Two-Dimensional CsAg₅Te_{3- x}S_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.