Revealing the Hidden Role of Cd in p‐Type Mg ₃ Sb ₂ : Enhanced Thermoelectric Performance by Grain Boundary Segregation Engineering

ABSTRACT Grain boundary (GB) scattering of charge carriers plays an important role in the electrical properties of materials. For thermoelectrics, this scattering can significantly reduce the carrier mobility and output power. Various GB engineering strategies, such as increasing the grain size and decreasing the GB potential barrier height, have been demonstrated in n‐type Mg 3 Sb 2 ‐based thermoelectrics. Yet, similar effects in p‐type Mg 3 Sb 2 have been less reported, and effective methods to modify the GB potential are elusive. Here, we reveal that Cd can reduce the GB barrier height via segregation to the GB. The enrichment of Cd suppresses the formation of hole‐killer defects (Sb Mg + ) and thus reduces the GB resistance, leading to significant improvements in the electrical conductivity and power factor. A two‐phase model shows a progressive decrease in the GB barrier height with increasing Cd content. Simultaneously, the GB Cd segregation and grain‐interior Cd alloying strengthen the phonon scattering and reduce the sound velocity, substantially reducing the thermal conductivity. Consequently, a maximum ZT of 0.84 is achieved in Mg 2.5 Cd 0.5 Sb 2– 1 at% NaF at 773 K. These findings unravel the hidden role of Cd in tuning the GB characteristics and the transport properties of Mg 3 Sb 2 , revisiting the functionality of conventional dopants in materials design.