Stereochemical Modulation of Lone-Pair Electrons and Its Impact on Lattice Thermal Conductivity

Crystalline solids containing lone-pair electrons (LPEs) are of fundamental interest because the existence of LPEs makes physical properties (e.g., mechanical, optical, electrical, and thermal properties) of materials distinct from conventional materials. Among them, thermal properties have gathered significant attention from thermoelectric researchers because of the intrinsically low lattice thermal conductivity of LPE-containing solids, which is crucial for achieving high thermoelectric conversion efficiency. Conventional thermoelectric materials were intentionally doped with several impurities to suppress a propagation of phonons and reduce the lattice thermal conductivity. However, an intrinsically low lattice thermal conductivity of LPE-containing materials provided a room for optimizing electrical properties within a limited solubility of dopants, leading to a success in discovery of high-performance thermoelectric materials such as AgSbTe₂.<br/>ABX₂-type compounds (A=Cu, Ag, B=Sb, Bi, X=S, Se, Te) are one of the typical LPE-containing materials, where s-orbital electrons of B-site cations remain unpaired. These LPEs are known to exhibit strong lattice anharmonicity, which leads to the intrinsically minimal lattice thermal conductivity at room temperature. However, a clear demonstration of the effect of LPEs on the lattice thermal conductivity has not been reported. For example, it is not obvious whether the lattice thermal conductivity increases or decreases when some LPEs are eliminated from the lattice. Identifying the consequence of aforementioned question is important in that some dopants may reduce the concentration of LPEs due to their bonding characteristics.<br/>Here, we experimentally demonstrate a rise of the lattice thermal conductivity with doping in Cd-doped AgSbSnSe₃, which is contradictory to the result expected from a conventional phonon scattering theory. We found that despite of an increased point defects scattering, more LPEs become stereochemically inactive with increasing Cd concentration, and this makes the lattice more harmonic. Systematic pair distribution function analysis by powder X-ray diffraction measurements reveal that the off-centering of cations is reduced and the crystal structure changes symmetrically with Cd doping. Density functional theory calculations support that these “harmonic” environment does not allow LPEs to suppress the phonon propagation, and ironically leads to higher lattice thermal conductivities with a doping. This study corroborates an importance of preserving LPE configurations in ABX₂-type compounds for maintaining intrinsically low thermal conductivity when designing thermal-glass materials.