Relation between crystal structure and superconductivity in La2-xBixCuO4+δ with excess oxygen

The relation between the crystal structure and superconductivity has been investigated for La 2-x Bi x CuO 4+δ with excess oxygen, in which the phase separation observed in La 2 CuO 4+δ is suppressed. In the range of 0.03≲δ≲0.1 and T≲100 K, the crystal structure is orthorhombic for x = 0.05, while it is tetragonal for x =0.10. With increasing δ, T c increases and then reaches the maximum value, 20–25 K, at δ ≳ 0.05 for 0.05⩽x⩽0.10, independent of the crystal structure. The overall symmetry of the crystal structure is concluded not to be so responsible for the appearance of high-T c superconductivity.