Analysis and Implications of Structural Complexity in Low Lattice Thermal Conductivity High Thermoelectric Performance PbTe-PbSnS₂ Composites

The high-performance PbTe-SnTe-PbS thermoelectric system forms a completely new composite PbTe-PbSnS₂ with high n-type figure of merit. Electron diffraction and high-resolution electron microscopy characterization of the thermoelectric composite PbTe + 25% PbSnS₂ reveals that the system is nanostructured, with PbSnS₂nanocrystals in the range of 80 to 500 nm in size. In most of the cases, they are endotaxially grown within the PbTe matrix. Three independent crystal superstructures were observed for the PbSnS₂ inclusions, originating from the same parent SnS-type structure. The presence of the parent structure is not excluded. Modified structural models for two of the superstructures observed in the PbSnS₂ precipitates are proposed. Often, more than one of the structural phases are observed in the same nanocrystal, providing one extra phonon scattering factor in the system. Evidence was also found for the growth process of the nanocrystals, starting from PbS and followed by gradual dissolving of SnS. Our findings suggest that this nanostructured thermoelectric composite exhibits unique structural complexity, which contributes to the low lattice thermal conductivity reported for these nanocomposite materials.