Ferrodistortive sulfur modes in the BiS₂ superconductors

The discovery of phonon-mediated superconductivity in the BiS₂ family has renewed interest in intrinsically disordered compounds with strong structure–property coupling. We investigated ReO_1-xF_xBiS₂ (Re = La, Nd, Pr) using neutron and synchrotron X-ray diffraction to probe the electron–phonon interactions. In these materials, in-plane sulfur distortions split the Bi–S bonds, yielding inequivalent Bi–S distances and an uneven local charge distribution around Bi that drives charge fluctuations. In the superconducting compositions, an additional out-of-plane (z-axis) motion of the apical sulfur emerges, which likely functions as a charge-transfer conduit between the electron-doped blocking layers and the superconducting BiS₂ planes.