Ferro-lattice-distortions in Bismuth Sulfide Superconductors

ReO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2}}$ (Re $=$ La, Nd and Pr) is an electron-phonon coupled superconductor with the maximum transition temperature of 10.8 K in LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2}}$ at x $=$ 0.5. The parent phases, ReOBiS$_{\mathrm{2}}$ (Re $=$ La, Nd and Pr) compounds are either bad metals or insulators. The crystal structure of ReO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{\mathrm{2}}$ is investigated using single crystal neutron and synchrotron X-ray diffraction experiments. In all compositions, a superlattice Bragg pattern was observed on hk0 plane. The Bragg patterns challenge the long-presumed nominal symmetry of BiS$_{\mathrm{2}}$ superconductors P4/nmm, and other theoretically suggested symmetries. The Bragg structure can be reproduced by a model involving coherent in-plane displacements of the sulfur in superconducting BiS$_{\mathrm{2}}$ planes. The sulfur displacements produce different Bi-S bond lengths in-plane giving rise to charge fluctuations. The lattice distortions that arise from unstable phonon modes can trap the charge carriers decreasing the number of pairing electrons.