Reports of superconductivity in doped LnOBiS2 (Ln = La, Ce, Pr, Nd, and Yb), doped SrOBiS2, and Bi4O4S3 have generated interest in a new family of layered superconductors where the electronically active layers are comprised of BiS2 units. Interestingly, the preliminary refinements to the laboratory X-ray diffraction data for the reported Bi4O4S3 structure yielded a model which contains both sulfides (S2-) and sulfates (S6+). Such disparate, disproportionated, oxidation states are not usually chemically stable, especially in solid state reactions performed at elevated temperatures. Furthermore, the small X-ray scattering factors of sulfur and oxygen relative to that of bismuth make the precise structural assignment of these light elements difficult to determine. We have used a wide variety of average and local structure techniques to gain insight into the actual chemistry of the Bi-O-S superconductor. We find that the compound contains mixed sulfide/disulfide units, not sulfide and sulfate, and has the actual chemical formula Bi3O2S3. Furthermore, we find that all samples contain random intergrowths of two layered phases, Bi3O2S3 and Bi2OS2 (BiOBiS2), with the latter having a detrimental effect on the superconductivity. Additionally, our results imply that the superconductivity in this system arises from incomplete charge transfer between S22- dimer layers and BiS2 layers in Bi3O2S3, in agreement with recent Density Functional Theory calculations motivated by our findings.

Influence of average and local crystal structures on superconductivity in the Bi-O-S system

247th National Meeting (2014)

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