Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2-xSex.

We examined the crystal structure of the new thermoelectric material LaOBiS_2-xSe_x, whose thermoelectric performance is enhanced by Se substitution, by using powder synchrotron X-ray diffraction and Rietveld refinement. The emergence of metallic conductivity and enhancement of the thermoelectric power factor of LaOBiS_2-xSe_x can be explained with the higher in-plane chemical pressure caused by the increase of Se concentration at the in-plane Ch1 site (Ch = S, Se). High-temperature X-ray diffraction measurements for optimally substituted LaOBiSSe revealed anomalously large atomic displacement parameters (U_iso) for Bi and Ch atoms in the BiCh₂ conduction layers. The anisotropic analysis of the atomic displacement parameters (U₁₁ and U₃₃) for the in-plane Bi and Ch1 sites suggested that Bi atoms exhibit large atomic displacement along the c-axis direction above 300 K, which could be the origin of the low thermal conductivity in LaOBiSSe. The large Bi vibration along the c-axis direction could be related to in-plane rattling, which is a new strategy for attaining low thermal conductivity and phonon-glass-electron-crystal states. [ABSTRACT FROM AUTHOR]