Showing papers on "Perovskite (structure) published in 1998"

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

Abstract: Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

Two-dimensional ferroelectric films

Abstract: Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films1,2, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature3. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature4. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 A in diameter5, in perovskite films 100 A thick6 and in crystalline ferroelectric polymers as thin as 25 A (7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size11,12,13. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 A (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

Synthesis and Characterization of Organic−Inorganic Perovskite Thin Films Prepared Using a Versatile Two-Step Dipping Technique

Abstract: A convenient two-step technique for preparing thin films of the organic−inorganic perovskites (RNH3)2(CH3NH3)n-1MnI3n+1 (R = butyl, phenethyl; M = Pb, Sn; n = 1, 2, ∞) has been achieved. Films of the metal halide, MI2, were first deposited onto ash glass or quartz disks using vacuum evaporation or spin-coating. By dipping these inorganic films, at room temperature, into an organic ammonium iodide solution for a short period of time (1−5 min for the layered perovskites), single-phase samples of the corresponding organic−inorganic perovskite thin films were formed. While a variety of solvents can be used for the dipping process, toluene/2-propanol mixtures have been shown to work well for many of the present compounds. The layered organic−inorganic perovskite films exhibited uniform surfaces and strong photoluminescence at wavelengths that were consistent with the corresponding materials made by single-crystal growth from solution. However, dipped organic−inorganic films made from evaporated PbI2 exhibited ...

Crystal orientation dependence of piezoelectric properties of lead zirconate titanate near the morphotropic phase boundary

Abstract: The piezoelectric and dielectric constants in different crystal orientations of the lead zirconate titanate (PZT) have been phenomenologically calculated for the compositions near the morphotropic phase boundary at room temperature. For a tetragonal PZT, the effective piezoelectric constant d33 monotonously decreases as the crystal cutting angle from the spontaneous polarization direction [001] increases. However, for a rhombohedral PZT, the effective piezoelectric constant d33[001]// along the perovskite [001] direction was found to be much larger than those along the spontaneous polarization direction [111]. This crystal orientation-related enhancement is emphasized as the composition approaches the morphotropic phase boundary. This suggests that by adopting the perovskite [001] orientation with a rhombohedral composition near the morphotropic phase boundary, the piezoelectric constant d33 for PZT can be greatly enhanced.

Lead-free high-strain single-crystal piezoelectrics in the alkaline–bismuth–titanate perovskite family

Abstract: Doped alkaline–bismuth–titanate perovskite single crystals have been grown in ferroelectric phases with high piezoelectric actuation. Rhombohedral-phase Na1/2Bi1/2TiO3–BaTiO3 crystals exhibit up to 0.25% free strain with low hysteresis along the cubic 〈001〉 direction (d33∼450 pC/N). Tetragonal phase crystals exhibit free strains as high as 0.85% with greater hysteresis characteristic of domain switching; low field d33 exceeds 500 pC/N. Strain energy densities exceed those of optimized polycrystalline lead perovskites, and actuation capability is retained at compressive stresses >100 MPa.

Intergrain tunneling magnetoresistance in polycrystals of the ordered double perovskite Sr2FeRe06

Abstract: Intergrain tunneling magnetoresistance (TMR) subsisting up to room temperature has been observed for polycrystalline ceramics of ${\\mathrm{Sr}}_{2}{\\mathrm{FeReO}}_{6},$ which has ordered double perovskite structure with Curie temperature above 400 K. The first-principles band calculation predicts that ${\\mathrm{Sr}}_{2}{\\mathrm{FeReO}}_{6}$ shows the half metallic ground state with the ferrimagnetic coupling of Fe and Re spins. The experimental results of electronic and magnetic properties are in accord with this picture. In fact, the magnitude of intergrain TMR with the magnetic field of 7 T at 4.2 and 300 K is as large as 21 and 7%, respectively, reflecting high spin polarization of carriers.

Domain patterns in epitaxial rhombohedral ferroelectric films. I. Geometry and experiments

Abstract: Possible domain patterns are developed for (001) oriented (pseudocubic indexing) epitaxial rhombohedral perovskite ferroelectric (FR) films. We assume that the films are grown above their Curie temperature (TC) in a cubic paraelectric (PC) state. The rhombohedral distortion consists of a “stretch” along one of the four 〈111〉 crystallographic directions of the cubic perovskite unit cell. Domain pattern formation is concurrent with the PC→FR transformation on cooling from the growth temperature. The domain patterns form to minimize elastic energy in the film, at the energetic expense of both forming domain boundaries and developing local stresses in the substrate. Eight possible domains may form, half of which are related by inversion, thus leading to four mechanically distinct variants. The possible domain walls are determined by mechanical and charge compatibility and follow closely from the analysis of Fousek and Janovec [J. Appl. Phys. 40, 135 (1969)]. Domain patterns may develop with either {100} or {1...

Elastic anomalies in minerals due to structural phase transitions

Abstract: Landau theory provides a formal basis for predicting the variations of elastic constants associated with phase transitions in minerals. These elastic constants can show substantial anomalies as a transition point is approached from both the high-symmetry side and the low-symmetry side. In the limiting case of proper ferroelastic behaviour, individual elastic constants, or some symmetry­ adapted combination of them, can become very small if not actually go to zero. When the driving order parameter for the transition is a spontaneous strain, the total excess energy for the transition is purely elastic and is given by: which has the same form as a Landau expansion. In this case, the second-order elastic constant Cik softens as a linear function of temperature with a slope in the low-symmetry phase that depends on the thermodynamic character of the transition. If the driving order parameter, Q, is some structural feature other than strain, the excess energy is given by: G = ia(T Tc )Q2 + ±bQ4 + . . . + .LAi,m,neFQn + i � Ci%eiek l,m,n l,k In this case, the effect of coupling, described by the term in AemQn, is to cause a great diversity of elastic variations depending on the values of m and n (typically 1, 2 or 3), the thermodynamic character of the transition and the magnitudes of any non-symmetry-breaking strains. The elastic constants are obtained by taking the appropriate second derivatives of G with respect to strain in a manner that includes the structural relaxation associated with Q. The symmetry properties of second-order elastic constant matrices can be related to the symmetry rules for individual phase transitions in order to predict elastic stability limits, and to derive the correct form of Landau expansion for any symmetry change. Selected examples of \"ideal\" behaviour for different types of driving order parameter, coupling behaviour and thermodynamic character have been set out in full in this review. Anomalies in the elastic properties on a macroscopic scale can also be understood in terms of the properties of acoustic phonons. These microseopie processes must be considered if elastic anomalies due to dynamical effects are to be accounted for 0935122 1/98/00 10-0693 $ 30.00 001:1 0.1127/ejm/1 0/4/0693 © 1998 E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart

Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO3 Based Oxide Ion Conductor I. Doped as a New Cathode Material

Abstract: ‐based perovskite oxides doped with Sr and Mg exhibit high ionic conductivity over a wide range of oxygen partial pressure. In this study, the stability of ‐based oxide was investigated. The ‐based oxide was found to be very stable in reducing, oxidizing, and atmospheres. Solid oxide fuel cells (SOFCs) using ‐based perovskite‐type oxide as the electrolyte were studied for use in intermediate‐temperature SOFCs. The power‐generation characteristics of cells were strongly affected by the electrodes. Both Ni and (Ln:rare earth) were suitable for use as anode and cathode, respectively. Rare‐earth cations in the Ln site of the Co‐based perovskite cathode also had a significant effect on the power‐generation characteristics. In particular, a high power density could be attained in the temperature range 973–1273 K by using a doped for the cathode. Among the examined alkaline earth cations, Sr‐doped exhibits the smallest cathodic overpotential resulting in the highest power density. The electrical conductivity of increased with increasing Sr doped into the Sm site and attained a maximum at . The cathodic overpotential and internal resistance of the cell exhibited almost the opposite dependence on the amount of doped Sr. Consequently, the power density of the cell was a maximum when was used as the cathode. For this cell, the maximum power density was as high as 0.58 W/cm2 at 1073 K, even though a 0.5 mm thick electrolyte was used. This study revealed that a ‐based oxide for electrolyte and a ‐based oxide for the cathode are promising components for SOFCs operating at intermediate temperature.

Microscopic study of oxygen-vacancy defects in ferroelectric perovskites

Abstract: We investigate the nature of atomic relaxations around oxygen-vacancy defects in the ferroelectric perovskite ${\mathrm{PbTiO}}_{3}$ through first-principles pseudopotential total energy calculations. A tail-to-tail polarization is one of the patterns that emerges from atomic relaxations around oxygen vacancies and its stability is found to be enhanced by charge trapping. Oxygen vacancies in Ti-O-Ti chains along the polarization axis are more favorable than those in Ti-O-Ti planes normal to the axis. The possible role of oxygen vacancies in fatigue and aging is discussed.

Method for switching the properties of perovskite materials used in thin film resistors

Abstract: A method for switching properties of perovskite thin film materials, including the colossal magnetoresitive (CMR) and high temperature superconducting (HTSC) materials, is provided. Short electrical pulses are applied to the materials in thin films to change both reversibly and non-reversibly the electrical, thermal, mechanical and magnetic properties of the material. Reversible resistance changes of over a factor of 10 are induced in CMR materials at room temperature and in zero external magnetic field by electrical pulsing. Applications of the method and materials to form memory devices, resistors in electronic circuits which can be varied in resistance and other applications are disclosed.

Molten salt synthesis of lead-based relaxors

Abstract: The molten salt synthesis (MSS) of lead-based relaxors which have a perovskite structure, A(BIBII)O3 where BI is Mg2+, Fe3+, Zn2+, Ni2+ or Co2+, and BII is Nb5+, has been reviewed with regard to the formation of the perovskites, phase stability and morphology characteristics. Two relaxor materials, Pb(Mg1/3Nb2/3)O3 and Pb(Fe1/2Nb1/2)O3 were found to be successfully synthesized at a low temperature in a very short time by the MSS method. Using the example of Pb(Mg1/3Nb2/3)O3, the phase stability has been discussed on the basis of thermal and chemical analyses. The influences of the processing parameters, such as temperature, time, type and amount of salt, and non-stoichiometry, on the formation and the powder characteristics of the perovskite phase were investigated with possible explanations for the observed differences which were induced by changing the parameters. Finally, densification behaviour and dielectric properties resulting from the MSS powder were examined and compared to those of powders obtained by using the conventional mixed oxides (CMO) method. © 1998 Kluwer Academic Publishers

Order-disorder phenomena in new LaBaMn2O6-x CMR perovskites. Crystal and magnetic structure

Abstract: Three new perovskites in the LaBaMn2O6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn2O6 is cubic (a = 3.906 A), with a disordered distribution of La3+ and Ba2+ cations, whereas a second form of LaBaMn2O6 is tetragonal (a = 3.916 A; c = 7.805 A), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn2O5, which is also tetragonal (a = 5.650 A; c = 7.808 A) and adopts a YBaCuFeO5-related structure. Elucidation of the magnetic structure of LaBaMn2O5, from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn2O6, the PND data show a ferromagne...

Methane combustion on some perovskite-like mixed oxides

Abstract: Some perovskite-like mixed oxides of general formula La1−xA′xBO3 were prepared by the amorphous citrate method and tested for methane combustion within the 300–600°C temperature range. Substitution at A-site with a bivalent (Eu, Sr) or tetravalent (Ce) metal cation led to a decrease or increase of catalytic activity, respectively. La0.9Ce0.1CoO3 proved to be the most active catalyst, showing complete conversion at 500°C. The nature of the metal cation introduced modifies the oxidation state of cobalt, which leads to the formation of cationic or anionic vacancies. TPD-MS analysis confirmed that the catalytic activity is related to the oxygen storage properties of the catalyst. The substitution at B-site (B=Fe, Co, Ni) allowed to find interesting correlations between catalytic activity and the temperature Tmax of maximum oxygen desorption rate.

A neutron diffraction investigation into the rhombohedral phases of the perovskite series

Abstract: Rietveld refinements using neutron powder profiles are reported for a series of samples (commonly known as PZT), with x ranging from to . Cation shifts, octahedral distortion and tilts are determined with varying composition across the ferroelectric rhombohedral regions, and , of the PZT phase diagram. These parameters are then used in conjunction with a simple Landau-Devonshire model to investigate the nature of the phase transition. It is found that the cation shifts, octahedral distortion and tilt angles decrease with increasing Ti content, but, surprisingly, the octahedral strain, as indicated by the rhombohedral angle, increases. This is in contrast to the case for all other known rhombohedral perovskites. Furthermore, the refined anisotropic displacement parameters of the cations are anomalous and cannot be accounted for by the average crystal structure. A model is presented in which a domain-type `local' structure is considered, containing `ordered' additional cation displacements, consistently with the reports of extra reflections observed in electron microscopy studies by Viehland et al, Dai et al and Ricote et al.

Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications

Abstract: The thermal expansion for the perovskite (La,Sr)(Al,Ta)O3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component.

Phase transitions in ferrimagnetic and ferroelectric ceramics by ac measurements

Abstract: Ac conductivity measurements were carried out on polycrystalline samples of a ferrimagnetic spinel (Zn0.44Mn0.56Fe2O4) and a ferroelectric perovskite (Sr0.25Bi4Ti3.25O12.75), in the temperature range 20–160 and 20–660 °C, respectively, and in the frequency range 5 Hz–13 MHz. The impedance response in both cases could be resolved into two contributions, associated with the bulk (grains) and the grain boundaries. An analysis by means of the ac conductivity power law showed evidence of a critical temperature of 132 and 536 °C, for the ferrimagnetic and the ferroelectric samples, respectively, which corresponds to the Curie temperature for each type of material. These results are interpreted in terms of the disorder increase approaching the phase transition.

Atomically defined epitaxy and physical properties of strained La0.6Sr0.4MnO3 films

Abstract: La0.6Sr0.4MnO3 thin films were fabricated on SrTiO3 (001) substrates using pulsed laser deposition with observing persistent intensity oscillation of reflection high-energy electron diffraction. By atomic force microscopy, the surface of resulting films was confirmed to be extremely flat, showing atomically smooth terraces and 0.4 nm high steps corresponding to a unit cell height of perovskite. The surface terminating atomic layer was unambiguously assigned to the MnO2 layer by coaxial impact collision ion scattering spectroscopy. Crystal symmetry of the films is distorted into a tetragonal one due to the strain to fulfill perfect in-plane matching with the substrate even for films as thick as 100 nm. Even for films as thin as 4 nm (10 unit cells), ferromagnetic transition takes place to induce a metallic state and large negative magnetoresistance is observed as well.

A simple approach to lattice effects in conducting perovskite-type oxides

Abstract: A method for parametrizing the A cation size disparity or mismatch in ATO3 perovskites through the A cation size variance has recently been proposed and experimental studies are reviewed. (A is a mixture of trivalent lanthanide and divalent alkaline-earth cations.) The metal−insulator transition temperature in AMnO3 perovskites and the critical temperature in A2CuO4 superconductors are both found to decrease linearly with this size variance at constant doping level and average A cation radius. This result enables a pair of quadratic relationships for the mean size and size variance effects to be proposed as the result of changing strain energies associated with small structural distortions at the electronic transitions. Doping level, mean A cation radius, and the size variance are thus the three major parameters that control electronic properties in oxygen stoichiometric perovskite oxides.

Energetics and Crystal Chemical Systematics among Ilmenite, Lithium Niobate, and Perovskite Structures

Abstract: Transitions from chain silicates to garnet, ilmenite, and perovskite structures are important in deep earth geophysics and solid-state chemistry. Titanates and other oxides also show polymorphism among ilmenite, lithium niobate (metastable), and perovskite structures. This review brings together the evidence from the recent mineralogical and materials science literature linking the crystal chemistry, thermodynamics, and occurrence of these polymorphs at high pressure and temperature. A-site ordered multicomponent titanate perovskites are also discussed. A common feature of the perovskite structure is its high vibrational entropy.

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part I, Bi4Ti3O12

Abstract: Ferroelectric thin films of bismuth-containing layered perovskite Bi4Ti3O12 have been fabricated by a metalorganic decomposition (MOD) method. Crack-free and crystalline films of ∼5000 A thickness have been deposited on Pt/Ti/SiO2/Si substrates. Different heat treatments have been studied to investigate the nucleation and growth of perovskite Bi4Ti3O12 crystallites. If the same composition and final annealing temperature are used, films with different orientations are obtained by different heating schedules. These films show a large anisotropy in ferroelectric properties. Theoretical considerations are presented to suggest that nucleation control is responsible for texture and grain-size evolution. Moreover, the origin of the ferroelectric anisotropy is rooted in the two-dimensional nature of layered polarization.

Influence of Precursor Chemistry on the Formation of MTiO3 (M = Ba, Sr) Ceramic Thin Films

Abstract: Thin films of BaTiO3 and SrTiO3 were prepared by a chemical solution deposition method. The impact of the precursor on the processing, on the microstructure, and on the dielectric properties has been studied by systematically varying the alkyl chain length of the used Ba- and Sr-carboxylates. In addition, the effect of stabilizing the Ti-alkoxide precursor by acetylacetone has been investigated. The decomposition process, the crystallization behavior, and the film morphology were analyzed by glancing incidence XRD, reflectance FT-IR and field emission SEM. Distinct precursor effects on the thin film morphology and properties were revealed. Part of this influence can be attributed to an intermediate complex carbonate phase which forms for selected carboxylates with short alkyl chains. The high transformation temperature of this intermediate phase to the perovskite obviously has a marked influence on the crystallization and densification process of the alkaline earth titanate thin films. We correlate the morphological differences of the films to their dielectric properties.

Stability of La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite membranes in reducing and nonreducing environments

Abstract: The chemical stability of perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF-6428) has been investigated in connection with its potential use as a catalytic membrane for the oxidative coupling of methane (OCM). Once a steady-state oxygen-defect gradient is established (15−20 h), these membranes are found to be very stable under air/nitrogen gradients at temperatures up to 960 °C and they respond instantaneously to temperature changes with an apparent activation energy of 159 kJ/mol. However, serious near-surface etching occurs when one side of the membrane is exposed to 100% CH4 at 850 °C and atmospheric pressure, which resulted in dramatic increases in oxygen flux (a factor of 5). While this also caused an increase in the OCM reaction rate, the selectivity of C2+ hydrocarbons fell from 40 to 10%. It is also shown that stable operation under OCM conditions is possible if CH4 pressures are reduced to 0.1 atm but at a cost of C2+ production rate.

Stabilization of YMnO3 in a Perovskite Structure as a Thin Film

Abstract: Using pulsed laser deposition (PLD), metastable perovskite YMnO3 films were grown from a hexagonal YMnO3 target. The stabilization of the metastable phase is a result of the structural similarity between it and the perovskite substrates. X-ray and electron diffraction confirm the films' epitaxial nature but evince that the orientation and residual strains depend on the substrate. The implication of these findings is that PLD is a simple synthetic approach to stabilizing new, more complex, metastable perovskites.

Ceramic Microstructure and Oxygen Permeability of SrCo ( Fe , M ) O 3 − δ ( M = Cu or Cr ) Perovskite Membranes

Abstract: Stabilization of the cubic perovskite phase was demonstrated in the SrCo{sub 0.90{minus}x}Fe{sub 0.10}Cr{sub x}O{sub 3{minus}{delta}} oxide system at x {ge} 0.03. Oxygen permeability of SrCo(Fe, Cr)O{sub 3{minus}{delta}} solid solutions was independent of chromium content at x = 0.01--0.05. It was found that reducing ceramic grain size dimensions resulted in decreasing thermal expansion, electrical conductivity, and oxygen permeability of SrCo(Fe, Cu)O{sub 3{minus}{delta}} ceramic membranes. Oxygen transport through SrCo(Fe, M)O{sub 3{minus}{delta}} ceramics was shown to be limited by both the bulk ionic conductivity and oxygen exchange currents. The limiting effect of the membrane permeate-side surface on the permeation was higher in comparison with that on the feed-side surface.