Abstract
Chemical composition is often considered a key factor in governing thermoelectric performance. However, samples with the same composition can exhibit various transport behaviors depending on the form of dopant incorporation. Here, we compare the thermoelectric performance of Mg3(Sb, Bi)2 samples with the same nominal Cu2Se content, introduced either by elemental addition or compound addition. Elemental addition mainly induces multicomponent-like doping, while compound addition exhibits a slightly doping effect. Both strategies result in the formation of Cu-rich grain-boundary phases that facilitate electron transport. However, elemental addition leads to excessive carrier concentration and grain refinement, ultimately degrading the power factor. In contrast, compound addition significantly enhances electrical conductivity through Cu-rich grain-boundary phases while avoiding over-doping. Additionally, abundant Se-rich precipitates with core-shell structures are also formed in the Cu2Se-added sample, effectively suppressing lattice thermal conductivity. Consequently, the Cu2Se-added sample exhibits superior zT across the entire temperature range, reaching a peak zT of 1.71 at 723 K and an exceptional average zT of 1.21 over 323-723K. This work demonstrates that the form of additive incorporation, not just chemical composition, plays a decisive role in microstructural evolution and thermoelectric performance, offering a new dimension for the compositional design of advanced thermoelectric materials.