Ultrafast Solid-State Synthesis of Chevrel Phases for Microstructure-Enabled Thermoelectric Transport Optimization

Chevrel phases are promising medium- to high-temperature thermoelectrics owing to their structural stability and intrinsically low lattice thermal conductivity, yet prolonged high-temperature synthesis causes grain coarsening and limits microstructural control. Here, we report an ultrafast solid-state synthesis strategy for preparing phase-pure ternary Chevrel phases within 20 min, reducing the reaction time by up to two orders of magnitude. Phase-evolution analysis and density functional theory calculations suggest a possible formation pathway involving metal-vapor insertion into metastable Mo₆S₈ intermediates, potentially facilitated by reaction-induced local overheating. This strategy is successfully extended to multiple sulfide- and selenide-based Chevrel phases. The shortened synthesis process suppresses grain growth and enables microstructure-mediated transport optimization. As a model system, microstructure-refined Cu₂Mo₆S₈ exhibits suppressed lattice thermal conductivity and a peak zT of 0.40 at 900 K. Further increasing the Cu content to Cu_3.66Mo₆S₈ introduces enhanced structural disorder and interface-induced energy filtering, simultaneously improving the Seebeck coefficient and reducing lattice thermal conductivity at elevated temperatures. Consequently, Cu_3.66Mo₆S₈ achieves a peak zT of 0.44 at 900 K, the highest value reported for ternary sulfide-based Chevrel phases, together with a high average zT over 300-950 K.