In the beginning, we explored new superconductors near the BiOCuS system because a superconductivity signal was detected for Cu-deficient BiOCu
1−xS [
6]. Although Reference [
6] reported superconductivity, the shielding volume fraction of the sample was not high, indicating that the superconductivity was not bulk in nature. In addition, the other research group also examined the BiOCu
1−xS and its Pb-doped samples [
7]; superconductivity, however, was not discovered in those samples. Therefore, we modified the composition of Bi-O-Bu-S and finally synthesized Cu-free Bi
2O
2S, which is a layered compound, according to the material database [
8]. Surprisingly, the Bi
2O
2S sample prepared at 500 °C showed a superconducting signal at
T of ~5 K in the temperature dependence of magnetization. The X-ray diffraction (XRD) pattern of the sample was not what was expected from the Bi
2O
2S structure and indicated that the sample was not a single phase. This suggests the presence of a Bi–O–S superconducting phase stable at around 500 °C. Then, we explored the optimal composition of the unknown Bi–O–S superconducting phase. After synthesizing and evaluating over 100 samples of Bi
xO
yS
z (with various synthesis temperatures), we obtained the optimal composition of Bi
4O
4S
3, in which the superconducting volume fraction is the highest, and the XRD pattern seemed to be a single phase.
Although we obtained the single-phase sample of Bi
4O
4S
3, the crystal structure was unknown, and no similar XRD pattern could be found in the database [
8]. The XRD pattern was analyzed by Fujihisa and Gotoh of AIST (National Institute of Advanced Industry Science and Technology), who are specialists in crystal structure determination. From the XRD pattern, the framework of Bi and S was determined. Since the XRD pattern was collected with a powder sample, the determination of the O site was difficult. Therefore, we determined the final structure model with the combination of calculation and Rietveld refinement. Finally, we concluded that Bi
4O
4S
3 had a layered structure as depicted in
Figure 1e. In addition, we analyzed the band structure with the support of Kuroki’s group (Osaka University). The crystal structure of Bi
4O
4S
3 is composed of alternate stacking of the BiS
2 bilayer, which is electrically conducting, and the Bi
4O
4(SO
4)
x blocking layer. The parent phase (
x = 0) was expected to be a band insulator, but the present phase, Bi
4O
4S
3, corresponded to
x = 0.5. Namely, there are 50% SO
4 defects in the blocking layer, and, hence, electrons are doped to the BiS
2 layers, resulting in metallic conductivity and superconductivity [
9]. Interestingly, the stable phase (composition) contains defects at the SO
4 site, and the control of the doping level is very difficult in this Bi
4O
4SO
4Bi
2S
4 structure. Motivated by this difficulty, we tried to synthesize other BiS
2-based superconductors with a simpler structure.