Unlocking the Full Potential of MgAgSb by Unravelling the Interrelation of Phase Constitution and Thermoelectric Properties

Abstract A thorough understanding and optimization of microstructure are key for enhancing performance in thermoelectric materials. By combining high‐resolution microstructural analysis with bulk transport characterization and spatially resolved Seebeck microprobe measurements, the interrelation between phase constitution and thermoelectric properties of MgAgSb, a promising p‐type room temperature material is established. This study analyzes 35 MgAgSb samples with varying compositions and synthesis conditions. Scanning transmission electron microscopy with energy‐dispersive X‐ray spectroscopy and selected area electron diffraction reveals that remnants of constituent elements from the initial synthesis step can cause the formation of detrimental secondary phases, including a previously unnoticed, highly dispersed Ag ₃ Sb nanophase uniformly distributed throughout the samples. Analysis of the temperature dependence of the weighted mobility reveals differences in carrier scattering as the main reason for the different thermoelectric performance. Furthermore, a statistical analysis of weighted mobility and material quality factor versus type and amount of secondary phase reveals the secondary phases to predominantly affect the electronic (not thermal) transport, ranks them according to impact and further distinguishes classical mixing from grain boundary effects. This approach is crucial for understanding how specific phases affect material performance, identifying Mg ₃ Sb ₂ and (Ag) as the most detrimental, but also providing guidelines for further material improvement.