Two-Dimensional CsAg₅Te_3–xS_x Semiconductors: Multi-anion Chalcogenides with Dynamic Disorder and Ultralow Thermal Conductivity

Metal chalcogenides underpin a wide variety of energy-related applications and are ideal systems for probing lattice dynamics and fundamental transport phenomena. Here we describe the synthesis and transport properties of CsAg₅TeS₂ and its solid solution CsAg₅Te_3–xS_x (x = 1–2), new semiconductors with tunable band gaps ranging from 0.17 to 0.30 eV. CsAg₅TeS₂ has a fully ordered two-dimensional structure that includes a group of Ag atoms in a heteroleptic tetrahedral coordination geometry (AgTe₂S₂). Single-crystal X-ray diffraction indicates that the compounds crystallize in the tetragonal space group P4/mmm, while pair distribution function (PDF) analysis reveals off-centering at the heteroleptic Ag sites, signifying the lower-symmetry I4/mcm space group. The underlying disorder acts as a phonon-blocking mechanism that helps facilitate an ultralow lattice thermal conductivity below 0.40 W·m^–1·K^–1 at ∼300 K , highlighting the importance of local disorder in thermal transport. Density functional theory provides additional insight into the electronic and thermal properties of the materials, which are good candidates for p-type thermoelectrics.