Role of self-organization, nanostructuring, and lattice strain on phonon transport in NaPb_18-xSn_xBiTe₂₀ thermoelectric materials

The composition and microstructure of five thermoelectric materials, PbTe, SnTe, Pb_0.65Sn_0.35Te and NaPb_18-xSn _xBiTe₂₀ (x = 5, 9), were investigated by advanced transmission electron microcopy. We confirm that the pure PbTe, SnTe, and Pb_0.65Sn_0.35Te have a uniform crystalline structure and homogeneous compositions without any nanoscale inclusions. On the other hand, the nominal NaPb₉Sn₉BiTe₂₀ phase contains extensive inhomogeneities and nanostructures with size distribution of 3-7 nm. We find that the chemical architecture of the NaPb₁₃Sn ₅BiTe₂₀ member of the series to be more complex; besides nanoscale precipitates, self-organized lamellar structures are present which were identified as PbTe and SnTe by composition analysis and transmission electron microscopy image simulations. Density functional theory calculations suggest that the arrangement of the lamellar structures conforms to the lowest total energy configuration. Geometric-phase analyses revealed large distributed elastic strain around the nanoscale inclusions and lamellar structures. We propose that interface-induced elastic perturbations in the matrix play a decisive role in affecting the phonon-propagation pathways. The interfaces further enhance phonon scattering which, in turn, reduces the lattice thermal conductivity in these systems that directly results directly in improvement in the thermoelectric figure of merit.