Phase equilibria and superconducting properties of BiSr2YCu2O7 (1212 phase)

Phase equilibria of the quasi-quaternary system BiO1.5-SrO-YO1.5-CuO have b een studied at a temperature of 950 degrees C in air, with special regard t o the 1212 phase. The 1212 phase reveals only very small changes in the cat ion ratio. Single-phase samples exist for (Bi0.24-0.36Cu0.42-0.55)-Sr2Y1.27 Cu2Oy compositions. The bismuth-rich composition of the 1212 pi;ase is in t hermodynamic equilibrium with a liquid and the 2212 phase, whereas the copp er-rich composition is in equilibrium with five other phases. The influence of combined calcium and lead doping also has been studied. Exceeding the c alcium saturation of the 1212 phase increases the amount of 2212 as a secon dary phase. Single-phase 1212 samples do not show any superconductivity in either the as-prepared or the post-annealed state. The only compositions wi th bulk superconductivity are those with calcium and lead doping after anne aling at a temperature of 980 degrees C. The superconductivity is attribute d to the 2212 phase crystallizing from the melt during slow cooling.