Showing papers on "Perovskite (structure) published in 1987"
TL;DR: Polarization and electrical conductivity measurements were made at 800°C in open air using the perovskite-type oxides La 1− x Sr x MO 3 (M Cr, Mn, Fe, Co) sputtered on ytteia stabilized zirconia electrolyte.
498 citations
TL;DR: In this paper, the cathodic polarization of the electrodes sputtered on yttria stabilized zirconia electrolyte was studied at 800°C in air and the system showed high electrode activity for oxygen reduction.
Abstract: The cathodic polarization of the electrodes sputtered on yttria stabilized zirconia electrolyte was studied at 800°C in air. The system showed high electrode activity for oxygen reduction. The study of the electrode resistance as functions of and temperature revealed that the rate‐determining steps for oxygen reduction were as follows: the charge transfer process for , the dissociation of oxygen molecules on the surface for and , and the oxygen diffusion on the electrode surface for . The marked electrode activity of was explained by the large effective reaction surface area, which is caused by the high oxide ion diffusivity and the high dissociation ability of oxygen molecules. In , the ability of the dissociation of oxygen molecules was considered to be lower than that in . The low activity of Cr‐perovskite electrode was explained by the low diffusivity of oxygen ions.
473 citations
TL;DR: It is demonstrated that perovskite can exist throughout the pressure range of the lower mantle and that it is therefore likely to be the most abundant mineral in Earth.
Abstract: Silicate perovskite of composition (Mg0.88Fe0.12) SiO3 has been synthesized in a laser-heated diamond-anvil cell to a pressure of 127 gigapascals at temperatures exceeding 2000 K. The perovskite phase was identified and its unit-cell dimensions measured by in situ x-ray diffraction at elevated pressure and room temperature. An analysis of these data yields the first high-precision equation of state for this mineral, with values of the zero-pressure isothermal bulk modulus and its pressure derivative being K0T = 266 ± 6 gigapascals and K90T = 3.9 ± 0.4. In addition, the orthorhombic distortion of the silicate-perovskite structure away from ideal cubic symmetry remains constant with pressure: the lattice parameter ratios are b/a = 1.032 ± 0.002 and c/a = 1.444 ± 0.006. These results, which prove that silicate perovskite is stable to ultrahigh pressures, demonstrate that perovskite can exist throughout the pressure range of the lower mantle and that it is therefore likely to be the most abundant mineral in Earth.
355 citations
TL;DR: The crystal structure of the high-T/sub c/ superconducting phase Ba/sub 2/YCu/sub 3/O/sub 9-//sub delta/(delta = 2.0)= has been determined by single-crystal x-ray diffraction.
Abstract: The crystal structure of the high-T/sub c/ superconducting phase Ba/sub 2/YCu/sub 3/O/sub 9-//sub delta/(delta = 2.0)= has been determined by single-crystal x-ray diffraction. The compound crystallizes in an orthorhombic, oxygen-deficient perovskite superstructure with space group Pmmm, a = 3.856(2) A b = 3.870(2) A, and c = 11.666(3) A. The ordering of Ba and Y in a (Ba-Y-Ba)/sub infinity/ sequence along the c axis is responsible for the tripled cell. Two (CuO/sub 2/)/sub infinity/ planes with copper in distorted square planar coordination are the key to conductivity.
298 citations
201 citations
Journal Article•
TL;DR: In this paper, the BaTiO 3 was added to BaTiVo 3, which permet d'eviter la formation de phase pyrochlore dans les ceramiques de PZN.
Abstract: L'addition de BaTiO 3 permet d'eviter la formation de phase pyrochlore dans les ceramiques de PZN. On etudie la preparation, les relations de phases et les proprietes dielectriques de plusieurs compositions du systeme binaire PZN-BT
200 citations
TL;DR: In this paper, a series of layered perovskite oxides, AILaNb2O7 (A = Li, Na, K, Rb, Cs, NH4) constituting n = 2 members of the family A A′n−1BnO3n+1, has been prepared.
197 citations
Journal Article•
181 citations
TL;DR: Wu et al. as discussed by the authors reported a new material based on the starting composition Y/sub 1.2/Ba/sub 0.8/CuO/sub y/ with T/sub c/ well above 90 K.
Abstract: Over the last decade, the search for high-temperature superconducting materials remained virtually stagnant. This situation changed radically with the discovery of Bednorz and Mueller of superconductivity above 30 K in a layered perovskite oxide composed of La, Ba, and Cu. Improvements in the superconducting transition temperature (T/sub c/) to approx. 45 K by Sr substitution and identification of the phase responsible for superconductivity (La/sub 2-x/Ba(or Sr)/sub x/CuO/sub y/ where x is typically between 0.1-0.3) followed rapidly. The next major advance was immediate and dramatic. Wu, Chu, and co-workers reported a new material based on the starting composition Y/sub 1.2/Ba/sub 0.8/CuO/sub y/ with T/sub c/ well above 90 K. Nearly simultaneous reports by other groups confirmed these results. The Y-Ba-Cu material was a mixture of several unidentified phases and only a small fraction of the sample actually was superconducting. Recently, the structure of this new superconductor was identified as an oxygen-defect perovskite corresponding to the composition Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/. In this paper, they report on the synthesis of single-phase Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub y/ and show how the preparation conditions play a dramatic role in determining the superconducting properties. Also, based on their knowledge of the structure, they havemore » prepared a variety of new, single-phase high-temperature superconducting compounds in which Y and Ba are substituted by related elements, demonstrating that 90 + K superconductivity is a more general property of this structure.« less
147 citations
TL;DR: In this paper, a perovskite-type oxides were synthesized at temperatures as low as 550-650 °C by calcination of amorphous citrate complex precursors.
Abstract: Well-crystallized perovskite-type oxides could be synthesized at temperatures as low as 550–650 °C by the calcination of amorphous citrate complex precursors. The large specific surface areas of the oxides obtained suggest that the citrate process is very promising for preparing perovskite-type oxide catalysts for practical use.
144 citations
TL;DR: The 90-K superconducting phase of Y-Ba-Cu-O has been isolated, and its structure at room temperature determined by single-crystal x-ray diffraction, resulting in a quasi-two-dimensional structure in which sandwiches of Ba/sub 2/Cu/sub 3/O/sub 8-//sub y/ are separated by layers of Y.
Abstract: The 90-K superconducting phase of Y-Ba-Cu-O has been isolated, and its structure at room temperature determined by single-crystal x-ray diffraction. The material is an oxygen-defect perovskite, with composition YBa/sub 2/Cu/sub 3/O/sub 8-//sub x/. Entire planes of oxygen atoms between copper atoms are missing from the perovskite structure. The result is a quasi-two-dimensional structure in which sandwiches of Ba/sub 2/Cu/sub 3/O/sub 8-//sub y/ are separated by layers of Y.
TL;DR: In this paper, the authors investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x-ray diffraction.
Abstract: We have investigated the crystallography and microstructure of Y1Ba2Cu3O9−x with transmission electron microscopy and x‐ray diffraction. Y1Ba2Cu3O9−x is a distorted, oxygen‐defect perovskite with ordering of the yttrium and barium ions. Its unit cell is orthorhombic with space group Pmm2 and lattice parameters a=3.893 A, b=11.688 A, and c=3.820 A. The structure is heavily twinned on {101} type planes, possibly due to a tetragonal‐to‐orthorhombic transition above room temperature.
TL;DR: In this paper, the crystal structure of perovskite-type MgSiO3 has been studied up to 96 kbar, using a miniature diamondanvil pressure cell and by means of single-crystal four-circle diffractometry.
Abstract: The crystal structure of perovskite-type MgSiO3 has been studied up to 96 kbar, using a miniature diamondanvil pressure cell and by means of single-crystal four-circle diffractometry. The observed unit cell compression gives a bulk modulus of Ko=2.47 Mbar, assuming K′o=4. The unit cell compression is controlled mainly by the tilting of SiO6 octahedra. The effect of pressure is to change Mg polyhedron towards 8-fold coordination rather than 12-fold coordination. The polyhedral bulk moduli of SiO6 and MgO8 are 3.8 Mbar and 1.9 Mbar, respectively.
TL;DR: The structure of Ba2YCu3O6 has been determined by neutron diffraction powder profile analysis as mentioned in this paper, and the cell is tetragonal P4/mmm with a = 3.8570(1) A and c = 11.8194(3) A.
TL;DR: In this paper, the structural distortions, lattice dynamics, and equations of state of the high-pressure perovskite phases of MgSiO3 and CaSiO 3 are examined with a parameter-free theoretical model.
Abstract: The structural distortions, lattice dynamics, and equations of state of the high-pressure perovskite phases of MgSiO3 and CaSiO3 are examined with a parameter-free theoretical model. A theoretical ionic description of the crystal charge density is constructed from shell-stabilized ions, whose wavefunctions are calculated from Hartree-Fock theory. The short-range forces are then calculated in the pairwise-additive approximation from modified electron gas theory. The resulting many-body-corrected pair potentials are used to study the lattice dynamics in the quasiharmonic approximation. The cubic structure of MgSiO3 perovskite (Pm3m) is found to be dynamically unstable at all pressures, with imaginary quasiharmonic phonons occurring at the edge of the Brillouin zone. In contrast, the cubic phase of CaSiO3 perovskite is found to be stable at low pressures but becomes dynamically unstable at ∼ 109 GPa (1.09 Mbar). Energy minimization of MgSiO3 in an orthorhombic cell (Pbnm) is performed to obtain a distorted perovskite structure that is dynamically stable. The calculated unit cell parameters at zero pressure and room temperature are within 2 percent of those determined by x-ray diffraction. The theoretical equation-of-state calculations predict a lower compressibility and thermal expansivity for the two silicate perovskites than does the available experimental data on these compounds. Extensions of the present ionic model for more accurate predictions will require the inclusion of polarization of charge density and vibrational anharmonicity.
TL;DR: In this paper, a high-resolution transmission electron microscope study of YBa2Cu3O6.9 is presented, which elucidates the microstructure of this system on an atomic scale.
Abstract: Several of the theories proposed to account for the unusually high critical temperature (Tc ) of YBa2Cu3O6.9 rely on a reduced dimensionality of this system: the stronger electron–phonon coupling in lower-dimensional systems of marginal stability may allow high-Tc superconductivity by the conventional phonon-mediated mechanism1. X-ray powder diffraction experiments have shown the structure to consist of a tripled perovskite unit cell2 which, in the presence of nine oxygen atoms, is three-dimensionally connected. The theoretical proposals therefore depend crucially on the details of the atomic arrangement to reduce the dimensionality of the system. Recent X-ray work on small single-crystal grains3,4 has established the structure of the metal framework in this system, but disagreements persist regarding the disposition of the oxygen vacancies needed to achieve the experimentally observed stoichiometry. Moreover, no conclusive information is available regarding the presence of long-range order in the arrangement of the oxygen vacancies, which could give rise to further reduced-dimensional features in this system5,6. Here we report the results of a high-resolution transmission electron microscope study of YBa2Cu3O6.9, which elucidates the microstructure of this system on an atomic scale. Our results can be consistently interpreted in terms of a particular form of long-range order in the disposition of the oxygen vacancies. We also observe twins, and planar defects that can be modelled as extrinsic faults. These defects reduce the O/Cu ratio of the material from 2.33 (for the inferred unit cell) towards the experimentally observed value of 2.3 (ref. 2).
TL;DR: In this article, a superconducting oxides, close to the composition of Bi2Bi2Cu207+δ has been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.
Abstract: During the investigation of the system Bi—Sr—Cu—O a novel family of superconducting oxides, close to the composition Sr2Bi2Cu207+δhas been isolated, with a midpoint critical temperature ranging from 7 K to 22 K. The X-ray diffraction and high-resolution electron microscopy studies have shown its relationships with perovskite and demonstrated its bidimensional character. The possible relationships of this lamellar oxide with Aurivillius phases is discussed.
TL;DR: The perovskite solid solutions BaPb1-xBixO3 exhibit unique properties in both metallic and semiconducting phases as discussed by the authors, and it turns out that the high T c originates from extremely strong electron-phonon interaction.
Abstract: The perovskite solid solutions BaPb1-xBixO3 exhibit unique properties in both metallic and semiconducting phases. In the metallic phase (0 ≤ x ≤ 0.35) they are superconducting with high transition temperatures. It turns out that the high T c originates from extremely strong electron-phonon interaction. At the metal-semiconductor transition the superconductivity disappears and the semiconducting properties persist over a wide range of Bi composition (0.35 ≤ x ≤ 1). Recent studies of the optical spectrum, as well as Raman scattering experiments, have given direct evidence for charge-density-wave formation in BaBiO3. The CDW is found to be local in the present system and be stable throughout the semiconducting phase. The local CDW instability appears to manifest itself even in the metallic phase, giving rise to various precursory effects.
TL;DR: In this paper, the authors derived an estimated value of -1.9, in good agreement with the thermodynamic value of 1.77 measured at zero pressure, which is the best known value.
Abstract: MgSiO 3 in the perovskite structure, thought to be the dominant mineral phase of the earth's mantle, has been studied by both zero-pressure Raman spectroscopy and by infrared spectroscopy to pressures of 27 GPa. A total of eight bands are observed: four in the Raman spectrum between 250 and 500 cm -i, and four in the mid-infrared between 500 and 800 cm -1. These bands are divisible into three separate groupings: high frequency (-800 cm-1), caused by asymmetric silicon-oxygen stretching motions; intermediate frequency (370--690 cm-1), modes predominantly due to octahedral stretching and bending vibrations; and low frequency (200--300 cm-1), involving Mg 2+ motion. The pressure shift of the four mid-infrared modes yields an average mode Grtineisen parameter of 1.36 + 0.15. Systematic trends observed between mode Grtineisen parameters and mode frequency are used to derive an estimate of the thermodynamic Grtineisen parameter from the spectroscopic data. In this way, we derive an estimated value of -1.9, in good agreement with the thermodynamic value of 1.77 measured at zero pressure.
Journal Article•
TL;DR: The crystal structure of the high-temperature superconductor YBa2Cu3O7 is derived from the ideal perovskite structure by the presence of ordered vacancies in the oxygen sublattice.
Abstract: The crystal structure of the high-temperature superconductor YBa2Cu3O7 is derived from the ideal perovskite (YBa2Cu3O9) structure by the presence of ordered vacancies in the oxygen sublattice. Owing to the ordering of these vacancies, the coordination polyhedra around the copper cations are either pyramids or slightly distorted squares1–5; the one-dimensional character and the orthorhombic symmetry of the structure are due to the existence of the squares. We have synthesized single crystals of the further-reduced compound Y0.9Ba2.1Cu3O6, and we report here a structural determination using single-crystal X-ray diffraction data. Y0.9Ba2.1Cu3O6 has a tetragonal oxygen-deficient perovskite structure related to that of YBa2Cu3O7. The additional oxygen vacancies transform the CuO4 squares into O–Cu–O linear bonds, while the pyramids are not affected. As Y0.9Ba2.1Cu3O6 does not exhibit any superconducting transition down to 4.2 K, we conclude that the chains of CuO4 squares are essential for the superconductivity mechanism.
TL;DR: The structure of an oxygen-deficient perovskite BaLa 4 Cu 5 O 13+δ has been determined by neutron powder diffraction and high-resolution electron microscopy.
TL;DR: In this paper, superconducting cuprate perovskites with Ba2YCu3O7 incorporating a variety of rare earth ions are reported, and they show high temperature superconductivity in phases containing ions carrying magnetic moments.
TL;DR: In this paper, the superconductivity in alloyed compounds YBa2Cu3-yNiyO7-δ has been studied in order to gain direct insights on the roles different Cu sites play in electrical conduction.
Abstract: Superconductivity in alloyed compounds YBa2Cu3-yNiyO7-δ has been studied in order to gain direct insights on the roles different Cu sites play in electrical conduction. Electron probe microanalysis as well as X-ray powder diffraction analysis shows that Ni is actually substituted for Cu in the oxygen-deficient perovskite compound for y at least as much as 0.27. Electrical resistivity measurements revealed that the substituted Ni does not substantially destroy the superconductivity.
TL;DR: The elastic properties of YBa2Cu3O7- delta ceramic have been determined from ultrasonic wave velocity measurements between 5 K and 260 K as discussed by the authors, showing that the elastic stiffness of this material is substantially smaller than that of ideal perovskite materials, a feature which can be abscribed to the oxygen defect.
Abstract: The elastic properties of YBa2Cu3O7- delta ceramic have been determined from ultrasonic wave velocity measurements between 5 K and 260 K. In the vicinity of the superconducting transition temperature both longitudinal and shear mode velocities show decreases with temperature, an indication of long-wavelength acoustic mode softening. Below Tc the mode velocities increase quite steeply. The elastic stiffness of this material is substantially smaller than that of ideal perovskite materials, a feature which can be abscribed to the oxygen defect, layer-like structure.
TL;DR: In this paper, electrical conductivity measurements of Perovskite and a Perovskiite-dominated assemblage synthesized from pyroxene and olivine were used to demonstrate that these high-pressure phases are insulating to pressures of 82 GPa and temperatures of 4500 K.
Abstract: Electrical conductivity measurements of Perovskite and a Perovskite-dominated assemblage synthesized from pyroxene and olivine demonstrate that these high-pressure phases are insulating to pressures of 82 GPa and temperatures of 4500 K. Assuming an anhydrous upper mantle composition, the result provides an upper bound of 0.01 S/m for the electrical conductivity of the lower mantle between depths of 700 and 1900 km. This is 2 to 4 orders of magnitude lower than previous estimates of lower-mantle conductivity derived from studies of geomagnetic secular variations.
TL;DR: In this paper, the extent of bulk reduction of mixed oxides in hydrogen, at temperatures between 250 and 500 °C, was measured using a gas recirculating batch reactor connected to a standard highvacuum system (10−3 Pa).
TL;DR: In this paper, the authors studied the evolution of oxygen by electrolysis of aqueous solutions on nonstoichiometric perovskite-related ferrites of composition and found that at low potential the acid base reaction is rate determining whereas at high potential it is the first electron transfer.
Abstract: Oxygen evolution by electrolysis of aqueous solutions has been studied on nonstoichiometric perovskite‐related ferrites of composition . Electrochemical measurements have been carried out according to a methodology previously proposed for ceramic‐type materials. The physical chemical and geometrical parameters are preliminarily fixed. A maximum of the electrocatalytic activity appears for , which also corresponds to the highest value of Fe4+ ratio . Two Tafel regions have been observed and kinetic parameters show that: at low potential the acid‐base reaction is rate determining whereas at high potential it is the first electron transfer . This change with potential implies that the transport properties of the materials play a decisive role in the reaction mechanism.
TL;DR: The structure of the high-temperature superconductor YBa2Cu3O7 has now been determined independently by neutron powder diffraction by several groups as mentioned in this paper, and these results all agree about the basic structure.
Abstract: The structure of the high-temperature superconductor YBa2Cu3O7 has now been determined independently by neutron powder diffraction by several groups1–4. These results all agree about the basic structure. It is an oxygen-deficient perovskite, with Ba and Y cations ordered over the A-sites of the A3B3O9−δ structure, in the sequence Ba–Ba–Y. The two oxygen vacancies (δ = 2) are perfectly ordered. There are no oxygen atoms on the Y-planes, and the oxygen vacancies on the Cu-planes, between the two Ba-planes, leave large tunnels along the (010) direction. This is only the average structure, as it was determined by a diffraction technique; it is useful to verify by electron microscopy the local degree of order and crystal perfection. In particular, electron microscopy will reveal the existence of any very long-range superstructures, or of isolated imperfections, such as interstitial planes of Ba and/or Y atoms, or shear planes. Here we report high-resolution electron microscopic observations of the YBa2Cu3O7 crystals used in the neutron powder determination of the structure by Capponi et al.1. The crystals are seen to be highly twinned on the (110) planes, but are very well ordered. The Ba–Ba–Y ordering of planes along the c-axis is perfect; no interstitial Ba and Y planes, or shear planes, were seen. Simulated images, using the structural data of ref. 1, are shown to be in good agreement with observed images.
TL;DR: The Nd1+xBa2-xCu3Oy system and its superconductivity were studied in this paper, where it was shown that there appears to be little influence of the magnetic ion Nd3+ on the Tc in the Nd 1 +xBa 2 -xCu 3 Oy system.
Abstract: The Nd1+xBa2-xCu3Oy system and its superconductivity were studied. This system forms a solid solution within the x range of 0.00 to 0.80. At room temperature, it has orthorhombic symmetry for x ranging from 0.00 to 0.14 and tetragonal symmetry for x not less than 0.20. All specimens at 900°C in air have tetragonal symmetry. It is a superconductor in the region of x less than 0.50. The superconducting onset temperature decreases from 93.5 to 40.0 K with increasing x. In comparison with the superconductivity of the La1+xBa2-xCu3Oy system, there appears to be little influence of the magnetic ion Nd3+ on the Tc in the Nd1+xBa2-xCu3Oy system.