Role of bond lengths in the 90-K superconductor: A neutron powder-diffraction study of YBa2Cu3-xCoxO7-y

We present the results of a neutron powder-diffraction study on YBa2Cu3-xCoxO7-y. We find that Co preferentially occupies the Cu(1) site, simultaneously attracting extra oxygen into these layers in an amount consistent with Co(III) replacing Cu(II). This compound displays a transition from orthorhombic-to-tetragonal structure for x between 0.05 and 0.1, while Tc decreases continuously and the compound becomes nonsuperconducting for x=0.4. We note several important relationships between bond-length changes, which reflect charge transfer among the ions. Also, we find that Tc changes systematically with the Cu-O bond length.