Discovery of a New Cu-Based Chalcogenide with High zT Near Room Temperature: Low-Cost Alternative for the...

Copper-based chalcogenides are cost-effective and environmentally friendly thermoelectric (TE) materials for waste heat recovery. Despite demonstrating excellent thermoelectric performance, binary Cu₂X (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 Cu₆Te_{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 Cu₆Te₃S, 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 Cu₆Te₃S at room temperature due to optimized carrier concentration and increased effective mass. Cu₆Te_{3- x}S_{1+ x} materials also demonstrate ultralow thermal conductivity (≈0.25 Wm^-1K^-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 Bi₂Te₃-based materials. The developed Cu₆Te_{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.