H.W. Zandbergen

Models for the modulation in A2B2CanCu1+nO6+2n, A, B=Bi, Sr OR Tl, Ba and n=0, 1, 2

Abstract A number of models for the modulation in the title compounds are postulated. For the Bi compounds periodic changes are considered due to the following structural models: (a) extra oxygen in the BiO layers, (b) partial substitution of Sr by a vacancy, (c) partial substitution of Bi by Cu, and (d) changes in the orientations of the lone pairs of Bi. Computer calculated images for these models are compared with high resolution images. Also general structural considerations and structural details from the reported subcells are taken into account. It is concluded that the best agreement with experimental data is obtained for the model with extra oxygen in the BiO layers. Several performed experiments, which support this conclusion, are reported and discussed; the most important of which are the existence of solid solutions Bi2Sr2−xBaxCuO6+δ, with x

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Abstract Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba 2 YCu 3 O 7− δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.

The vacancy order — disorder transition in Ba2YCu3O7−δ observed by means of electron diffraction and electron microscopy

Abstract It is shown by means of electron microscopy and electron diffraction that the “structural” vacancies in Ba 2 YCu 3 O 7−δ undergo an order-disorder transformation accompanied by a change in symmetry from orthorhombic to tetragonal. A superstructure due to the ordering of vacancies was found in certain crystal parts; it leads to doubling of the a 0 parameter. It is shown that the ordering of the vacancies is important for the superconducting behaviour. In order to obtain a high T c supraconductor the final heat treatment is crucial.

Superconductivity in (Pb, Bi)2Sr2−xLaxCu2O6+δ

Abstract Pb 2 Sr 2− x La x Cu 2 O 6+δ has a structure, which can be derived from that of Pb 2 Sr 2 Ca 1− x Y x Cu 3 O 8+δ , by removing a CuO 2 and (Ca, Y) layer. The until cell parameters for the composition with x = 1.0 are a = 0.5333 (2), b = 0.5421 (2), c = 1.2609 (6) nm with space group P22 1 2. The onset of the superconducting transition temperature is 32 K with zero resistance at 26 K for x = 1.2, whereas for other compositions a broader transition is observed. Partial replacement of Pb by Bi leads to a decrease in T c . Heating in air at 500°C leads to a transformation to tetragonal symmetry, which can be reversed by heating in nitrogen at 500°C.

High-temperature superconductivity in LaBaCaCu3O6.85

Abstract A new superconducting compound with composition LaBaCaCu3O6.85 has been prepared. It crystallizes in a pseudo-tetragonal structure related to YBa2Cu3O7. The onset superconducting temperature is 78 K. Electron diffraction clearly shows the presence of a superstructure in this material. Conductivity and superconductivity data are presented and possible structures of the unit cell are discussed.

Heavy atom disorder in the high Tc superconductor Ba2YCu3O7−δ studied by means of electron microscopy and electron diffraction

Abstract It is shown that on rapid cooling of the compound Ba 2 YCu 3 O 7-δ one obtains a highly disordered material in which the perovskite framework is conserved but in which barium and yttrium atoms exhibit a high degree of disorder. This leads to lattice deformations, which can be visualized in the electron microscope. The electron diffraction patterns also reveal the disorder. As-quenched specimens are no longer high T c superconductors, but the superconducting properties can be recovered by an adequate heat treatment whereby the vacancies become ordered.

Structural and transport properties of the triple-layer compounds Ba4(Pb1−xBix)3O10 (0≤ x<0.3)

A series of new compounds Ba4(Pb1−xBix)3O10 with 0≤x<0.3 has been prepared and characterized by X-ray and electron diffraction. The d.c. resistivity vs. temperature data for different x values are presented and structural features are compared to those of the superconducting phase Ba(Pb1−xBix)O3. The “mother-compound” Ba4Pb3O10 is tetragonal, with lattice constants a=0.4280(1) and c=3.017(1) nm. Its structure can be regarded as a stacking of Ba(PbBi)O3 triple-layers, separated by single BaO layers. Electron diffraction reveals the presence of a weak superstructure with the a-axis related to that of the perovskite by a=√2ap. Frequent intergrowth of this phase with the perovskite structure is observed. The conductivity of the samples Ba4Pb3O10 is nearly independent of temperature, while Bi-doped samples exhibit semiconductor-like behaviour at low temperature. No superconducting transition is observed down to 2K.

Preparation and crystal structure of Pb2Ba2YCu3O8+δ

Abstract The structure of Pb2Ba2YCu3O8+δ has been determined by electron diffraction and neutron powder diffraction. This compound adopts the Pb2Sr2YCu3O8+δ structure, having orthorhombic symmetry, space group P2212, with unit cell parameters a=0.54654(3) b=0.54973(3) and c=1.61729(9) nm. The orthorhombic deviation from the tetragonal symmetry is caused by a shift of the oxygen atoms in the PbO layer, leading to a distorted PbO5 pyramid with three short and two large Pb-O distances. Pb2Ba2YCu3O8+δ shows a semiconductor-like behavior.

A study of Bi2Sr2−xLaxCuO6+δ with X-ray powder and electron diffraction

Abstract In the superconducting compound Bi2Sr2CuO6+δSr is partly substituted by La. Up to 65% of Sr2+ can be replaced by La3+. The effect of this substitution on the structure is studied by means of X-ray powder diffraction, and electron diffraction in combination with element analysis. It is concluded that in order to maintain charge balance extra oxygen is incorporated in the structure.

High resolution electron microscopy and electron diffraction on LaCaBaCu3O6.85

Abstract High resolution electron microscopy has been carried out on LaCaBaCu3O6.85. This compound has a structure very similar to that of YBa2Cu3O7, except that this compound is tetragonal according to X-ray diffraction. Electron diffraction on materials annealed in oxygen shows the presence of a weak superstructure having a doubled a and c axis with a B centering, and which is orthorhombic, whereas in air annealed samples diffuse streaks are observed along a ∗ and b ∗ , corresponding to a doubled c axis. HREM suggests that the Y site is for approximately 75% occupied by La, when it is assumed that all Ba is located at the Ba positions. It is argued that the absence of an orthorhombic structure similar to that of YBa2Cu3O7 is caused by twinning on a very fine scale due to twin boundary pinning of small regions which exhibit an ordering of La, Ca and Ba, or to La on the Ba site requiring a different surrounding viz. extra oxygen in the CuO layer at z=0.