Crystal phases of bisrcacu₂o_x heat treated in a vacuum-sealed system

Heat treatment in a vacuum-sealed system on BiSrCaCu₂O_x was studied in comparison with that in air. The melting temperature was lowered from 880°C in air to 850°C in the vacuum-sealed system. The formation of the phase with 37 Å c spacing was enhanced in the vacuum-sealed system by heating above the melting temperature and the infsequent annealing in air improved the sharpness of the superconducting transition at 110 K. The heat treatment in the reduced pressure also caused the reduction of the Cu ion and it was suggested that the oxidation state of the Cu ion played an important role in the formation of the phase with 37 Å c spacing.