Showing papers on "Perovskite (structure) published in 1988"

Superconductivity near 30 K without copper: the Ba 0.6 K 0.4 BiO 3 perovskite

Abstract: It is well known that the breakthrough of Bednorz and Muller1 in discovering superconductivity in (La, Ba)2CuO4 was inspired in part by their knowledge of the superconducting properties of Ba(Pb, Bi)O3 (ref. 2). With a transition temperature, Tc, of ∼12 K, that compound was not generally considered anomalous despite the fact that its Tcis 3–5 times higher than that of traditional superconductors with comparable density of states3–5. The increases in Tc for copper-oxide-based materials continue to generate worldwide excitement, but from both a chemical and theoretical point of view it would also be exciting if high-Tcsuperconductivity were observed in another class of materials. Here we report the results of experiments leading us to the single-phase perovskite Ba0.6K0.4BiO3, which has a magnetically determined onset temperature of 29.8 K—a Tc considerably higher than that of conventional superconductors and surpassed only by copper-containing compounds. Superconductivity in this compound occurs within the framework of a three dimensionally connected bismuth-oxygen array. These results suggest that further research toward exploring the limiting Tcs for bismuth-oxide-based, high-temperature superconductors might be fruitful.

The parent structure of the layered high-temperature superconductors

Abstract: Oxide superconductors in the system Tl2Ba2Can–1CunO4+2n (ref. 1) have transition temperatures (Tc) above 100 K, increasing with n. So far, stacking sequences up to n = 3 have been found in small crystals, and sequences with n > 3 have been seen in electron microscopy studies1. For large n, the stoichiometry of Tl2Ba2Can–1CunO4+2n approaches CaCuO2, a structure expected to consist only of CuO2 planes separated by Ca atoms. By analogy with Tl2Ba2Can–1CunO4+2n, the unit cell of this hypothetical phase is expected to be tetragonal with a = 3.86 A. Such a compound is not known in the Ca–Cu–O system, but Roth2 recently reported that small amounts of Sr on the Ca site can stabilize this simple structure. Here we report the growth of small single crystals of this phase, with composition (Ca0.86Sr0.14)CuO2, and their characterization by single-crystal X-ray diffraction. The crystals are tetragonal with space group P4/mmm, and the structure contains planar [CuO2]∞ layers separated by Ca and Sr atoms. The structure is a simple defect perovskite with ordered oxygen vacancies and can be regarded as the n = ∞ parent of the A2B2Can–1CunO4+2n (A = Bi, Tl; B = Sr, Ba) superconductors.

Superconductivity in the high-Tc Bi-Ca-Sr-Cu-O system - Phase identification

Abstract: Four phases are observed in superconducting Bi-Ca-Sr-Cu-O samples. The superconducting phase, with onset temperature near 120 K, is a 15.4-A-layered compound with composition near Bi2Ca1Sr2Cu2O9 and an A-centered orthorhombic unit subcell 5.41 x 5.44 x 30.78 A. X-ray diffraction and electron microscopy data are consistent with a structure of alternating perovskite and Bi2O2 layers. High-resolution transmission electron microscopy images reveal a b-axis superstructure of 27.2 A, numerous (001) stacking faults, and other defects.

Perovskite‐Type Oxides—the New Approach to High—Tc Superconductivity

Abstract: The review dwells on the problems of high-temperature superconductivity. Ideas encouraging the search for high-temperature superconductivity are elucidated. The way from cubical alloys, containing niobium, to laminated copper-containing oxides with the perovskite-type structure is shown. Properties of new laminated oxide superconductors are described.

Effect of Cation Substitution on the Oxygen Semipermeability of Perovskite-type Oxides

Abstract: Oxygen semipermeability of LnCoO3-based perovskite-type oxides changed drastically by the partial substitution of A site and B site cations, Ba and Cu being the most effective substituents, respectively. Host rare earth cations (Ln) also affected the oxygen semipermeability, which increased with a decrease in ionic radius of Ln.

High- T c superconducting phases in the series Bi 2.1 (Ca, Sr) n+l Cu n O 2n+4+δ

Abstract: Following the report of Maeda et al.1 on high-Tc superconductivity in samples of nominal composition BiSrCaCu2Ox we have identified three distinct phases in the homologous series Bi2.1(Ca, Sr)n+lCunO2n+4+δ with n = 1,2 and 3. These have zero resistance Tcs of 80 K, 91 K and 105 K respectively. The structures in the homologous series appear to be based on alternating double bismuth Bi2O2 layers and perovskite (Sr, Ca)O–CuO2 layers, with higher members obtained by intercalating additional 3.1 A Ca-CuO2 bilayers. The structures can be indexed on a pseudo-tetragonal subcell with a = b = 5.4 A and c = 24.4 A (n = 1), 30.76 A (n = 2) or 36 A (n = 3). We were unable to prepare the n =4 member. In each structure a 2× superlattice structure in the c-direction arises as a natural consequence of a 19/4 incommensurate structure in the b direction, which accounts for 2.1 Bi atoms in the unit formula. The critical temperature, Tc, for these phases is sharply dependent on the Sr/Ca ratio and oxygen stoichiometry, as determined by heat treatment.

Synthesis, structure and superconductivity in the Ba1-xKxBiO3-y system

Abstract: The recent discovery of superconductivity near 30 K in Ba1−xKxBiO3−y (x≈0.4)1,2 is remarkable for two reasons. It is the first copper-free oxide superconductor that has a transition temperature (Tc) above that for the best intermetallic superconductor; and the structure is reported to be cubic, which excludes a two-dimensional metal–oxygen sublattice analogous to the CuO2 planes believed to be responsible for superconductivity in the copper-oxide-based superconductors. Cava et al.1 described a synthesis technique which involved starting with a 100% excess of KO2. At least part of the excess potassium was found to be present in the final sample (in a form not detectable by X-ray diffraction), resulting in samples that were not suitable for resistivity measurements and making a precise determination of the potassium and oxygen content in the superconducting phase impossible. Here we describe a two-step synthesis technique starting with a stoichiometric oxide composition, which yields single-phase samples suitable for transport measurements. Neutron powder diffraction studies of samples with varying potassium concentration show that superconductivity in Ba1−xKxBiO3−y, occurs only in a cubic perovskite phase which is stable at ≲600°C and which forms only for x > 0.25. Within this cubic phase, Tc is highest for compositions near the structural phase transition (x ≈ 0.25) and decreases with increasing x.

Catalytic Properties of Perovskite-Type Mixed Oxides (ABO3) Consisting of Rare Earth and 3d Transition Metals. The Roles of the A- and B-Site Ions

Abstract: The catalytic activities of perovskite-type mixed oxides (LnBO3 and Ln0.8Sr0.2CoO3, Ln=rare-earth (lanthanoid) elements, B=3d transition metals) for the oxidation of propane and methanol have been studied comparatively. It has been found that the catalytic activities of LnBO3 were principally determined by the B-site elements and were similar to those of the corresponding oxides of the B-site elements. The roles of the rare-earth ions of the A-site were secondary as long as they were trivalent. Upon the partial replacement of Ln3+ by Sr2+, the catalytic activities of LnCoO3 increased several times, the magnitude of the increase being similar among all the rare-earth ions. These results demonstrate that the kind and the valence of a B-site metal are of primary importance for the control of the catalytic activity for oxidation, almost regardless of the kind of trivalent rare-earth elements at the A-site.

Antiferromagnetism and superconductivity in oxygen-deficient YBa2Cu3O(x)

Abstract: Positive-muon spin-rotation and -relaxation measurements of the oxygen-deficient perovskite YBa2Cu3O(x) have revealed local antiferromagnetic order for x = 6.0-6.4 with a Neel temperature TN that decreases rapidly with increasing oxygen content x. For slowly annealed samples with x = 6.35-6.5 the superconducting transition temperature Tc increases smoothly with x from 25 K at x = 6.348 to 60 K at x = 6.507. Two such samples with x = 6.348 and x = 6.400 appear to 'switch' from superconductivity to antiferromagnetic order at lower temperatures.

Versatile new metalorganic process for preparing superconducting thin films

Abstract: A new technique for producing thin films of the high‐temperature perovskite superconductors, LnBa2Cu3O7−x, where Ln represents a rare‐earth element, using spin‐on metalorganic precursors is described. Pyrolysis of the spin‐coated films, consisting of mixed metal (2‐ethyl hexanoates), leads to black films up to several microns thick whose degree of orientation is a function of the processing temperature and duration. Representative films of YBa2Cu3O7−x on MgO begin to exhibit orientation with the c axis perpendicular to the film plane at heat treatments above 900 °C. The superconducting behavior of a highly oriented film produced at 990 °C is characterized by Tc(onset)=89 K, Tc(R=0)=77 K, and Jc=103 A cm−2 at 65 K.

Chemical durability of high‐temperature superconductor YBa2Cu3O7−x in aqueous environments

Abstract: The stability of YBa2Cu3O7−x in water and 100% humidity has been investigated at three temperatures using pH measurements, x‐ray diffraction, and scanning electron microscopy. The oxide‐ceramic superconductor is highly unstable and reacts rapidly with water and also degrades in moisture. Dissolution of the oxide perovskite in water is highly incongruent. The corrosion products are found to be BaCO3, CuO, O2, etc. Barium hydroxide is first formed which further reacts with atmospheric CO2 to form needle‐shaped crystals of BaCO3. For any practical applications, devices made from these materials would have to be protected with an impermeable coating to prevent deterioration from atmosphere.

Incommensurate structure in the Bi-Sr-Ca-Cu-O 80-K superconductor.

Abstract: We report on the structure and characterization of the major phase in the Bi-Sr-Ca-Cu-O system that becomes superconducting below 80 K. A superlattice modulation in the structure that is incommensurate with a tetragonal perovskite-based subcell is observed using electron diffraction. High-resolution imaging indicates that the modulation occurs primarily in bismuth-rich layers in the structure.

Growth and structural characterization of superconducting Ba1–xKxBiO3 single crystals

Abstract: Superconductivity near 30 K was discovered recently in a copper-free phase, Ba1–xKxBiO3 (refs 1, 2). This material has structural and electronic properties in common with both the cuprate high-transition-temperature (high-Tc) superconductors3 and with BaPb1–xBixO3, which has a Tc of ˜12 K (ref. 4). Here we report the growth and structural characterization of single crystals of Ba1–xKxBiO3. A maximum Tc of 30.5 K is found from d.c. susceptibility measurements for crystals of approximate composition Ba0.6K0.4BiO3. The structure for a crystal with x = 0.37 is determined by X-ray diffraction and is found to be a simple cubic perovskite with a = 4.2869 (6) A. A commensurate lattice distortion occurs for x = 0.04, which is identified as a distortion of the BiO6 octahedra. Effects of structural distortions and concomitant changes in electronic properties in this non-magnetic system may contain clues to the role of phonons and electronic interactions in the microscopic pairing mechanism of high-Tc superconductors.

Powder X-ray and neutron diffraction study of the superconductor Bi2Sr2CaCu2O8

Abstract: A sample of the new BiO-perovskite superconductor has been synthesized. X-ray diffraction has been used to locate the cations while neutron diffraction has been used to determine the precise oxygen co-ordination. The stricture contains CuO 2 planes with Cu coordinated to four oxygens at 1.92A within the plane, plus more distant oxygen at 2.65A perpendicular to the plane. Two such copper oxide (perovskite) planes are intercalated with planes of BiO. The stricture is described in an Fmmm subcell (5.4A, 5.4A, 30.8A), except for the oxygen within the BiO layers, and to a lesser extent the Bi itself, which require a x5 larger b-axis. The X-ray refinement in Bbmm indicates that this Bi displacement is 0.27A.

Thermodynamic fluctuations in the high-Tc perovskite superconductors.

Abstract: The role of fluctuations in the breakdown of mean-field theory in the new high-${T}_{c}$ perovskite superconductors is analyzed. Both the breakdown of the classical critical exponents (the Ginzburg criterion) and the quantitative breakdown of mean-field theory (the Brout criterion) are discussed. It is found that for certain sets of observed parameters for these materials, a breakdown of mean-field theory is to be expected.

The Raman spectra of several orthorhombic calcium oxide perovskites

Abstract: We have obtained Raman spectra for a number of orthorhombic perovskites CaBO3, where B=Ti, Ge, Zr or Sn. The room temperature Raman spectrum of CaTiO3 was compared with cubic SrTiO3 to assign first- and second-order features. Partially polarized micro-Raman spectra were obtained for CaTiO3 perovskite. The CaBO3 perovskites showed a sequence of increasing complexity in their Raman spectra with increasing degree of orthorhombic distortion from the ideal cubic structure. The spectral changes cannot easily be correlated with changes in chemistry or structure of the perovskite. High temperature micro-Raman spectra for CaGeO3 perovskite were obtained by laser-heating a 15 μm sample. None of the low frequency Raman modes were soft, but showed only normal anharmonicity up to approximately 700 K.

Dopant effects on the superconductivity of YBa2Cu3O7 ceramics

Abstract: We investigated the effects of SrTiO3 and 17 different oxide dopants, namely Ca, Sc, La, and Zr with possible substitution in the yttrium sites, K, Sr, and Pb in the barium sites, Li, Mg, Fe, Co, Ni, Zn, Al, Ti, Nb, and Si in the copper sites on the superconductivity of YBa2Cu3O7. These dopants can be classified into four categories depending on their solubilities and substitution in the different cation sublattices of YBa2Cu3O7 perovskite. Dopants in the first category dissolve and substitute in the copper sublattice and they can significantly reduce the transition temperature of the superconductivity of YBa2Cu3O7. For example, three dopants in this category, namely Zn, Mg, and Li, at 2 mole % dopant level, reduce the transition temperatures to 66, 65, and 82 K, respectively. The second category dopants substitute in the barium and yttrium sites, and these dopants, e.g., Sr at 2 mole % concentration, do not have any significant effect on the transition temperature of the superconductivity of YBa2Cu3O7. While dopants, e.g., AlO1.5 and SiO2, in the third category have a limited solubility in YBa2Cu3O7, they have a strong tendency to decompose the superconducting phase by leaching out some components of YBa2Cu3O7. The transition temperature of the undecomposed superconducting phase remains unaffected at ∼91 K even though the susceptibility signal is reduced substantially. Since the dopants in the fourth category have a limited solubility and very little reactivity with YBa2Cu3O7, they are present simply as an inert second phase among the superconducting matrix.

Oxygen intercalation in the perovskite superconductor YBa2Cu3O6+x

Abstract: We have studied superconductivity and the thermodynamics of oxygen insertion in the oxygen intercalation system YBa2Cu306+„. The superconducting transition temperature decreases as x decreases, and the transition is sharp at T, 90 K near x 1 and at 55 K near x 0.5. The thermodynamics of oxygen intercalation at high temperatures (both isotherms and isobars), and the orthorhombic-tetragonal transition, can be explained in terms of a lattice-gas model that includes a large energy of repulsion between oxygen atoms on nearest-neighbor sites. The high-temperature behavior is also consistent with smaller interactions, some attractive, between second neighbors. Such energies are needed to explain the behavior at low temperatures.

Superstructure of the superconductor Bi2Sr2CaCu2O8 by high-resolution electron microscopy

Abstract: The structure of the 80-K superconductor Bi2Sr2CaCu2O8 (ref. 1) determined by X-ray and neutron diffraction2–4 is an Aurivillius-related phase as suggested by Michel et al.5, with approximate unit-cell dimensions a = √2ap, b = 5√2ap and c = 8ap, where the perovskite lattice parameter ap = 3.8 A. It consists of layers of perovskite sandwiched between BiO layers. Although electron microscopy does not have sufficient resolution to unambiguously determine the basic structural unit of such a compound, it is an invaluable tool in determining the superstructure6. Here we present a high-resolution electron microscope study which reveals waves of distortion along the b-axis, giving a superlattice slightly larger or smaller than 5√2ap. The superlattice is composed of building blocks four, five, and six times √2ap/2 in approximately periodic combinations. The very low 'twin' density in Bi2Sr2CaCu2O8 and the nature of the 'twins' (successive planes with the a and b axes interchanged, also called twist boundaries) implies that twinning does not play an essential role in the superconducting phenomenon.