Structural relationships in high temperature superconductors

The recent discovery of two types of metallic copper oxide compounds which are superconducting to above 90/sup 0/K has renewed interest in the search for new high temperature superconducting materials. It is significant that both classes of compounds, La/sub 2-x/Sr/sub x/CuO/sub 4-y/ and YBa/sub 2/Cu/sub 3/O/sub 7-delta/ are intimately related to the extensively studied perovskite family. Both compounds contain highly oxidized, covalently bonded Cu-O sublattices, however, they differ in geometry. In this paper we discuss the relationship of these features to the superconducting properties. 30 refs., 6 figs

Solubility and superconductivity in RE(Ba sub 2-x RE sub x )Cu sub 3 O sub 7+. delta. (RE = Nd, Sm, Eu, Gd, Dy)

Solid solutions of RE(Ba{sub 2-x}RE{sub x})Cu{sub 3}O{sub 7- {delta}}(RE=Nd,Sm,Eu,Gd,Dy) for x=0 to x=0.5 have been investigated. X-ray and resistivity measurements show that there exists a solid solution region, through which, the structure changes from orthorhombic to tetragonal and the superconducting properties are depressed. The solubility limits depend strongly on the size of the rare-earth ion, with the smallest (Dy) showing no appreciable solubility. The superconducting transition temperature versus x for all of the rare-earth ion substitutions falls on a universal curve, indicating that the Ba sites are extremely ionic and magnetically isolated. 20 refs., 4 figs

Amorphous W S N thin films The atomic structure behind ultra low friction

Amorphous W-S-N in the form of thin films has been identified experimentally as an ultra-low friction material, enabling easy sliding by the formation of a WS2 tribofilm. However, the atomic-level structure and bonding arrangements in amorphous W-S-N, which give such optimum conditions for WS2 formation and ultra-low friction, are not known. In this study, amorphous thin films with up to 37 at.% N are deposited, and experimental as well as state-of-the-art ab initio techniques are employed to reveal the complex structure of W-S-N at the atomic level. Excellent agreement between experimental and calculated coordination numbers and bond distances is demonstrated. Furthermore, the simulated structures are found to contain N bonded in molecular form, i.e. N-2, which is experimentally confirmed by near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy analysis. Such N-2 units are located in cages in the material, where they are coordinated mainly by S atoms. Thus this ultra-low friction material is shown to be a complex amorphous network of W, S and N atoms, with easy access to W and S for continuous formation of WS2 in the contact region, and with the possibility of swift removal of excess nitrogen present as N-2 molecules. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

A microscopic description of the evolution of the local structure and an evaluation of the chemical pressure concept in a solid solution

Extended x-ray absorption fine-structure studies have been performed at the Zn K and Cd K edges for a series of solid solutions of wurtzite Zn1-xCdxS samples with x = 0.0, 0.1, 0.25, 0.5, 0.75, and 1.0, where the lattice parameter as a function of x evolves according to the well-known Vegard's law. In conjunction with extensive, large-scale first-principles electronic structure calculations with full geometry optimizations, these results establish that the percentage variation in the nearest-neighbor bond distances are lower by nearly an order of magnitude compared to what would be expected on the basis of lattice parameter variation, seriously undermining the chemical pressure concept. With experimental results that allow us to probe up to the third coordination shell distances, we provide a direct description of how the local structure, apparently inconsistent with the global structure, evolves very rapidly with interatomic distances to become consistent with it. We show that the basic features of this structural evolution with the composition can be visualized with nearly invariant Zn-S-4 and Cd-S-4 tetrahedral units retaining their structural integrity, while the tilts between these tetrahedral building blocks change with composition to conform to the changing lattice parameters according to the Vegard's law within a relatively short length scale. These results underline the limits of applicability of the chemical pressure concept that has been a favored tool of experimentalists to control physical properties of a large variety of condensed matter systems

A new graphitic carbon nitride-coated dual Core–Shell sulfur cathode for highly stable lithium–sulfur cells

Lithium-sulfur (Li–S) batteries have promise to deliver energy density two to three times higher than that of currently used lithium-ion batteries. The commercialization of Li–S batteries is primarily hindered due to the polysulfide shuttle (PSS) effect, which not only leads to the loss of active materials from the cathode, but also causes serious irreversible reactions between the polysulfide intermediates and the lithium metal anode, resulting in low coulombic efficiency, high self-discharge and short cycle life. In this paper, we report the design and synthesis of a new graphitic carbon nitride-coated dual core–shell structured sulfur cathode (S@HCS@g-C3N4) to address these issues, leading to superior capacity retention properties in Li–S cells. This structural design allows confinement of polysulfide intermediates within a dual-core electrically conductive structure consisting of a hollow mesoporous carbon sphere (HCS) core, and a peripheral graphitic carbon nitride (g-C3N4) layer, which is known to suppress the PSS effect by enhanced chemical interactions with polysulfide intermediates. Indeed, the S@HCS@g-C3N4 cathode displayed excellent electrochemical performance in terms of high initial specific capacity (1446 mA h g−1 at 0.2C) and very good long-term cycling performance (capacity decay rate of 0.049% per cycle after 500 cycles at 1C). © 2020 Elsevier B.V

Field-induced valence transition in EuPtP1−xAsx

The ternary compound EuPtP exhibits two valence transitions at T1 = 235 K and T2 = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP1−xAsx compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T1 and T2, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP0.5As0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T1