Understanding nanostructures in thermoelectric materials: An electron microscopy study of AgPb₁₈SbSe₂₀ crystals

The characterization and understanding of the presence of nanostructuring in bulk thermoelectric materials requires real space atomic level information. We report electron diffraction and high-resolution transmission electron microscopy studies of crystals of the system AgPb₁₈SbSe₂₀ (=18PbSe + AgSbSe₂) which reveal that this system is nanostructured rather than a solid solution. Nanocrystals of varying sizes are found, endotaxially grown in the matrix of PbSe (phase A), and consist of two phases, a cubic one (phase B) and a tetragonal one (phase C). Well-defined coherent interfaces between the phases in the same nanocrystals are observed. On the basis of the results of combined electron crystallography techniques, we propose reasonable structural models for the phases B and C. There are significant differences in the nanostructuring chemistry between AgPb₁₈SbSe ₂₀ and the telluride analog AgPb₁₈SbTe₂₀ (LAST-18).