Nanoscale Atomic Distortions in the BiS2 Superconductors: Ferrodistortive Sulfur Modes

The discovery of phonon-mediated superconductivity in the BiS 2 class led to a renewed interest in compounds that exhibit a strong structure-property relationship due to its intrinsic disorder. The ReO1-xFxBiS2 (Re = La, Nd, and Pr) systems were investigated using neutron and synchrotron X-ray diffraction to reveal the nature of electron-phonon coupling that leads to unconventional states with intricate microstructures and physical properties. In ReO1-xFxBiS2 the distortions are manifested in the form of in-plane sulfur distortions which split the in-plane Bi-S bonds while the splitting gives rise to different bond lengths around Bi atoms. The proposed distortion modes create an unequal charge distribution around the Bi atom giving rise to charge fluctuations. The superconducting phases of ReO1-xFxBiS2 also manifest z-motion of apical sulfur which could act as a charge transfer conduit between the electron doping layers and superconducting layers.