The relationship between the crystal structure and superconductivity has been investigated for La1.9Bi0.1CuO4+δ with excess oxygen. A phase diagram in the T-δ plane is given for this system, where the phase separation observed in La2CuO4+δ is suppressed. For very small values of δ the crystal structure is orthorhombic, and an orthorhombic-tetragonal phase transition occurs markedly at δ∼0.03 in the measured temperature range between 13 and 293 K. A superconducting phase is observed in the range of 0.04≤δ≤0.11. This is clear evidence that high-Tc superconductivity also appears in the tetragonal phase.
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