Phase Equilibria and Superconducting Properties of BiSr₂YCu₂O₇ (1212 Phase)

Abstract

Phase equilibria of the quasi‐quaternary system BiO1.5–SrO‐YO1.5–CuO have been studied at a temperature of 950°C in air, with special regard to the 1212 phase. The 1212 phase reveals only very small changes in the cation ratio. Single‐phase samples exist for (Bi0.24–0.36Cu0.42‐0.55)–Sr2Y1.27Cu2Oy compositions. The bismuth‐rich composition of the 1212 phase is in thermodynamic equilibrium with a liquid and the 2212 phase, whereas the copper‐rich composition is in equilibrium with five other phases. The influence of combined calcium and lead doping also has been studied. Exceeding the calcium saturation of the 1212 phase increases the amount of 2212 as a secondary phase. Single‐phase 1212 samples do not show any superconductivity in either the as‐prepared or the post‐annealed state. The only compositions with bulk superconductivity are those with calcium and lead doping after annealing at a temperature of 980°C. The superconductivity is attributed to the 2212 phase crystallizing from the melt during slow cooling.

Powder synthesis of (Bi, Pb)-1212 superconductors

“…As a grain boundary phase the 2212 would be able to build a percolating network although the volume content is only a few per cent or even less. Kaeschel et al [12] also claim that 2212 impurities are responsible for measured superconducting signals. Like many other authors [15][16][17][18][19][20] they measured Ca-free Bi-1212 compounds, which never showed superconductivity at all.…”

“…Results for the (Bi, M)-1212 have been reported for several dopant cations such as Cu [6], Pb [7], Cd [8], In [9], Mo [10] and Zn [11]. The critical temperature T c is in the range from non-superconducting [12] up to 110 K for a Pb-doped Bi-1212 [13]. Three important results can be extracted from a literature review: (1) no single crystals of (Bi, M)-1212 have been synthesized yet and also in highly phase pure (Bi, M)-1212 samples there was a percentage of superconducting secondary phases.…”