Neutron study of the crystal structure and vacancy distribution in the superconductorBa2YCu3

Abstract

Two samples of the high-temperature superconductor Ba2YCu309with 8 2.0 and 2.2 have been studied at room temperature and at 10 K, with the neutron powder diA'raction method and profile analysis. The structure of the compound is orthorhombic. The oxygen atoms are located on four sets of sites and not on five sets as reported earlier in x-ray-difraction studies. Of the two sets of copper atoms, one is surrounded by four oxygen atoms at distances 1.9299(4) and 1.9607(4) A. This arrangement forms a two-dimensional framework of Cu-0 atoms that extends indefinitely along the a and b axis. A fifth oxygen atom is located at 2.295(3) A from the copper atom, giving a pyramidal configuration.In the other set, the Cu atoms are also surrounded by four oxygen atoms, with distances 1.9429(1) and 1.846(2) A. In this case, however, the oxygen atoms form rectangles connected by vertices, and resulting in chains along the b axis. In the compound with b 2.0 all oxygen sites are fully occupied. %'hen b 2.2 there are oxygen vacancies, but these are confined to one set of positions only, specifically to the oxygen atoms of the chains, located on the b axis. No detectable change of the structure has been observed between room temperature and low temperature.The discovery of superconductors with values of the critical temperature of the order of 90-95 K in the Y-Ba-Cu-0 system has stimulated intense research work aimed at the identification and characterization of the materials with high values of T,.It has been found in a number of investigations' that the superconducting phase has the composition Ba2YCu309-s (with 8= 2) and that the structure of the compound can be described as a distorted oxygendeficient perovskite. ' Structural investigations of Ba2YCu309q have been carried out using singlecrystal and pounder x-ray-diffraction techniques. The structure of the compound has been refined in space groups Pmmm, P4/mmm, and P4m2 (Ref. 4) with no significant diA'erences in the assignment of the metal atoms. The crystallographic features of the oxygen atoms, however, are quite diA'erent in the three space groups. Adopting the nomenclature of Ref. 3 (which has been followed throughout this paper), we have a situation where atoms O(2) and O(3) are crystallographically equivalent in space group P4/mmrn but not in Pmmm and P4m2, while atoms O(4) and O(5) occupy equivalent positions in space groups P4/mmm and P4m2, but not in Pmmm, where they split into two inequivalent sets. It was reported in Refs. 2 and 3 that only half of the sites of atoms O(4) and O(5) are occupied, with the oxygen vacancies randomly distributed in the plane of the Cu(1) atoms. The x-ray powder-diffraction intensities of Beyers et al. were best fitted by a model having oxygen vacancies ordered on the barium planes at z =0.18, while the high-resolution transmission electron microscopy suggests that these vacancies are present on the copper planes between the barium planes.The discrepancies between the above x-ray studies may reAect true differences in the samples prepared un...

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