Effect of rattling motion without cage structure on lattice thermal conductivity in LaOBiS_(2-x)Se_x

摘要

Low energy phonons in LaOBiS_(2-x)Se_x are studied using inelastic neutron scattering. Dispersionless flat phonon branches that are mainly associated with a large vibration of Bi atoms are observed at a relatively low energy of E = 6-6.7 meV. The phonon energy softens upon Se doping presumably owing to its heavier atomic mass than the S atom and the expansion of the lattice constant. Simultaneously, the lattice thermal conductivity lowered upon Se doping as the same manner of the phonon softening. These suggest that despite the lack of an oversized cage in LaOBiS_(2-x)Se_x, rattling motions of Bi atoms can scatter phonon like rattling in cage compounds, contributing to enhance the thermoelectric property.