Hidden Structures and Unusual Fourth-Nearest-Neighbor Interactions Drive Atomic Ordering in Thermoelectric AgSbTe2

Abstract AgSbTe2 is a leading mid-temperature thermoelectric material, with several doped variants achieving a figure of merit (zT) > 2. However, the microscopic mechanism by which dopants facilitate cation ordering for enhanced thermoelectric performance remains elusive. Here, we combine synchrotron X-ray pair distribution function analysis and machine-learned force field molecular dynamics simulations to investigate pristine AgSbTe2 and its highest-performing variant AgSb0.94Cd0.06Te2. Our analysis reveals ∼0.2 Å local rhombohedral distortions due to cation off-centering within the average rock-salt structure. Upon Cd doping, the fourth-nearest-neighbor (4NN) atom pair correlations are distinctly modified, evidencing the emergence of local cation ordering along <100>. We find that the unusually strong long-range interactions along <100> arising from multicenter p-bonding promote bonding coherence and drive 4NN cation ordering. By understanding how Cd reshapes 4NN correlations and bonding topology, we establish a direct link between local structural order, chemical bonding, and charge transport in AgSbTe2. Our insights offer a unified structural and chemical bonding-based framework for understanding dopant effects and guiding the rational design of next-generation AgSbTe2 thermoelectrics.