Abstract
We have measured x-ray diffraction (XRD), high-resolution electron microscopy (HREM), resistivity and susceptibility for a series of differently prepared YBa2Cu3O7-δ samples. The high-temperature superconductivity was studied as a function of variable quench temperature. Quenching above 820 °C leaves the sample in the tetragonal phase and destroys the superconductivity. We suggest that the supercondutivity is located in the CuO chains which are extremely sensitive to oxygen loss.