THE superconducting transition temperature, Tc, in the YBa2Cu3O6+x system is known to depend not only on the oxygen stoichiometry1, but also on the degree of oxygen order2,3. Observations of oxygen ordering structures by electron microscopy4,5 have led to the suggestion that the '60-K plateau' near x = 0.5 in the curve of Tc versus x arises from an ordered structure (the 'ortho-II phase') in which Cu–O and Cu chains alternate in the basal plane6. Until now, however, the ortho-II phase has been seen only by electron diffraction, except for one observation at an oxygen stoichiometry of x = 6.7 by X-ray diffraction7. Compared with true bulk methods such as X-ray and neutron diffraction, electron diffraction suffers from the disadvantage that only a very small portion of the sample is probed. Here we report the observation, by neutron diffraction, of local ortho-II ordering in an YBa2Cu3O6.4 crystal (Tc = 38 K). The observed correlation lengths along the principal axes are 24b (along the chains), 10a and 2c. Our observation of the existence of the ortho-II phase for an oxygen stoichiometry close to the minimum for which superconductivity is observed (x≈0.4) supports existing theoretical models8,9, in particular one8 that relates Tc to the presence of two types of ordered domain.

Local oxygen ordering in superconducting YBa 2 Cu 3 0 6.4 observed by neutron diffraction

We have measured the superconducting transition temperature Tcof oxygen deficient single crystals of YBa2Cu3Ox, where x = 7−δ in the range 6.5 T ∗ . We associate T ∗ with the onset of a transition from a vacancy ordered low temperature (ortho II and/or ortho III) phase to a high temperature ortho I phase. The nature of the vacancy ordering below T ∗ varies with x. We believe that, upon heating above T ∗ , a signigicant concentration of chain fragments develops in normally vacant chains of the ordered structure. When the crystals were quenched from temperatures above T ∗ , and subsequently permitted to anneal at 51°C, two relaxation processes were found. The two processes have relaxation times differing by about two orders of magnitude. The faster relaxation process is associated with changes in vacancy short-range disorder. The slower process is attributed to restoration of long-range chain vacancy order as misplaced fragments anneal out. T ∗ is determined for different oxygen concentrations and compared to theoretical phase diagram calculations. Electron diffraction measurements on well annealed single crystals with stoichiometries x = 6.50 and 6.65 show that different chain ordered phases (primarily ortho II and ortho III, respectively) exist at each of these compositions. Chain vacancy order is also observed at temperatures above T ∗ .

New observation of a phase boundary in oxygen deficient YBa2Cu3O7−δ single crystals

From the intensities of superstructure reflections the structure of the ortho-II phase in YBa 2 Cu 3 O 6.51(5) (T c =55.8 K ) was derived by single crystal X-ray diffraction. In the basal plane of the 2 a × b × c supercell the oxygen atoms order in alternating CuOCu and CuCu chains. The correlation lengths of the ortho-II domains are 68 A, 520 A, 65 A in a , b , and c - direction. Due to the oxygen order Ba atoms are displaced in an antiferrodistortive way towards the CuO chains by 0.03 A. In this way the Coulomb energy of the ortho-II structure is reduced. The antiferrodistortive displacements of the Ba atoms could be important for the superconducting pairing mechanism.

Barium displacements in the superconducting ortho-II phase of YBa2Cu3O6.51. An X-ray structure determination

The structural properties of YBa2Cu3O6+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 phahe 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 equilibrium 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.

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

The superstructure of ordered oxygen atoms in a single domain of YBa2Cu3O6.35 has been determined by X-ray diffraction. The 45o rotated superstructure cell is orthorhombic—spacegroup Pbam, lattice constants\(2\sqrt 2 a \times \sqrt 2 a \times c\), witha=3.8652(3) A,c=11.815(2)A the dimensions of the fundamental tetragonal cell. The arrangement of the oxygen atoms in the basal plane minimizes their electrostatic repulsion by avoiding near and next-near neighbors. There are displacements of copper atoms of 0.13 A in the Cu(1) plane and 0.06 A in the Cu(2) plane. Only 12% of the oxygen atoms in the basal plane contribute to the superstructure.

Y. P. Lin

Papers

Defect ordering in YBa2Cu3O6.5 and YBa2Cu3O6.6 : synthesis and characterization by neutron and electron diffraction

A study of oxygen and vacancy ordering in YBa2Cu3O7−x