Structure ofYBa2Cu3O7−δversus temperature by x-ray-absorption spectroscopy (original) (raw)

Cu 3d9- LIGAND HOLE CONFIGURATION IN YBa2Cu3O~7BY X-RAY SPECTROSCOPIES

1987

Cu K edge and L3 edge x-ray absorption near edge structure (XANES) and Cu L3 x-ray photoelectron spectroscopy (XPS) of YBa2Cu307 have been measured to determine the local electronic structure. The K-edge XANES have been interpreted by multiple scattering approach. The data show the formation of the 3 d 9~ (where L is a hole in the oxygen derived band, ligand hole) many body configuration in the initial state of the superconducting oxide.

X-ray-absorption studies ofYBa2Cu3O7−δandGdBa2Cu3O7−δsuperconductors

Physical Review B, 1987

X-ray-absorption measurements of the high-T, superconductors YBa2Cu307and GdBazCu307q show that their near-edge and extended fine structures (EXAFS) are essentially identical over the temperature range of 4.2 to 688 K. The results indicate that not only are YBa2Cu30p-& and GdBa2Cu307-& very similar to each other in local bonding and electronic structure, but also remain virtually unchanged, electronically and structurally, over this wide range in temperature, from far below to well above T,. The positions of the Cu near-edge spectra for these superconductors suggest a Cu valence between that of Cu in CuO, which has a formal valence of II, and that in KCu02, with a formal valence of III. Our EXAFS structural results agree well with those from neutron diffraction data. Recently proposed alternate structures are shown to be inconsistent with our data. The characteristic vibration frequencies of the nearneighbor atoms show that the two-dimensional Cu-0 planes and the one-dimensional Cu-0 chains form the most rigid parts of the structure, and these layers are bound more weakly along the c axis via the Ba and Y planes. These oscillator frequencies are identified with specific nearneighbor pairs, and the values are consistent with those for some of the modes seen by inelastic neutron scattering as well as by infrared and Raman spectroscopy. No evidence of phonon softening is apparent in any of the phonon modes studied, providing evidence against a strong electron-phonon coupling mechanism.

Observation of temperature-dependent site disorder inYBa2Cu3O7−δbelow 150 °C

Physical Review B, 1990

We have observed annealing effects in both single-crystal and ceramic samples of oxygen-deficient YBa,Cu30& z that occur at temperatures as low as O'C. Oxygen stoichiometries were controlled by quenching, from 520 C, samples equilibrated in a controlled 0&-N& atmosphere. For single crystals, transition temperatures with any desired value between 0 and 92 K, and transition widths less than 2 K, were readily obtained with appropriate variation of 5. After quenching, superconducting transition temperatures of the reduced samples rise, as much as 15 K, when aged in ambient atmosphere for several days; the sample composition does not change with aging. With low-temperature aging, structural changes also occur; e.g. , orthorhombicity increases. These effects are attributed to oxygen-vacancy ordering that occurs in the chain region of the structure. The rise in T, with aging (ordering) is attributed to increased hole doping in the planes that results from an increased population of two-coordinated (monovalent) copper atoms. For samples with reduced stoichiometries, the degree of disorder can be reversibly controlled with secondary quenches in the temperature range 0-120'C. For stoichiometries 7-5=6.5, an approximate activation energy of 0.96 eV was obtained for the annealing process. This low-temperature annealing behavior, occurring in samples of constant composition, provides a remarkably simple and effective way to study the relationship between superconductivity, structure, and associated electronic properties.

New 1212 structural analog of YBa2Cu3O7

Physica C: Superconductivity, 1993

We have prepared a new superconducting copper oxide, CuSr2_xLaxYCu2Ov+a (x~0.4, J~0.2) with To~60 K. While the parent compound, x=O, can be prepared only at very high oxygen pressures (P(O2)>20 000 atm), the tetrqonal (a= 3.8027, cv 11.3806 A), La-doped, material can be obtained at relatively moderate pressures, P(O2) =600 atm at 1180°C. Rietveld refinements of neutron powder diffraction data show that La and Sr occupy the "Ba" site but with slightly different positions. The displacement pattern of neighboring oxygen atoms suggests that the La defects pair to form dimers perlx~dicular to the basal plane.

Dynamics of the Cu-O chain in YBa2Cu3O7??: Emission M�ssbauer studies

Journal of Superconductivity, 1990

The dynamics of the Cu-O chain in YBa2Cu307_~ was investigated by emission MiSssbauer spectroscopy, using S7Co dopant to microprobe the Cu(1) site. The transformation of the 5-coordinate trigonal-bipyramidal cobalt configuration (species B) to a square-pyramidal one (species C) is induced by the'zig-zag motion of the chain. The thermodynamical parameters associated with this conversion were determined, AH ~ 38 + 5 kJ/mol (0.4 eV) and AS ~ 127+20J/K. About 40% of species C does not convert into B at all. We suggest that the inconvertible C has a static square-pyramidal configuration at twin-boundaries where the chain motion is damped. In the 57Fe-doped compound, the iron atoms form linear clusters, and the Cu-O chain motion and also the B-to-C conversion is retarded in the temperature range 200-350 K. A new minor Co species similar to B (namely, B') is reported, Which is presumably situated on the grain boundaries. Its nonmetallic environment is reflected in the relatively large value of isomer shift. The properties of the electric field gradient tensor at major Co(Fe) sites are also discussed.

Structural properties of YBa2Cu3O6+x at elevated temperatures in controlled oxygen atmospheres

Physica C: Superconductivity, 1993

The structural properties of YBa2Cu306+x have been determined at elevated temperatures in controlled oxygen partial pressure by in situ neutron powder diffraction. Diffraction data were obtained as a function of the oxygen partial pressure at constant temperatures of 490 ° C and 440 ° C. These data, taken in thermodynamic equilibrium, provide information that cannot be obtained from metastable oxygen-deficient samples studied at room temperature. For example, the orthorhombic-to-tetragonal phase transition is observed to be continuous, as expected for a second order phase transition. Sequential diffraction measurements following each change in experimental conditions were used to monitor the approach to equilibrium by observing changes in lattice constants. A considerable increase in equilibration time (following a step-like change in oxygen partial pressure) was found as the orthorhombic-to-tetragonal phase transition was approached. No phases other than the high-temperature orthorhombic (ortho-I) and tetragonal phases were observed.