Effect of Oxidation on the Mechanochemical Synthesis of Bismuth Telluride Nanophases, Ex Situ and In Situ Structural Characterization, and Thermoelectric Properties

The Bi–Te system remains one of the most extensively studied thermoelectric materials and is among the few commercially viable solutions for waste heat recovery. However, the role of synthesis conditions particularly precursor oxidation on phase formation and transport properties remains insufficiently understood. In this work, we investigate the effect of chemical composition and precursor oxidation on the polymorphism and thermoelectric performance of mechanochemically synthesized, low-density BixTe100−x (x = 50 and 70) nanomaterials. High-energy ball milling enables rapid phase formation within minutes, yielding distinct phase compositions depending on precursor form and oxidized contaminations. For Bi50Te50, highly oxidized precursors stabilize Bi2Te3, moderately oxidized conditions favor BiTe, and low-oxide precursors promote the formation of Bi4Te3, while Bi2Te is obtained for Bi70Te30. In situ X-ray diffraction reveals rapid phase formation on short timescales, strongly dependent on precursor oxidation. Highly oxidized precursors also induce ternary oxide phases that significantly suppress electrical transport. Despite this, a compensating reduction in thermal conductivity leads to a maximum zT of 0.18 at 200 °C for the Bi2Te3 phase. Analysis of the thermoelectric quality factor demonstrates that oxidation ultimately degrades intrinsic electronic performance, indicating that zT alone can be misleading when comparing materials with different microstructures and porosity. Additional densification via spark plasma sintering improves room-temperature performance but highlights the competing increase in electronic thermal conductivity, further supporting the need to evaluate intrinsic material quality beyond zT. These results establish mechanochemistry as a versatile route for accessing a wide range of Bi–Te polymorphs and highlight the critical role of precursor oxidation in achieving optimal thermoelectric performance.