Potassium Molybdenum Chalcogenides: Electronic Structure Modulation through Cluster Dimensionality

Abstract Herein, we report a systematic structural and electronic investigation of potassium-inserted extended molybdenum chalcogenides synthesized via microwave-assisted solid-state routes, focusing on KMo6S8, K2Mo15S19, and K2Mo6S6. Using X-ray absorption spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy, we evaluate how systematic variations in Mo/S ratio modulate phase morphology and local electronic structure. Analysis of the S K-edge and Mo L3-edge XANES reveals distinct electronic environments across the extended clusters, demonstrating precise control over oxidation state and coordination as a function of Mo–Mo cluster extension. To probe the tunability of these materials, KxMo15S19 was employed as a platform to investigate the limits of potassium insertion. Synthesis of KxMo15S19, with a generalized product-phase thermodynamic model, indicates that compositions within the range of x = 2–2.5 demonstrate the highest stability, maintained through enthalpy and entropy effects. Finally, electrochemical studies demonstrate that stable KxMo15S19 phases enable modulation of proton adsorption and electron-transfer behavior during the hydrogen evolution reaction, establishing extended Mo–Mo chalcogenides as a versatile platform for studying electron density localization and reactivity. Altogether, this work uncovers design rules that are generalizable across multinary chalcogenides and that deepen our understanding of how compositional and structural variations govern functional properties in these solids.