Spatially Homogeneous Mg ₃ (Sb, Bi) ₂ With Suppressed Parasitic Transport Enables Ultrahigh Thermoelectric Performance

Chemical inhomogeneity widely exists in thermoelectric materials, yet its effects are mainly ascribed to local interface scattering and compositional fluctuations. Here, we show that the spatial connectivity of chemical inhomogeneity governs thermoelectric performance by enabling parasitic transport pathways. In Mg 3 (Sb, Bi) 2 , multiscale Bi‐related chemical inhomogeneity forms spatially connected percolation pathways that deteriorate both the Seebeck coefficient and thermal conductivity. Cyclic pressure sintering disrupts these percolative networks, thereby inhibiting parasitic transport and enhancing phonon scattering. As a result, a high zT of ∼2.08 at 673 K and an exceptional average zT of ∼1.58 over 323–723 K are achieved, placing these values among the highest reported for Mg 3 (Sb, Bi) 2 ‐based materials. Furthermore, a single‐leg device delivers a conversion efficiency of ∼12.1% (Δ T = 410 K). These findings establish the spatial connectivity of heterogeneity as a key parameter governing thermoelectric transport and provide a general strategy for performance optimization.