homogeneity. In contrast, a lanthanum hydroxide phase (hexagonal) is also evident in the sample obtained by co-precipitation method. In the nanocomposite richer in cobalt, besides the rhombohedral perovskite, the hexagonal lanthanum oxide and hydroxide phases are also detectable. Perovskite phase does not form when the amount of deposited cobalt oxide is too low. Both in the nanostructured sample prepared by citrate gel method and nanocomposite with [Co/La]nominal = 1.0 XPS, DRIFT and TGA outcomes suggest a lower presence of hydroxyl groups and carbonates species. The reactivity of the powders with respect to carbon monoxide was studied (at atmospheric pressure, between RT and 573 K) by means of the diffuse reflectance infrared Fourier transform spectroscopy. A significant signal around 2058 cm−1 testifies about the interaction of CO with the Lewis acidic sites distributed on the surface of all the samples. The two nanostructured bulk LaCoO3 show a high reactivity toward CO oxidation since 423 K. A similar oxidation capability is observed also for the nanocomposite richer in cobalt. In contrast, a rather low reactivity is displayed from the nanocomposite with lower Co/La nominal atomic ratio and La2O3.