Crystal structure and superconductivity in La1.95Bi0.05CuO4+delta oxidized by a KMnO4 aqueous solution

We have obtained La1.95Bi0.05CuO4+delta with various values of delta by the KMnO4 oxidation method. The largest value of delta approximate to 0.136 is about twice as large as that obtained by annealing under high pressures of oxygen, delta approximate to 0.056. A structural phase transition from the orthorhombic to the tetragonal phase with the increase of delta is observed at delta = 0.035-0.04. For delta greater than or similar to 0.08, another orthorhombic phase with large orthorhombic distortion appears and forms a mixture with the tetragonal phase. Superconductivity appears in the range of delta greater than or similar to 0.03. In the mixture range of delta greater than or similar to 0.08, however, the orthorhombic phase may exhibit no superconductivity due to the overdoping of hole carriers into the CuO2 plane. The maximum T-c increases up to similar to 35 K, which is about 10 K higher than that obtained by annealing under high pressures of oxygen, owing to the larger hole concentration in the CuO2 plane.