THE IMPORTANT ROLE OF PYRAMIDAL COPPER LAYERS OF THE 123-STRUCTURE IN SUPERCONDUCTIVITY - THE OXIDES BA2Y1-XCAXCU3-XFEXO7 AND BA2Y1-XCAXCU3-XFEXO6

Three series of oxides Ba2Y1-xCaxCu3-xFexO7, tetragonal and orthorhombic, and Ba2Y1-xCaxCu3-xFexO6 tetragonal, have been synthesized by successive appropriate thermal treatments under oxygen and argon, for x = 0.09, 0.12, 0.15, 0.24. The structure of these different forms has been determined by neutron powder diffraction, allowing for the "O7" and "O6" composition to be confirmed and the oxygen and cationic distribution to be determined. One of the important points of this structural study deals with the random distribution of iron on both Cu(1) and Cu(2) sites of the "123" structure of the "O7" tetragonal form whereas they are located on the pyramidal Cu(2) sites in the tetragonal "O6" and the orthorhombic "O7" form. The systematic study of the superconducting properties of the "O7" oxides shows that for a constant x value i.e. for a constant Cu (III)and a constant Fe(III) content the orthorhombic phases exhibit a critical temperature and a superconducting volume much smaller than those of the tetragonal phases. These results demonstrate the predominant role of pyramidal copper layers in the superconducting properties of the "123"-type oxides.