Structural and thermal properties of ultralow thermal conductivity Ba₃Cu₂Sn₃Se₁₀

The thermal properties of Ba₃Cu₂Sn₃Se₁₀ were investigated by measurement of the thermal conductivity and heat capacity. The chemical bonding in this diamagnetic material was investigated using structural data from Rietveld refinement and calculated electron localization. This quaternary chalcogenide is monoclinic (P2₁/c), has a large unit cell with 72 atoms in the primitive cell, and a high local coordination environment. The Debye temperature (162 K) and average speed of sound (1666 m s^-1) are relatively low with a very small electronic contribution to the heat capacity. Ultralow thermal conductivity (0.46 W m^-1 K^-1 at room temperature) is attributed to the relatively weak chemical bonding and intrinsic anharmonicity, in addition to a large unit cell. This work is part of the continuing effort to explore quaternary chalcogenides with intrinsically low thermal conductivity and identify the features that result in a low thermal conductivity.