Discovery of a New Cu‐Based Chalcogenide with High zT Near Room Temperature: Low‐Cost Alternative for the Bi 2 Te 3 ‐Based Thermoelectrics

Abstract Copper‐based chalcogenides are cost‐effective and environmentally friendly thermoelectric (TE) materials for waste heat recovery. Despite demonstrating excellent thermoelectric performance, binary Cu 2 X ( X = S, Se, and Te) chalcogenides undergo superionic phase transitions above room temperature, leading to microstructural evolution and unstable properties. In this work, a new γ‐phase of Cu 6 Te 3‐ x S 1+ x (0 < x ≤ 1) is discovered, a narrow‐bandgap semiconductor with outstanding thermoelectric performance and high stability. By substituting Te with S in metallic Cu 6 Te 3 S, the crystal symmetry is modified and structural phase transitions are eliminated. The γ‐phase exhibits a significantly higher Seebeck coefficient of up to 200 µVK −1 compared to 8.8 µVK −1 for Cu 6 Te 3 S at room temperature due to optimized carrier concentration and increased effective mass. Cu 6 Te 3‐ x S 1+ x materials also demonstrate ultralow thermal conductivity (≈0.25 Wm −1 K −1 ), which, in concert with improved power factors, enables a high zT of ≈1.1 at a relatively low temperature of 500 K. Unlike most Cu‐based chalcogenides, the γ‐phase exhibits excellent transport property stability across multiple thermal cycles, making it a cost‐effective and eco‐friendly alternative to Bi 2 Te 3 ‐based materials. The developed Cu 6 Te 3‐ x S 1+ x is a promising candidate for thermoelectric converters in waste heat recovery, and its potential can be further extended to cooling applications through carrier concentration tuning.