Ostwald Ripening of Ag₂Te Precipitates in Thermoelectric PbTe: Effects of Crystallography, Dislocations, and Interatomic Bonding

Nanostructuring is important for designing thermoelectrics. Yet, nanoprecipitates are thermodynamically unstable and coarsen through Ostwald ripening. Here, the Ostwald ripening of Ag2Te in PbTe and its resulting impact on thermoelectric performance is investigated. Numerous Guinier-Preston zones and platelet Ag2Te precipitates in the sample quenched from a single-phase region is observed. Upon annealing, these platelet precipitates grow into big lath-shaped second phases by consuming small Ag-rich clusters. The crystallographic orientation relationships between Ag2Te and PbTe are unraveled by scanning transmission electron microscopy and modeled by first-principles calculations. The interfaces with low lattice mismatch determine the morphology of Ag2Te in PbTe. Atom probe tomography reveals different chemical bonding mechanisms for PbTe and Ag2Te, which are metavalent and iono-covalent, respectively. This leads to an acoustic phonon mismatch at the precipitate-matrix interface. Yet, the electrons are also scattered by these interfaces, resulting in poor electrical properties in the as-quenched sample. In contrast, the annealed sample contains abundant Ag-decorated dislocations by activating the Bardeen-Herring source. These dislocations strongly scatter phonons while maintaining a good electron transmission, contributing to a higher thermoelectric performance. This work demonstrates the complex role of microstructure morphologies, compositions, and bonding mechanisms in thermoelectric response, providing insights into structural design for thermoelectrics.