Electronic structure of high-Tc Ba0.6K0.4BiO3 by x-ray photoelectron spectroscopy

We have investigated the electronic structure of Ba1-xKxBiO3 (0superconductivity, which predicts hole conduction in the Ba-K-Bi-O and Ba-Pb-Bi-O systems.

An investigation of the PrBa2Cu3O7-δ system

Members of the PrBa2Cu3O7–delta system possessing the orthorhombic structure over a wide range of oxygen stoichiometry (delta=–0.5 to +0.5) have been prepared and characterized. Similar compositions with a tetragonal structure have also been prepared. None of the oxides is superconducting, independent of the structure or stoichiometry. Praseodymium seems to be present to a small extent in the 4+ state in oxygen-excess (negative delta) samples. Orthorhombic PrBa2Cu3O7–delta samples show the presence of twins suggesting that twins arise from orthorhombicity and have no relation to the superconductivity. A neutron diffraction study of a near-stoichiometric sample has shown a disordered orthorhombic structure with 72% occupancy of the 01 (chain) sites and with no interchange between Pr and Ba sites.

A comparative study of superconducting LaBa2Cu3O7−δ and La3−xBa3+xCu6O14+δ

Based on X-ray diffraction and electron microscopy it is shown that oxides of the general formula La Ba2Cu3O7−δ become tetragonal when δ deviates slightly from 0. This tetragonal structure is similar to that of La3−xBa3+xCu6O14+δ, with a cubic perovskite subcell and triple periodicity. Electron micrographs of these tetragonal oxides show 90° microdomains. Orthorhombic LaBa2Cu3O7−δ with high Tc (not, vert, similar77 K) is found only when δ reverse similar, equals 0; this sample is subject to formation of twins. Fluorine substitution seems to favor superconductivity.

Evidence for double valence fluctuation in metallic oxides of lead

The possible role of double valence fluctuation in both lead and oxide ions with reference to metallization in oxides of lead is examined by x-ray-photoemission spectroscopy, ultraviolet-photoemission spectroscopy (UPS), and 207Pb NMR studies. The double valence fluctuations may be viewed as Pb4++2O2-⇄Pb2+O22-. While the insulating oxides PbO, Pb3O4, and Sr2PbO4 show a single oxide ion, O2- characterized by O(1s) at 529.7 eV, the insulating peroxide BaO2 is characterized by the ion O22- with a single O(1s) at 533 eV. The metallic PbO2, BaPbO3, BaBiPbO3, and SrPbO3 showed the occurrence of both O2- and O22- ions. The valence band in these compounds has also been studied by UPS, and clear evidence for the coexistence of O2- and O22- is seen in PbO2. A simultaneous study of 207Pb NMR suggests that the Pb ion could also exist in mixed-valence states. Qualitative arguments are presented to rationalize the existence of such mixed valences of the anion in metal oxides in general and their role in superconductivity.

Nature of the copper species in superconducting YBa2Cu3O7

The nominal composition of YBa2Cu3O7 oxide suggests that 33% of the copper should be in the 3+ state. In structural studies3–5, Cu3+ and Cu2+ ions have been assigned specific sites based on their preferred coordinations, and many of the models of superconductivity also require the presence of Cu3+. Our studies however show that Cu is present only in the Cu1+ and Cu2+ states, the proportion of the former increasing with decreasing temperature. It appears that there is no clear experimental evidence for the presence of Cu3+ in YBa2Cu3O7.

Solid-state chemistry of high-temperature oxide superconductors:The experimental situation

High-temperature superconductivity in oxides of the type(La, Ln)2?xBax(Sr)xCuO4, Y(Ln)Ba2Cu3O7??, La3?xBa3+xCu6O14, and related systems is discussed with emphasis on aspects related to experimental solid-state chemistry. All of these oxides possess perovskite-related structures. Oxygen-excess and La-deficient La2CuO4 also exhibit superconductivity in the 20�40 K just as La2?xBax(Srx)CuO4; these oxides are orthorhombic in the superconductivity phase. The crucial role of oxygen stoichiometry in the superconductivity ofYBa2Cu3O7?? (Tc = 95 ± 5K) is examined; this oxide remains orthorhombic up to ? ? 0.6 and becomes tetragonal and nonsuperconducting beyond this value of ?. Oxygen stoichiometry in this and related oxides has to be understood in terms of structure and disorder. The structure of La3?xBa3+xCu6O14 is related to that of YBa2Cu3O7, the orthorhombic structure manifesting itself when the population of O1 oxygens (along the Cusingle bondOsingle bondCu chains) is preponderant compared to that of O5 oxygens (along thea-axis); nearly equal populations of O1 and O5 sites give rise to the tetragonal structure. A transition from a high-Tc (95 K) superconductivity regime to a low-Tc (not, vert, similar60 K) regime occurs in YBa2Cu3O7?? accompanying a change in ?. There is no evidence for Cu3+ in these nominally mixed valent copper oxides. Instead, holes are present on oxygens giving rise to O? or O2?2 species, the concentration of these species increasing with the lowering of temperature. Certain interesting aspects of the superconducting oxides such as domain or twin boundaries, Raman spectra, microwave absorption, and anomalous high-temperature resistivity drops are presented along with the important material parameters. Preparative aspects of the superconducting oxides are briefly discussed. Phase transitions seem to occur atTc as well as at not, vert, similar240 K in YBa2Cu3O7.

Structure, stoichiometry and spectroscopy of oxide superconductors

In the new oxide superconductors, structure and oxygen stoichiometry play the most crucial role. Thus, all the high-temperature oxide superconductors are orthorhombic perovskites with low-dimensional features. Oxygen stoichiometry in YBa2Cu3O7-δ has an important bearing on the structure as well as superconductivity. This is equally true in the La3-xBa3+xCu 6O14+δ system of which only the 123 oxide (x = 1) with the orthorhombic structure shows high Tc. Orthorhombicity though not essential, is generally found ; it is necessary for the formation of twins. The nature of oxygen and copper in the cuprates has been examined by electron spectroscopy. Copper in these cuprates is only in 1 + and 2 + states. It seems likely that oxygen holes are responsible for superconductivity of the cuprates as well as Ba(Bi, Pb)O3. High Tc superconductivity is also found in oxides of the Bi-(Ca, Sr)-Cu-O and related oxides possessing Cu-O sheets.

Tl1-xPbxCaSr2Cu2O7: Another 1122 high-Tc superconductor

The 1122 member of the Tl(Ca, Sr)n+1CunO2n+3 system is stabilized by partial substitution of Tl by Pb. Tl1?xPbxCaSr2Cu2O7 with x=0.25 and 0.5 have both primitive tetragonal structures (c 12.1 Å) showing onset of superconductivity around 70 K and 90 K, respectively.

An X-ray and neutron diffraction study of cation substituted TlSr2CuO5

The structures of TlSr(2−x)LaxCuO(5+δ), with x=0.5, 0.75 and 1, and Tl.5Pb0.5Sr2CuO(5+δ) have been examined with X-ray and neutron powder Rietveld refinement. They are isostructural (P4/mmm) with the corresponding thallium-barium cuprate having one Cu-O layer with Cu3+ ions in octahedral coordination with oxygen (structure type 1201). The influence of cation substitution and disorder on the structure and superconducting properties of these phases have been investigated. La3+ substitution for Sr2+ stabilises the structure and reduces Cu3+, permitting superconductivity, while Pb2+ substitution for Tl3+ only stabilises the structure, without reducing Cu3+.

Disorder-driven electronic localization and phase separation in superconducting Fe1+yTe0.5Se0.5 single crystals

We have investigated the influence of Fe excess on the electrical transport and magnetism of Fe1+yTe0.5Se0.5 (y=0.04 and 0.09) single crystals. Both compositions exhibit resistively determined superconducting transitions (T-c) with an onset temperature of about 15 K. From the width of the superconducting transition and the magnitude of the lower critical field H-c1, it is inferred that excess of Fe suppresses superconductivity. The linear and nonlinear responses of the ac susceptibility show that the superconducting state for these compositions is inhomogeneous. A possible origin of this phase separation is a magnetic coupling between Fe excess occupying interstitial sites in the chalcogen planes and those in the Fe-square lattice. The temperature derivative of the resistivity d(rho)/d(T) in the temperature range T-c < T < T-a with T-a being the temperature of a magnetic anomaly, changes from positive to negative with increasing Fe. A log 1/T divergence of the resistivity above T-c in the sample with higher amount of Fe suggests a disorder-driven electronic localization.

Photoemission study of the high-Tc superconductor Bi1.8Pb0.4Sr2Ca2.2Cu3O10

Ultraviolet and X-ray photoemission spectroscopic (UPS and XPS) studies to characterize the electronic structure of bismuth cuprate superconductor with nominal composition of Bi1.8Pb0.4Sr2Ca2.2Cu3O10 have been carried out. The data clearly shows the metallic emission at the Fermi level (EF). The shoulder (-1.2 eV) near the EF is attributed to the Cu-O derived states. Cu satellite structures observed both in the UPS and XPS show the strongly correlated nature of the Cu 3d electrons. Core level shifts indicate that 3+ and 4+ are the main oxidation of Bi and Pb, respectively. The Pb core lines show two components indicating their inequivalent sites. Core level O 1s spectrum is deconvoluted to show the presence of structurally non-equivalent oxygen sites.

Relation between Tc and hole concentration in superconducting cuprates

The superconducting transition temperature, Tc, of several series of cuprates shows a nonlinear dependence on the hole concentration, nh, determined by chemical titrations. The tc becomes maximum when nh is in the 0.12-0.15 range in cuprates containing a single Cu-O layer and around 0.2 in cuprates containing two Cu-O layers.

Structure and superconducting properties of Tl1?yPbyY1?xCaxSr2Cu2O7star, open: Dependence of Tc on hole concentration

Structure and superconducting properties of Tl1?yPbyY1?xCaxSr2Cu2O7 (y=0.0, 0.1, 0.25 and 0.5) , derived from the parent insulator TlYSr2Cu2O7, have been investigated for different values of x. XANES studies show Pb to be in the 4+ state while Tl is in the 3+ state, suggesting thereby that in this series, (x?y) approximately corresponds to the hole concentration. The in-plane Cu---O distance decreases with increase in x for all values of y. The apical Cu---O distance as well as the Tl(Pb)---O(2) distance show anomalies at x=0.5 in the series with y=0.25; at this composition, Tc also reaches a maximum. In general, the composition (value of x) at which the Tc of Tl1?yPbyY1?xCaxSr2Cu2O7 reaches a maximum depends on the Pb content and the maximum Tc itself increases with increase in y, reaching a value of 105 K at y=0.5. More interestingly, a maximum Tc occurs at an (x?y) value of not, vert, similar0.25, which is close to the hole concentration at maximum Tc in other cuprate superconductors containing two CuO2 layers.

The Blackett Memorial Lecture, 1991: Chemical Insights into High-Temperature Superconductors

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.

A model for antiferromagnetic to superconductor transition

A model system capable of exhibiting both superconductivity of conduction electrons and “antiferromagnetic” order of site localised electrons is studied. Coexistence of both types of order seems a possibility and the model predicts a re-entrant behaviour of the transition temperature.