Reaction pathways in the formation of the Tl–Ca–Ba–Cu superconducting phases

Abstract

The 2223, 1223, 1324, and 1122 phases in the Tl–Ca–Ba–Cu system were prepared from sample mixtures having the same stoichiometry as the final phase. Powder x-ray diffraction analysis was used to follow the formation of these phases at 867 °C for up to 2 h. The formation of the 2223 phase is shown to occur via the sequential transformations 2021 → 2122 → 2223. For the 1–2–2–3 and 1–3–2–4 samples, the sequential transformations 1021 → 2021 → 2122 → 2223 occur before the final 1223 and 1324 phases are obtained. For the 1122 phase the formation is preceded by the 1021 → 2021 → 2122 transformations. Using scanning electron microscopy, a phase with a glassy appearance was detected in many of the reacted samples. This “glassy” phase appears to be a precursor in the formation of the Tl–Ca–Ba–Cu phases.