Experimental Determination of SmBa2Cu3O7-delta Stability Diagram at Low Oxygen Pressures

We experimentally determined the stability diagram of SmBa2Cu3O7-delta (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 degrees 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.