R. Sonntag

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

Abstract 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.

X-ray superstructure determination of ordered oxygen and Cu-displacements in a single domain of YBa2Cu3O6.35

AbstractThe 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

X-ray anomalous scattering on the superconducting ortho-II phase of YBa2Cu3O6.51: A verification of Ba displacements

2\sqrt 2 a \times \sqrt 2 a \times c

Crystallographic and magnetic structures of highT c related (Bi/Pb)2Sr2FeO6.25 determined by neutron powder diffraction

, witha=3.8652(3) Å,c=11.815(2)Å 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 Å in the Cu(1) plane and 0.06 Å in the Cu(2) plane. Only 12% of the oxygen atoms in the basal plane contribute to the superstructure.

X-ray anomalous scattering on the superconducting ortho-II phase of YBa

Abstract From an extended set of 346 observed superstructure reflections (227 symmetry independent) collected at a synchrotron radiation source, we were able to determine small antiferro-distortive displacements in the supercell 2 a . b . c of the ortho-II phase of YBa 2 Cu 3 O 6.50 . Barium is displaced by 0.041(2) A and yttrium by 0.011(1) A along the a -axis. The apical oxygen atoms are shifted by ±0.026(4) A and the copper atoms in the superconducting plane by ±0.016(1) A along the c -axis relative to averaged positions. The displacements along the a -axis are explained by an electrostatic interaction model.

_2

Abstract In YBa 2 Cu 3 O 6.5 with ortho-II structure the oxygen atoms in the basal plane order to alternating CuOCu and CuCu chains. Due to the oxygen order the Ba atoms are antiferro-distortively displaced towards the CuOCu chains. These displacements are confirmed by anomalous scattering near the Ba K-absorption edge observed by synchrotron X-ray diffraction on a YBa 2 Cu 3 O 6.51 single crystal. Changes in the structure factors of ( h /200) ortho-II superstructure reflections are due to Ba displacements. Possible contributions of displacements of other atoms were found to be 5(13)%. A rigid ion model shows that the Ba displacements stabilize the ortho-II structure by reducing its lattice energy. The Ba-shift calculated by the model, δ model Ba =0.039 A , is in close quantitative agreement with the experimental observation, δ exp Ba =0.034(3) A .

Cu

Abstract In a single crystal of YBa 2 Cu 3 O 6.35 we determined an oxygen superstructure with a 2√2 a X2√2 a cell by neutron diffraction. A 2√2 a X√2 a superstructure of a monodomain has been found by X-ray diffraction. In YBa 2 Cu 3 O 6.4 of T c =38K, we observed a short range ordered ortho-II structure by neutron single crystal diffraction.

_3

The structure of (Bi/Pb)2Sr2FeO6.25 was refined from high resolution neutron powder diffraction data taken at 300 K and 5 K. The average structure is well described in the orthorhombic spacegroup Amaa witha=5.4254 Å,b=5.4909 Å andc=23.2245 Å. The FeO6 octahedra are tilted by 2.7° towards (0 1 0). The oxygen positions in the (Bi/Pb)O-layers are consistent with the occurrence of (Bi/Pb)−O−(Bi/Pb) chains. Excess oxygen was located between these chains. Antiferromagnetic 3-dim ordering of the Fe-moments withM=(2, 1, 2) μB was observed at 5 K with the wavevector pointing along (1 0 0).