• • Ag incorporation at x = 0.01 in Mg3.3Sb0.5Bi1.497Te0.003 yields a power factor enhancement at 300 K and an average ZT of ~1.0 across 300–400 K, directly addressing the low Hall carrier mobility bottleneck in n-type Mg3(Sb,Bi)2 for near-room-temperature waste heat recovery.
• • Ag preferentially segregates at grain boundaries, forming Mg-rich MgAg alloy phases with limited solid solubility in the matrix, as confirmed by microstructural characterization and first-principles calculations, enabling simultaneous optimization of carrier transport and phonon scattering.
• • The engineered grain boundaries provide efficient carrier transport channels, enhancing carrier mobility, while MgAg phases and lattice disorders scatter phonons without disrupting electron transport, achieving a decoupling of electrical and thermal transport.
• • The material exhibits excellent mechanical properties and thermal stability, making it suitable for practical near-room-temperature thermoelectric applications, where commercial Bi2Te3 suffers from tellurium scarcity, toxicity, and poor mechanical robustness.