[en] We have investigated the relation between the crystal structure and superconductivity in La_1.9Bi_0.1CuO_4+δ, in which the phase separation observed in La₂CuO_4+δ is suppressed. A phase diagram in the T δ plane is given for La_1.9Bi_0.1CuO_4+δ with excess oxygen. For very small δ values, the crystal structure is orthorhombic, and an orthorhombic tetragonal phase transition occurs markedly at δ approximately 0.03 in the measured temperature range between 13 and 293 K. Superconductivity is observed in the range of 0.04 approx-lt δ approx-lt 0.11. This is clear evidence that high-Tc superconductivity also appears in the tetragonal phase. 5 refs., 2 figs