Orthorhombic-to-Tetragonal Phase Transition in the Y-Ba-(Cu, Co)-O System

X-ray powder diffraction, thermal analysis and resistivity measurements have been carried out for the YBa2(Cu1-x , Co x )3O7-δ system with 0<xT G) curve steadily decreases and the structure transforms from orthorhombic to tetragonal. Around this phase transition at x=0.02, T c starts to decrease. This phenomenon is considered to be ascribed to the disordering of oxygen sandwiched between Ba planes.