Experimental Determination of SmBa₂Cu₃O_7−δ Stability Diagram at Low Oxygen Pressures

We experimentally determined the stability diagram of SmBa2Cu3O7−δ (SmBCO) under a low oxygen pressure (PO2) regime of 1–100 mTorr. Sm–Ba–Cu–O amorphous films were used as precursors instead of SmBCO powder in order to avoid oxygen release problem during decomposition of SmBCO. The precursor films were annealed at 780–932 °C at a given PO2 region and then quenched. Under a PO2 of 20–100 mTorr, the stability line of SmBCO on the log PO2 versus 1/T (K–1) diagram is expressed as log (PO2 (Torr)) = 19.22–24292/T (K), with a reaction of SmBCO ↔ Sm2BaO4 + liquid (L1). Under the PO2 regime of 1–10 mTorr, the stability line of SmBCO is expressed as log (PO2 (Torr)) = 17.73–22582/T (K), with a reaction of SmBCO ↔ Sm2BaO4 + Sm2O3 + L2. At the phase boundary between SmBCO ↔ Sm2BaO4 + L1 and SmBCO ↔ Sm2BaO4 + Sm2O3 + L2, the reaction of L1 ↔ L2 + Sm2O3 occurs. Based on the above relationship, the stability diagram of SmBCO, including the solid–liquid phase equilibria above its stability line, is accurately constructed, which is crucial for understanding the fundamental mechanism involved in the fabrication of SmBCO-based high-temperature superconducting coated conductors.