Ag-Segregation to Dislocations in PbTe-Based Thermoelectric Materials

Dislocations
have been considered to be an efficient source for scattering midfrequency
phonons, contributing to the enhancement of thermoelectric performance.
The structure of dislocations can be resolved by electron microscopy
whereas their chemical composition and decoration state are scarcely
known. Here, we correlate transmission Kikuchi diffraction and (scanning)
transmission electron microscopy in conjunction with atom probe tomography
to investigate the local structure and chemical composition of dislocations
in a thermoelectric Ag-doped PbTe compound. Our investigations indicate
that Ag atoms segregate to dislocations with a 10-fold excess of Ag
compared with its average concentration in the matrix. Yet the Ag
concentration along the dislocation line is not constant but fluctuates
from ∼0.8 to ∼10 atom % with a period of about 5 nm.
Thermal conductivity is evaluated applying laser flash analysis, and
is correlated with theoretical calculations based on the Debye–Callaway
model, demonstrating that these Ag-decorated dislocations yield stronger
phonon scatterings. These findings reduce the knowledge gap regarding
the composition of dislocations needed for theoretical calculations
of phonon scattering and pave the way for extending the concept of
defect engineering to thermoelectric materials.