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

We experimentally determined the stability diagram of SmBa₂Cu₃O_7−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.