Ultralow Thermal Conductivity in Cs–Sb–Se Compounds: Lattice Instability versus Lone-Pair Electrons

The presence of lone-pair electrons (LPE) is known to lead to large anharmonicities in materials and thus to low lattice thermal conductivity (κ). Materials with LPE typically have κ values that are lower than the κ values of those with similar compositions but without LPE. In this study, we investigate thermal transport properties in Cs–Sb–Se ternary compounds by first-principles methods and show that LPE do not necessarily lead to the lowest κ. We find that the calculated κ in LPE free Cs3SbSe4 is smaller (∼0.1 W/mK) than the κ in CsSbSe2 (∼0.3 W/mK), which has LPE. This is unusual and contradictory to the traditional concept in which LPE lead to low κ values. We explain this difference in terms of the local structural instabilities. The small ionic radius of Sb5+ brings the Se2– closer. This leads to a Coulomb repulsion between them making the [SbSe4]3– polyhedra in Cs3SbSe4 less stable. In addition, the bonding topology disconnects these units as compared to CsSbSe2 where there are shared Se atoms. This results in soft phonons and large thermal displacements for the Sb and Se for this material. This gives larger anharmonicity and higher scattering rates in Cs3SbSe4 than those from LPE-driven anharmonicity of CsSbSe2. Higher scattering rates and softer modes due to weaker interactions result in the κ of Cs3SbSe4 being lower than that of CsSbSe2.