Advancing n-type Mg3+δSb1.5Bi0.47Te0.03-based thermoelectric Zintls via sc-doping-driven band and defect engineering

[Display omitted] •A high ZT of > 1.5 is achieved at 703 K in Sc/Te co-doped Mg3.2Sb1.5Bi0.5.•Both ScMg and TeBi point effects improve the overall thermoelectric performance.•First-principles DFT calculations are introduced to guide the overall material design.•Single parabolic band model confirms the optimized carrier density and ZT. Mg3+δX2 (X = Sb, Bi) based Zintl phases possess characteristics, such as non-toxicity, cost-effectiveness, ease of fabrication, and a wide range of working temperatures, showing high thermoelectric potential as a n-type thermoelectric material. However, there exists a relative research gap in understanding the underlying mechanisms behind improving their thermoelectric performance through doping, and the corresponding influence on the micro/nanostructures of the materials. Here, guided by the first-principles calculations, we design Mg3.2Sb1.5Bi0.5-based thermoelectric Zintl phases co-doped with Sc and Te. Computational results indicate that the substitution of Mg with Sc as a cationic site dopant and Bi with Te as an anionic site dopant can effectively shift the Fermi level into the conduction band, leading to a significant increase in carrier concentration. Experimental results confirm the effectiveness of the co-doping, exhibiting a high power factor exceeding 20 μW cm−1 K−2 at 703 K. Furthermore, detailed micro/nanostructural characterizations revealed that doping induces a high density of point defects causing extensive lattice distortions. These lattice imperfections effectively scatter phonons, validating the achieved low thermal conductivity of 0.93 W m−1 K−1. As a result, an improved ZT value of 1.53 was obtained at 703 K in Sc0.01Mg3.19Sb1.5Bi0.47Te0.03 bulk material. Additionally, we found that Sc doping significantly enhances mechanical properties, including elastic modulus, hardness, shear strength, and compressive strength, which is practically meaningful for its application in real device scenarios.