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

Abstract

We experimentally determined the stability diagram of SmBa 2 Cu 3 O 7−δ (SmBCO) under a low oxygen pressure (PO 2 ) 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 PO 2 region and then quenched. Under a PO 2 of 20−100 mTorr, the stability line of SmBCO on the log PO 2 versus 1/T (K −1 ) diagram is expressed as log (PO 2 (Torr)) = 19.22− 24292/T (K), with a reaction of SmBCO ↔ Sm 2 BaO 4 + liquid (L 1 ). Under the PO 2 regime of 1−10 mTorr, the stability line of SmBCO is expressed as log (PO 2 (Torr)) = 17.73−22582/T (K), with a reaction of SmBCO ↔ Sm 2 BaO 4 + Sm 2 O 3 + L 2 . At the phase boundary between SmBCO ↔ Sm 2 BaO 4 + L 1 and SmBCO ↔ Sm 2 BaO 4 + Sm 2 O 3 + L 2 , the reaction of L 1 ↔ L 2 + Sm 2 O 3 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.