Solid‐State Janus Nanoprecipitation Enables Amorphous‐Like Heat Conduction in Crystalline Mg3Sb2‐Based Thermoelectric Materials

Solid‐state precipitation can be used to tailor material properties, ranging from ferromagnets and catalysts to mechanical strengthening and energy storage. Thermoelectric properties can be modified by precipitation to enhance phonon scattering while retaining charge‐carrier transmission. Here, unconventional Janus‐type nanoprecipitates are uncovered in Mg3Sb1.5Bi0.5 formed by side‐by‐side Bi‐ and Ge‐rich appendages, in contrast to separate nanoprecipitate formation. These Janus nanoprecipitates result from local comelting of Bi and Ge during sintering, enabling an amorphous‐like lattice thermal conductivity. A precipitate size effect on phonon scattering is observed due to the balance between alloy‐disorder and nanoprecipitate scattering. The thermoelectric figure‐of‐merit ZT reaches 0.6 near room temperature and 1.6 at 773 K. The Janus nanoprecipitation can be introduced into other materials and may act as a general property‐tailoring mechanism. Bi‐/Ge‐rich Janus nanoprecipitates in Mg3Sb1.5Bi0.5 compounds are uncovered by electron microscopy and atom probe tomography. This complex Janus nanoprecipitate results from local comelting of Bi and Ge during sintering and enables an amorphous‐like lattice thermal conductivity near room temperature. The mechanistic understanding of thermal‐conductivity reduction is supported by modelling the material systems with and without precipitates.