Crystal structure, lattice vibrations, and superconductivity of LaO _1-xF_xBiS₂

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO_0.5F _0.5BiS₂ and its nonsuperconducting parent compound LaOBiS₂. The crystal structures and vibrational modes have been examined. Bragg peaks from the superconducting sample exhibit pronounced broadening compared to those from the nonsuperconducting sample. In the inelastic measurements, a large difference in the high-energy phonon modes was observed upon F doping. Alternatively, the low-energy modes remain almost unchanged between nonsuperconducting and superconducting states either by F doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.