Crystal structure and superconductivity of iodine-intercalated Bi₂Sr₂CaCu₂O₈I_x (0≦x≦1). Dependence on the iodine concentration

Variations of the crystal structure and superconductivity with iodine concentration have been investigated for iodine-intercalated sintered-samples of Bi₂Sr₂CaCu₂O₈I_x with 0≦x≦1. It has been found from powder X-ray diffraction that samples with 0.4≲x≲0.7 are composed of mixed phases of the host compound Bi₂Sr₂CaCu₂O₈ and the stage-1 iodine-intercalation compound Bi₂Sr₂CaCu₂O₈I. Diffraction peaks due to the stage-3 structure are vaguely observed for a sample with x=0.24. In spite of the mixed-phase structure for 0.4≲x≲0.7, the superconducting phase is single and of the bulk for all x values. The superconducting transition temperature decreases with increasing x for 0≦x≲0.4 and is almost constant for 0.4≲x≦1. A model of the x dependence of the spatial distribution of intercalated iodine atoms is proposed, and the x dependence of T_c is discussed in relation to that of the charge transfer between iodine atoms and the CuO₂ sheets.