In the past two decades, the exploitation of new material and innovating engineering strategies have substantially increased the figure of merit of TE materials, zT (=S 2 T/ρκ, in which ρ, S, κ, and T are the electrical resistivity, Seebeck coefficient, total thermal conductivity, and temperature), the primary metric to measure the heat-to-electric power conversion efficiency. High zT values have been constantly reported in PbTe, [2,3] SnSe, [4-6] GeTe, [7-17] CoSb 3 , [18] Zn 4 Sb 3 , [19-21] Mg 3 Sb 2 , [22,23] and related materials in the mid-temperature range. Several important theories and concepts have also been proposed to elucidate the mechanisms leading to their extraordinary TE performance. [24-28] GeTe is a narrow bandgap semiconductor with a large hole carrier concentration of ≈10 21 cm-3 due to native Ge vacancies. It adopts a cubic structure (Fm3m, β-GeTe) at high temperatures, which undergoes a ferroelectric phase transition to a non-centrosymmetric Phase transition in thermoelectric (TE) material is a double-edged swordit is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-torhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb 2 Te 3 significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.

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