Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting LaO1−xFxBiS2 is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing x that may lead to a charge transfer conduit between the doping layers and the superconducting BiS2 planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically, that induces spin polarization.

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The local atomic structure of superconducting NdO 1−x F x BiS 2 (x =0.2 and 0.4) is investigated using neutron diffraction and the pair density function analysis technique. In the nonsuperconducting x=0.2 composition, ferrodistortive displacements of the pyramidal sulfur ions break the tetragonal symmetry and a superlattice structure emerges with peaks appearing at h + k odd reflections superimposed on the even reflections of the P 4/nmm symmetry. In the superconducting x=0.4 composition, similar ferrodistortive displacements are observed but with different magnitudes coupled with in-plane Bi distortions. The distortions are indicative of charge fluctuations that may lead to charge localization and suppression of superconductivity.