Neutron study of the crystal structure and vacancy distribution in the superconductor ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$

Two samples of the high-temperature superconductor ${\mathrm{Ba}}_2$ ${\mathrm{YCu}}_3$ ${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$ with δ=2.0 and 2.2 have been studied at room temperature and at 10 K, with the neutron powder diffraction 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 diffraction studies. Of the two sets of copper atoms, one is surrounded by four oxygen atoms at distances 1.9299(4) and 1.9607(4) Å. This arrangement forms a two-dimensional framework of Cu-O atoms that extends indefinitely along the a- and b-axis. A fifth oxygen atom is located at 2.295(3) Å 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) Å. In this case, however, the oxygen atoms form rectangles connected by vertices, and resulting in chains along the b axis. In the compound with δ=2.0 all oxygen sites are fully occupied. When δ=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 and low temperature.