Thermoelectric materials are promising alternative energy sources, suitable for applications in thermoelectric generators and refrigerators, due to their low cost and environmentally friendly heat-to-power generation. Their wide-scale utilization is limited because of their low efficiency; however, nanoscale inclusions can improve it by suppressing the lattice thermal conductivity.

The high performance PbTe-SnTe-PbS thermoelectric system forms a new PbTe – PbSnS₂ composite with high n-type figure of merit. Electron Diffraction (ED) through tilting and Precession Electron Diffraction (PED) experiments as well as High Resolution Transmission Electron Microscopy (HRTEM) characterization of the thermoelectric composite PbTe + 25% PbSnS₂ reveal that the system is nanostructured in a unique way, with PbSnS₂ nanocrystals in the range of 80 to 500 nm in size. In most of the cases, they are endotaxially grown within the PbTe matrix (Figure 1).

Three independent crystal superstructures were observed for the PbSnS₂ inclusions, originating from the same parent SnS-type structure. Specifically, the PbSnS₂ inclusions appear with all three known structural modifications within the same matrix and often within the same nanocrystal. HRTEM images were qualitatively improved by Fast Fourier Transform (FFT) and inverse FFT (IFFT) processing, as well as by averaging of selected parts of the images. We then compared the processed images with computer image simulations and a very good resemblance between them was found. Figure 3 depicts a HRTEM image taken with the electron beam parallel to [110]PbSnS₂ direction and is a part of a nanocrystal of about 300 nm in size. Two types of FFT patterns were observed and are shown as insets in Figure 3. Modified structural models for two of the superstructures observed in the PbSnS₂ precipitates are proposed (one of them is presented in Figure 2).

Finally, evidence was also found for the growth process of the nanocrystals. The presence of some PbS nanocrystals implies that the growth process of the minor PbSnS₂ phase starts from PbS crystals where SnS is gradually dissolved to eventually form PbSnS₂ crystals.

Our findings suggest that this nanostructured thermoelectric composite exhibits unique structural complexity, which contributes to the low lattice thermal conductivity reported previously for these nanocomposite materials, by introducing extra scattering mechanisms. Phonon scattering can occur not only at the interfaces between the nanocrystals and the matrix, but also within the nanocrystals, due to their structural heterogeneity.

Acknowledgements

This work was supported by “IKY Fellowships of Excellence for Postgraduate Studies in Greece – SIEMENS Program”. Work at Northwestern is supported by the Department of Energy, Office of Science Basic Energy Sciences under grant DE-SC0014520.

Figures:

Fig. 1 PED pattern with the electron beam parallel to [001]PbTe direction, revealing the endotaxial growth of PbSnS2 in the PbTe matrix.

Fig. 2 Processed HRTEM image along the [110]PbSnS2 direction deriving from area 01 of Figure 3. The simulated image, shown as inset, was calculated with defocus and thickness values of -36nm and 3.5nm, respectively.

Fig. 3 A HRTEM image taken from a PbSnS2 nanocrystal with the electron beam parallel to the [110] direction. The FFTs from areas 01 and 02 are shown as insets. The two areas correspond to different structural phases of PbSnS2.

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EMC Abstracts - https://emc-proceedings.com/abstract/structural-characterization-of-the-high-thermoelectric-performance-pbte-pbsns2-system-and-implications-of-its-structural-complexity-in-low-lattice-thermal-conductivity/