Morphological evolution of Ag2Te precipitates in thermoelectric PbTe

The precipitation of Ag2Te in a PbTe matrix is investigated using electron microscopy and atom probe tomography. We observe the formation of oriented nanoscale Ag2Te precipitates in PbTe. These precipitates initially form as coherent spherical nanoparticles and evolve into flattened semi-coherent disks during coarsening. This change in morphology is consistent with equilibrium shape theory for coherently strained precipitates. Upon annealing at elevated temperatures these precipitates eventually revert to an equiaxed morphology. We suggest this shape change occurs once the precipitates grow beyond a critical size, making it favorable to relieve the elastic coherency strains by forming interfacial misfit dislocations. These investigations of the shape and coherency of Ag2Te precipitates in PbTe should prove useful in the design of nanostructured thermoelectric materials.