High Thermoelectric Performance in the New Cubic Semiconductor AgSnSbSe₃ by High-Entropy Engineering

We investigate the structural and physical properties of the AgSnSbSe system with m = 1-20 (i.e., SnSe matrix and ∼5-50% AgSbSe₂) from atomic, nano, and macro length scales. We find the 50:50 composition, with m = 1 (i.e., AgSnSbSe₃), forms a stable cation-disordered cubic rock-salt p-type semiconductor with a special multi-peak electronic valence band structure. AgSnSbSe₃ has an intrinsically low lattice thermal conductivity of ∼0.47 W m^-1 K^-1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on Se sites creates a quinary high-entropy NaCl-type solid solution AgSnSbSe_3-Te with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ∼0.32 W m^-1 K^-1 at 723 K and higher hole carrier concentration than AgSnSbSe₃. Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe_1.5Te_1.5, the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ∼9.54 μW cm^-1 K^-2 (400-773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K, and a high average ZT of ∼1.0 over a wide temperature range of 400-773 K in AgSnSbSe_1.5Te_1.5.