Ultra-low lattice thermal conductivity and enhanced thermoelectric performance in Ag_2−xSe_1/3S_1/3Te_1/3 via anion permutation and cation modulation

Silver chalcogenides Ag₂X (X = S, Se, and Te) bear intrinsically low lattice thermal conductivity and decent electrical properties due to their ionic conduction feature. In this work, we report that even lower lattice thermal conductivities and enhanced thermoelectric performance can be realized in moderately porous Ag_2−xSe_1/3S_1/3Te_1/3 (0 ≤ x ≤ 0.15) samples synthesized via high-energy ball milling and spark plasma sintering (SPS) methods. Lattice thermal conductivity for sample Ag_1.9Se_1/3S_1/3Te_1/3 can get down to ~0.2 W·m⁻¹·K⁻¹ at room temperature; maximum figure of merit ZT value appears at 423 K to be ~0.55, and the calculated average ZT is 0.46 from 300 to 423 K (~10% higher than state-of-art Ag₂Se). We attribute the enhanced figure of merit ZT largely to the significantly reduced lattice thermal conductivity, which results from the synergetic effect of strong alloy scattering at anion site due to Te/Se/S permutation and moderate sample porosity up to 14.42%.