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

ABSTRACT 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 Mo 6 S 8 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 Cu 2 Mo 6 S 8 exhibits suppressed lattice thermal conductivity and a peak zT of 0.40 at 900 K. Further increasing the Cu content to Cu 3.66 Mo 6 S 8 introduces enhanced structural disorder and interface‐induced energy filtering, simultaneously improving the Seebeck coefficient and reducing lattice thermal conductivity at elevated temperatures. Consequently, Cu 3.66 Mo 6 S 8 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.