Changes in the oxygen stoichiometry as well as the La:Ba ratio in the La4BaCu5O13+δ family bring about metal-insulator transitions. The cross over from the metallic to the semiconducting behaviour occurs around 3×10−3 ohm.cm.
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In this article the author discusses commonalities in structure and superconducting properties among high-temperature superconducting cuprates discovered since 1987. The nature of oxygen holes in these high-temperature oxide superconductors is described and their importance in determining whether a cuprate is a superconductor or an insulator is addressed.
This article sets out to describe and account for the chemical and physical consequences of the presence of gross disorder in solids. Knowledge of the structure of such disordered materials is an obvious prerequisite to a further understanding of other properties and behavior, and our current knowledge of the structure of various noncrystalline systems is discussed together with the experimental techniques which need to be employed in order to obtain such information. The so‐called glass transition, which takes place as a liquid is supercooled below the crystallization temperature, is discussed in terms of the various models which have been proposed to account for this phenomenon. The effect of noncrystallinity on electronic properties is also discussed, and we highlight new developments in the understanding of electron localization and transport processes. Finally, two applications of amorphous solids are considered in some detail: optical fibers for use in communication networks and “superionic” glasses for possible use in solid‐state batteries.
Graphene has generated great sensation owing to its fascinating properties with possible potential applications. This two-dimensional material exhibits half-integer quantum Hall effect and an ambipolar electric field effect, along with ballistic conduction of charge carriers. In this article, we provide a overview on some aspects of graphene devoting the special attention to synthesis, functionalization, self-assembly, surface properties, gas adsorption and fluorescence quenching ability of graphene. Graphenes with varying number of layers can be synthesized by using different strategies. Graphene can be functionalized by different means in order to disperse it in various solvents. We also present the self-assembly of graphene at the liquid-liquid interface besides its surface properties including adsorption of hydrogen, carbon dioxide and methane. The remarkable property of graphene of quenching fluorescence of aromatic molecules is shown to be associated with photo-induced electron transfer.
Magnetic and electrical properties of Ln0.5Ba0.5CoO3−δ with Ln=Dy and Er have been investigated to examine the effects of large cation size-disorder. While the Dy compound shows the small magnetic anomaly around 290K just as the Gd derivative, the Er compound is essentially paramagnetic due to the large cation size-disorder. Compositions with the same average A-site cation radii as Dy0.5Ba0.5CoO2.91 and Er0.5Ba0.5CoO2.9, but with smaller size-disorder, show progressive evolution of ferromagnetism and metallic properties with decreasing disorder.
ZnO nanoparticles have been synthesized from the cupferron complex by a solvothermal route in toluene solution. The nanoparticles have been prepared in the presence of various capping agents, of which the best results were obtained with tri-n-octylphosphine oxide, polyethylene glycol, and sodium bis (2-ethylhexyl) sulphosuccinate. The particles obtained with these capping agents have diameters in the 8-14 nm range. The nanoparticles have been characterized by electron microscopy, UV absorption spectroscopy, and photoluminescence spectroscopy, besides x-ray diffraction. Optical spectra of the small nanoparticles show evidence for quantum confinement. ZnO nanoparticles doped with 5% and 10% Mn could be prepared by the solvothermal route starting with a Zn(1-x)Mnx cupferron complex, and the Mn-doped nanoparticles remain paramagnetic down to 5K.
An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.
Superconducting YBa2Cu3O7-δ (0.3 ≤ δ ≤ 0.4) compositions seem to transform to a metastable YBa2Cu4O8 (124)-like phase with a c parameter of 27.2 Å on annealing at 473 K for a few hours.
Co-crystallization of maleic acid, 1, with 4,4′-bipyridine, 2, in acetone, chloroform, ethylacetate and methanol gives a hydrogen bonded 2:1 adduct of 1 with 2 (monoclinic space group, C2/c, a=24.056(8), b=6.891(2), c=11.474(6)Å, β=116.10(4)°). Co-crystallization in dimethylformamide and dimethylsulfoxide, however, yields a 1:1 adduct of fumaric acid with 2 (triclinic space group P̄1, a=3.875(1), b=8.906(1), c=10.016(1)Å, α=109.60(1)°, β=98.64(1)°, γ=96.64(1)°).
The high-temperature superconductors are complex oxides, generally containing two-dimensional CuO2 sheets. Various families of the cuprate superconductors are described, paying special attention to aspects related to oxygen stoichiometry, phase stability, synthesis and chemical manipulation of charge carriers. Other aspects discussed are chemical applications of cuprates, possibly as gas sensors and copper-free oxide superconductors. All but the substituted Nd and Pr cuprates are hole-superconductors. Several families of cuprates show a nearly constant n(h) at maximum T(c). Besides this universality, the cuprates exhibit a number of striking common features. Based on Cu(2p) photoemission studies, it is found that the Cu-O charge-transfer energy, DELTA, and the Cu(3d)-O(2p) hybridization strength, t(pd), are key factors in the superconductivity of cuprates. The relative intensity of the satellite in the Cu(2p) core-level spectra, the polarizability of the CuO2 sheets as well as the hole concentration are related to DELTA/t(pd). These chemical bonding factors have to be explicitly taken into account in any model for superconductivity of the cuprates.
Some materials exhibit large changes in electrical resistance in the presence of a magnetic field, and this change can be used in applications from sensor technology to magnetic data storage. In their Perspective, Rao and Cheetham discuss magnetoresistance in perovskite manganates, where the effect is unusually strong. Much has been learned about these materials, and this understanding is driving the search for new materials with even more impressive properties.
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Preparation and characterization of the fullerenes, C60 and C70, are described in detail, including the design of the generators fabricated locally. The characterization techniques employed are UV-visible, IR, Raman and C-13 NMR spectroscopies, scanning as well as transmission electron microscopy and mass spectrometry. The electron energy level diagram of C60 as well as the one-electron reductions of C60 and C70 leading to various anions are discussed. Electronic absorption spectra of C60- and C60(2-) are reported. Phase transitions from the plastic to the crystalline states of C60 and C70 are examined. Based on a C-13 NMR study in a mixture of nematic liquid crystals, it has been demonstrated that C60 retains its extraordinary symmetry in solution phase as well. Interaction of C60 and C70 with strong electron-donor molecules has been investigated employing cyclic voltammetry. Superconductivity of K(x)C60 has been studied by non-resonant microwave absorption; Na(x)C60 as well as K(c)C70 are shown to be non-superconducting. Doping C60 with iodine does not make it superconducting. Interaction of C60 with SbCl5 and liquid Br2 gives rise to halogenated products.
