Lattice Dynamics Study of Thermoelectric Oxychalcogenide BiCuChO (Ch = Se, S)

The BiCuSeO-based compounds with very low thermal conductivity are among the most promising thermoelectric materials recently discovered. Their lattice dynamics, which remains mostly unexplored experimentally, is investigated in this paper. We report Raman experiments on BiCuSe 1– x S x O solid solutions and infrared experiments on BiCuSeO and BiCuSO alloys coupled to first-principles-based calculations. We have observed that the high-energy A 1g Raman-active mode strongly depends on the chalcogen content. We also report the density functional theory calculations of the dielectric and elastic constants, the phonons in the whole Brillouin zone, and the thermodynamic properties. We have determined the thermal expansion of BiCuSeO at room temperature from neutron diffraction experiments and evaluated its thermodynamic Grüneisen parameter and found a quite large value compared to other thermoelectric materials, which confirms the large anharmonicity of BiCuSeO.