Room‐Temperature Cubic Ag 2 S 1−2 x S e x T e x with Promising Ductility and Thermoelectric Properties Enabled by Entropy Engineering

Since the discovery of superior ductility in semiconducting Ag 2 S at room temperature, Ag 2 S‐based inorganics attract ever‐increasing attention as ductile thermoelectrics (TEs) for flexible electronics, while the monoclinic to cubic structure transition near room temperature (≈455 K for Ag 2 S) of these materials leads to instability of their structures and properties. In this work, single‐phase cubic Ag 2 S 1−2 x Se x Te x ( x = 0.13–0.33) samples are stabilized at room temperature via entropy engineering. In comparison with pure Ag 2 S, the random mixing of S, Se, and Te at the anion site results in increased configuration entropy, improved electrical conductivity, decreased lattice thermal conductivity, and thus significantly enhanced TE properties of cubic Ag 2 S 1−2 x Se x Te x samples. By further optimizing the carrier concentration through introducing Ag vacancies, the slightly Ag‐deficient Ag 1.98 S 0.34 Se 0.33 Te 0.33 sample achieves a power factor of 6.1 µW cm −1 K −2 and a dimensionless figure of merit zT of 0.4 at room temperature. In the measured temperature range of 300–500 K, this cubic sample with excellent ductility shows not only a record average zT value of 0.62 in ductile inorganics but also very stable TE properties, demonstrating the great potential of entropy engineering in the design of high‐performance ductile TE inorganics.