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.