SUPERCONDUCTIVITY IN THE TETRAGONAL PHASE OF LA1.9BI0.1CUO4+DELTA ANNEALED UNDER HIGH OXYGEN-PRESSURE

We have investigated the relation between the crystal structure and superconductivity in La1.9Bi0.1CuO4+delta, in which the phase separation observed in La2CuO4+delta is suppressed. A phase diagram in the T-delta plane is given for La1.9Bi0.1CuO4+delta with excess oxygen. For very small delta values, the crystal structure is orthorhombic, and an orthorhombic tetragonal phase transition occurs markedly at delta approximately 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04 less-than-or-similar-to delta less-than-or-similar-to 0.11. This is clear evidence that high-T(c) superconductivity also appears in the tetragonal phase.