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

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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.