Showing papers on "Perovskite (structure) published in 2000"

Reproducible switching effect in thin oxide films for memory applications

Abstract: Thin oxide films with perovskite or related structures and with transition metal doping show a reproducible switching in the leakage current with a memory effect. Positive or negative voltage pulses can switch the resistance of the oxide films between a low- and a high-impedance state in times shorter than 100 ns. The ratio between these two states is typically about 20 but can exceed six orders of magnitude. Once a low-impedance state has been achieved it persists without a power connection for months, demonstrating the feasibility of nonvolatile memory elements. Even multiple levels can be addressed to store two bits in such a simple capacitor-like structure.

The perovskite structure—a review of its role in ceramic science and technology

Abstract: Starting with the history of the fundamental science of the relation of structure to composition delineated completely by Goldschmidt, we use the perovskite structure to illustrate the enormous pow...

Designing fast oxide-ion conductors based on La2Mo2O9

Abstract: The ability of solid oxides to conduct oxide ions has been known for more than a century, and fast oxide-ion conductors (or oxide electrolytes) are now being used for applications ranging from oxide fuel cells to oxygen pumping devices1,2. To be technologically viable, these oxide electrolytes must exhibit high oxide-ion mobility at low operating temperatures. Because of the size and interaction of oxygen ions with the cationic network, high mobility can only be achieved with classes of materials with suitable structural features. So far, high mobility has been observed in only a small number of structural families, such as fluorite3,4,5, perovskites6,7, intergrowth perovskite/Bi2O2 layers8,9 and pyrochlores10,11. Here we report a family of solid oxides based on the parent compound12 La2Mo2O9 (with a different crystal structure from all known oxide electrolytes) which exhibits fast oxide-ion conducting properties. Like other ionic conductors2,13, this material undergoes a structural transition around 580 °C resulting in an increase of conduction by almost two orders of magnitude. Its conductivity is about 6 × 10-2 S cm-1 at 800 °C, which is comparable to that of stabilized zirconia, the most widely used oxide electrolyte. The structural similarity of La2Mo2O9 with β-SnWO4 (ref. 14) suggests a structural model for the origin of the oxide-ion conduction. More generally, substitution of a cation that has a lone pair of electrons by a different cation that does not have a lone pair—and which has a higher oxidation state—could be used as an original way to design other oxide-ion conductors.

Water Splitting into H2 and O2 on New Sr2M2O7 (M = Nb and Ta) Photocatalysts with Layered Perovskite Structures: Factors Affecting the Photocatalytic Activity

Abstract: Sr2Ta2O7 and Sr2Nb2O7 with similar layered perovskite structure showed activities for water splitting into H2 and O2 in pure water without any additives under UV irradiation. The band gaps of Sr2Ta2O7 and Sr2Nb2O7 were 4.6 and 3.9 eV, respectively. Sr2Ta2O7 gave H2 and O2 from pure water under UV irradiation even in the absence of a cocatalyst. The activity of Sr2Ta2O7 was much increased by loading NiO as a cocatalyst even without pretreatment. The quantum yield of the NiO(0.15 wt %)/Sr2Ta2O7 photocatalyst was 12% at 270 nm. On the other hand, native Sr2Nb2O7 did not possess the activity. The high activity was obtained for the Sr2Nb2O7 photocatalyst when NiO was loaded and pretreated. Factors affecting the photocatalytic activities were discussed by using the Sr2Ta2O7 and Sr2Nb2O7 powder with similar layered perovskite structure. A predominant factor affecting the photocatalytic behavior of Sr2Ta2O7 and Sr2Nb2O7 is the conduction band levels formed by Ta5d and Nb4d.

Evolution of the Jahn-Teller distortion of MnO6 octahedra in RMnO3 perovskites (R = Pr, Nd, Dy, Tb, Ho, Er, Y): a neutron diffraction study.

Abstract: Stoichiometric RMnO3 perovskites have been prepared in the widest range of R3+ ionic sizes, from PrMnO3 to ErMnO3. Soft-chemistry procedures have been employed; inert-atmosphere annealings were required to synthesize the materials with more basic R cations (R = Pr, Nd), in order to minimize the unwanted presence of Mn4+. On the contrary, annealings in O2 flow were necessary to stabilize the perovskite phases for the last terms of the series, HoMnO3, ErMnO3, and YMnO3, thus avoiding or minimizing the formation of competitive hexagonal phases with the same stoichiometry. The samples have been investigated at room temperature by high-resolution neutron powder diffraction to follow the evolution of the crystal structures along the series. The results are compared with reported data for LaMnO3. The distortion of the orthorhombic perovskite (space group Pbnm), characterized by the tilting angle of the MnO6 octahedra, progressively increases from Pr to Er due to simple steric factors. Additionally, all of the pe...

Surface properties and catalytic performance in methane combustion of Sr-substituted lanthanum manganites

Abstract: Perovskite type La1 − xSrxMnO3 (x = 0–0.5) oxides were prepared by the amorphous citrate process, characterised by X-ray diffraction, oxygen desorption, temperature-programmed reduction, infrared and X-ray photoelectron spectroscopic techniques, and tested for methane combustion within the 473–1073 K temperature range. Since catalyst activity was found to depend strongly on BET areas and to a lesser extent, on the degree of substitution (x), intrinsic activities were computed for La1 − xSrxMnO3 catalyst series. Among the compositions investigated, the degree of substitution x = 0.2 showed the highest intrinsic activity within the temperatures explored. Characterisation techniques made possible to correlate catalytic performance with the structural characteristics of the oxides. The stability of Mn4+ is probably the most important parameter, but excess of oxygen and atomic surface composition should also be taken into account.

Oxygen-vacancy ordering as a fatigue mechanism in perovskite ferroelectrics

Abstract: We present a paradigm for fatigue in ferroelectric perovskite oxides: That of a structural phase transition in which oxygen vacancies order into two-dimensional planar arrays capable of pinning domain wall motion.

An x-ray diffraction and Raman spectroscopy investigation of A-site substituted perovskite compounds: the (Na1-xKx)0.5Bi0.5TiO3 (0 x1) solid solution

Abstract: The (Na1 - x Kx )0.5 Bi0.5 TiO3 perovskite solid solution is investigated using x-ray diffraction (XRD) and Raman spectroscopy in order to follow the structural evolution between the end members Na0.5 Bi0.5 TiO3 (rhombohedral at 300 K) and K0.5 Bi0.5 TiO3 (tetragonal at 300 K). The Raman spectra are analysed with special regard to the hard modes and suggest the existence of nano-sized Bi3+ TiO3 and (Na1 - 2x K2x )+ TiO3 clusters. The complementary use of XRD and Raman spectroscopy suggests, in contrast to previous reported results, that the rhombohedral tetragonal phase transition goes through an intermediate phase, located at 0.5 x 0.80. The structural character of the intermediate phase is discussed in the light of sub- and super-group relations.

A model for fatigue in ferroelectric perovskite thin films

Abstract: An analytic expression in closed form is given for “fatigue,” the dependence of P(N), the switched charge per unit area P versus switching event number N, in ABO3 perovskite structure ferroelectric thin films is given. The analysis is based upon Arlt’s model for fatigue in bulk perovskites, which involves preferential electromigration of oxygen vacancies to sites parallel to the electrode–ferroelectric interface plane, together with some arguments by Brennan on the effectiveness of such defect planes in pinning domain walls. The model is applied to PZT/Pt with no adjustable parameters and yields in complete agreement with experimental data the dependence of P(N) at different frequencies, different voltages, and different temperatures. Notably, unlike other proposals in the literature, the model does not involve any charge injection from the electrodes.

Thermoelastic properties and crystal structure of MgSiO3 perovskite at lower mantle pressure and temperature conditions

Abstract: A new synchrotron X-ray diffraction study of MgSiO3 perovskite at high-pressure and high-temperature has been carried out in a laser-heated diamond-anvil cell to 94 GPa and temperatures above 2500 K. MgSiO3 perovskite is shown to be stable in this P–T range and adopts an orthorhombic structure (space group Pbnm), thus ruling out any phase transition or decomposition to an assemblage of denser oxides. Structural refinements show an increase of the orthorhombic distortion with increasing pressure, counterbalanced by a decrease of this distortion at high temperature, as evidenced by the Si-O-Si angle evolution. In addition, thermoelastic parameters identified from this new pressure-volume-temperature data set indicate that a significant amount of magnesiowustite is required to match PREM bulk modulus profile, thus making a pure perovskite lower mantle unlikely.

Influence of foreign ions on the crystal structure of BaTiO3

Abstract: The influence of dopants (Cr, Co, Fe, Ni, Y, Er, Tb, Gd, Pr and La) on the crystal structure of barium titanate was studied at room temperature and at 250°C using X-ray diffraction. Fine BaTiO3 powders (Ba/Ti=1.02, size ≈30 nm) have been doped with 1 at% of the foreign element and annealed for 14 and 62 h at two different temperatures: 950 and 1350°C. The room temperature structure of doped BaTiO3 was in any case tetragonal with c/a ratio lower than in the undoped perovskite, but dependent on dopant nature and particle size. For powders calcined at 1350°C, the particle size was in the range 1–5 μm and the decrease in tetragonality was mainly determined by dopant incorporation. Powders treated at 950°C had particles more than one order of magnitude finer (0.1–0.2 μm) and a systematic lowering of the c/a ratio in comparison to the samples annealed at higher temperature was observed. Comparison of the experimental variations of the unit cell edge of cubic BaTiO3 (250°C) with the results of atomistic computer simulations gives indication on the preferential incorporation site of the dopant. In particular, La3+ and Pr3+ prefer to substitute at the Ba site, whereas Tb3+ and Gd3+ give partial substitution at the Ti site. For Er3+ and Y3+ preferential substitution at the Ti site is predicted. For transition metal ions, substitution at the Ti site with oxygen vacancy compensation is confirmed, although their behaviour is less accurately reproduced.

Defects in yttrium aluminium perovskite and garnet crystals: atomistic study

Abstract: Native and impurity point defects in both yttrium aluminium perovskite (YAP) and garnet (YAG) crystals are studied in the framework of the pair-potential approximation coupled with the shell model description of the lattice ions. The calculated formation energies for native defects suggest that the antisite disorder is preferred over the Frenkel and Schottky-like disorder in both YAP and YAG. The calculated values of the distortion caused by the antisite YAl x in the lattice turn out to be in an excellent agreement with the EXAFS measurements. In non-stoichiometric compounds, the calculated reaction energies indicate that excess Y2 O3 or Al2 O3 is most likely to be accommodated by the formation of antisites rather than vacancies or interstitials in the lattice. Enthalpies of the reactions for impurity (Ca2+ , Mg2+ , Sr2+ , Ba2+ , Cr3+ , Fe3+ , Nd3+ , Si4+ ) incorporation into both YAP and YAG lattices are calculated. The relevant experimental data are discussed.

Neutron powder diffraction study of rhombohedral rare-earth aluminates and the rhombohedral to cubic phase transition

Abstract: Neutron powder diffraction has been used to examine the structural changes of the rare-earth aluminates LaAlO3 , PrAlO3 and NdAlO3 over a wide range of temperatures. At room temperature, all three aluminates adopt the rhombohedral perovskite structure in space group R c (a = 5.3647(1) A, c = 13.1114(3) A for LaAlO3 , a = 5.3337(2) A, c = 12.9842(4) A for PrAlO3 , a = 5.3223(2) A, c = 12.9292(5) A for NdAlO3 ). The rhombohedral structure is characterized by rotation of the oxygen atom octahedra about the threefold axis, and compression of these octahedra parallel to the same axis. As the temperature is increased, the rotation angle and the compression decrease, indicative of an approach to the cubic symmetry of the ideal perovskite. Only for LaAlO3 , however, was the transition at a low enough temperature to unequivocally obtain the cubic phase. For PrAlO3 the transition was closely approached before the sample can failed, but for NdAlO3 the transition appeared to be inaccessible within the available temperature range. The rotation angle is taken to represent the order parameter, and its temperature variation is well described by a generalized mean field approach. Such a description suggests the transitions are continuous, being at 820 K and second order for the transition in LaAlO3 , and at 1768 K and tricritical for the transition in PrAlO3 . In the proximity of the phase transition, the octahedral compression varies with the square of the rotation angle, though this description is inadequate remote from the transition, and the constant of proportionality is different for the different compounds.

Organic−Inorganic Perovskites Containing Trivalent Metal Halide Layers: The Templating Influence of the Organic Cation Layer

Abstract: Thin sheetlike crystals of the metal-deficient perovskites (H2AEQT)M2/3I4 [M = Bi or Sb; AEQT = 5,5‘ ‘‘-bis(aminoethyl)-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene] were formed from slowly cooled ethylene glycol/2-butanol solutions containing the bismuth(III) or antimony(III) iodide and AEQT·2HI salts. Each structure was refined in a monoclinic (C2/m) subcell, with the lattice parameters a = 39.712(13) A, b = 5.976(2) A, c = 6.043(2) A, β = 92.238(5)°, and Z = 2 for M = Bi and a = 39.439(7) A, b = 5.952(1) A, c = 6.031(1) A, β = 92.245(3)°, and Z = 2 for M = Sb. The trivalent metal cations locally adopt a distorted octahedral coordination, with M−I bond lengths ranging from 3.046(1) to 3.218(3) A (3.114 A average) for M = Bi and 3.012(1) to 3.153(2) A (3.073 A average) for M = Sb. The new organic−inorganic hybrids are the first members of a metal-deficient perovskite family consisting of (Mn+)2/nV(n-2)/nX42- sheets, where V represents a vacancy (generally left out of the formula) and the metal cation valence,...

Synthesis and photocatalytic property of layered perovskite tantalates, RbLnTa2O7 (Ln = La, Pr, Nd, and Sm)

Abstract: The first example of an active layered tantalate photocatalyst containing partly filled lanthanide 4f shell is reported. A single phase of layered perovskite tantalates, RbLnTa2O7, could be obtained with Ln = La, Pr, Nd, and Sm; the ionic radii of these trivalent cations are required to be larger than 0.126 nm for constructing a perovskite slab. Under UV irradiation from a high-pressure Hg lamp, these layered tantalates demonstrated the photocatalytic activity for water splitting into stoichiometric H2/O2 mixtures even without loading metal catalysts. The rates of H2 and O2 evolutions were found to be strongly dependent on the lanthanoids, increasing in the sequence of La ≑ Pr ≪ Sm < Nd. The maximum rate of H2 evolution observed over RbNdTa2O7 reached 47.0 μmol/h. The absorption spectrum of RbNdTa2O7 consists of the internal 4f transitions in the visible region and a broad band overlapping the band-gap transition in the UV region. The latter band indicates that a possible excitation process including the ...

The tetragonal phase of Na0.5Bi0.5TiO3 – a new variant of the perovskite structure

Abstract: The structure of the tetragonal phase of the A-site-substituted perovskite sodium bismuth titanate, Na0.5Bi0.5TiO3, has been determined by neutron powder diffraction at 698 K. The structure was refined in space group P4bm with a (= b) = 5.5191 (1), c = 3.9085 (1) A, V = 119.055 (5) A3, Z = 2 and Dx = 5.91 Mg m−3. The structure exhibits an unusual combination of in-phase (a0a0c+) tilts and antiparallel cation displacements along the polar c axis, which results in a new variant of the perovskite structure.

Spinel and Perovskite Functional Layers Between Plansee Metallic Interconnect (Cr‐5 wt % Fe‐1 wt % Y 2 O 3) and Ceramic ( La0.85Sr0.15 ) 0.91MnO3 Cathode Materials for Solid Oxide Fuel Cells

Abstract: The degradation of the interface between Plansee Ducrolloy alloy (Cr-5 wt % Fe-1 wt %Y 2 O 3 ) (PD) and (La 0.85 Sr 0.15 ) 0.91 MnO 3 (LSM) ceramic was studied by means of electrical resistance measurements at 900°C in air humidified with 2% H 2 O and with a dc load of 100 mA/cm 2 and by scanning electron microscopy/energy-dispersive spectroscopy of cross sections of the samples. One sample of PD alloy with plasma-sprayed La 0.9 Sr 0.1 CrO 3 and a contacting layer of La 0.8 Sr 0.2 CoO 3 exhibited little degradation, estimated to be 0.066 Ω cm 2 after 10,000 h. Other samples of PD were coated by different spinel-forming slurries pretreated in H 2 /H 2 O/Ar mixtures at 950 and 1000°C. Coatings forming MnCr 2 O 4 spinel and using La 0.8 Sr 0.2 CoO 3 as a contacting layer showed a low estimated degradation of 0.04-0.06 Ω cm 2 after 10,000 h. Another spinel (Co,Mn) 3 O 4 appeared to reduce Cr diffusion significantly, and when used together with a mixed perovskite made of 1:1 of LSM and La 0.8 Sr 0.2 CoO 3 as a contacting layer, exhibited estimated degradation of 0.024 Ω cm 2 for 10,000 h.

Strain modification of epitaxial perovskite oxide thin films using structural transitions of ferroelectric BaTiO3 substrate

Abstract: Effects of induced biaxial strain on the electrical transport and magnetic properties of epitaxial thin films of SrRuO3 and La0.67Sr0.33MnO3 by structural transitions of ferroelectric BaTiO3 substrates have been studied. Large jumps of electrical resistivity (∼5% in SrRuO3 and ∼12% in La0.67Sr0.33MnO3) and low field magnetization (∼70% in La0.67Sr0.33MnO3) have been observed in the films at the structural transition temperatures of BaTiO3 substrate. The hysteretic jumps are reproducible through many thermal cycles, and they can be attributed to strain effects induced by the substrate. The use of phase transitions of ferroelectric substrates to manipulate lattice strain of epitaxial thin film heterostructures can be a useful way to modify the properties of perovskite oxides.

Self-assembly of Tiled Perovskite Monolayer and Multilayer Thin Films

Abstract: Dion−Jacobson phases are layer perovskites of the general formula A‘[An-1BnO3n+1], where A‘ is typically an exhangeable alkali or alkali earth ion. Because of their low layer charge density, acid-exchanged Dion−Jacobson phases are easily exfoliated to make single-sheet colloids. When the conditions temperature and pH were controlled, densely tiled monolayers of anionic sheets derived from HCa2Nb3O10, HSr2Nb3O10, HCaLaNb2TiO10, and HLaNb2O7 were grown on cationic substrates. Multilayer films were also assembled by alternately adsorbing these sheets and organic polycations.

AMnO3 (A=La, Nd, Sm) and Sm1−xSrxMnO3 perovskites as combustion catalysts: structural, redox and catalytic properties

Abstract: Catalytic combustion of methane has been investigated over AMnO3 (A = La, Nd, Sm) and Sm1−xSrxMnO3 (x = 0.1, 0.3, 0.5) perovskites prepared by citrate method. The catalysts were characterized by chemical analysis, XRD and TPR techniques. Catalytic activity measurements were carried out with a fixed bed reactor at T = 623–1023 K, space velocity = 40 000 N cm3 g−1 h−1, CH4 concentration = 0.4% v/v, O2 concentration = 10% v/v. Specific surface areas of perovskites were in the range 13–20 m2 g−1. XRD analysis showed that LaMnO3, NdMnO3, SmMnO3 and Sm1−xSrxMnO3 (x = 0.1) are single phase perovskite type oxides. Traces of Sm2O3 besides the perovskite phase were detected in the Sm1−xSrxMnO3 catalysts for x = 0.3, 0.5. Chemical analysis gave evidence of the presence of a significant fraction of Mn(IV) in AMnO3. The fraction of Mn(IV) in the Sm1−xSrxMnO3 samples increased with x. TPR measurements on AMnO3 showed that the perovskites were reduced in two steps at low and high temperature, related to Mn(IV) → Mn(III) and Mn(III) → Mn(II) reductions, respectively. The onset temperatures were in the order LaMnO3 > NdMnO3 > SmMnO3. In Sm1−xSrxMnO3 the Sr substitution for Sm caused the formation of Mn(IV) easily reducible to Mn(II) even at low temperature. Catalytic activity tests showed that all samples gave methane complete conversion with 100% selectivity to CO2 below 1023 K. The activation energies of the AMnO3 perovskites varied in the same order as the onset temperatures in TPR experiments suggesting that the catalytic activity is affected by the reducibility of manganese. Sr substitution for Sm in SmMnO3 perovskites resulted in a reduction of activity with respect to the unsubstituted perovskite. This behaviour was related to the reduction of Mn(IV) to Mn(II), occurring under reaction conditions, hindering the redox mechanism.