Entropy-Engineered Thermoelectrics: The Role of Symmetry-Driven Structure and Transport Property Evolution

High-entropy thermoelectrics are garnering considerable attention owing to their exceptional energy conversion efficiency, improved stability, and expanded composition space. Nevertheless, understanding the impact of entropy engineering on thermoelectrics remains a challenge. The presence of diverse atomic species can give rise to a variety of mixing reactions, potentially leading to structural and chemical inhomogeneity at multiple length scales. This heterogeneity can profoundly affect transport properties, whereas a comprehensive exploration of these effects is lacking.<br/><br/>IV–VI group semiconductors, including PbQ, SnQ (Q = S, Se, Te), and GeTe, are among the most promising thermoelectric materials. In light of their potential, we choose PbGeSnTe₃ as the model system for further research. Through alloying CdTe to PbGeSnTe₃, we progressively increase the configurational entropy and study the resulting evolution in structure and transport properties in PbGeSnCd_xTe_3+x. Our findings reveal that PbGeSnTe₃ adopts a rhombohedral structure (<i>R</i>3<i>m</i>) at room temperature and undergoes a phase transformation to a cubic structure (<i>Fm</i>-3<i>m</i>) above 373 K. Through transmission electron microscopy (TEM) analysis, we observe densely distributed polar domains mediated by inversion and twin boundaries, leading to structural inhomogeneity. <i>In situ </i>heating TEM results reveal the disappearance of domains during phase transformation at high temperatures, followed by reversible formation upon cooling. These observations indicate that the loss of inversion symmetry induces the formation of domains with different polarities. The polarity switching between each domain results in significant strain fluctuations that suppress phonon propagation. Variable temperature 4D-STEM (scanning transmission electron microscopy) analysis demonstrates the strain distribution evolves upon heating due to domain evolution, contributing to an abnormal temperature-dependent thermal conductivity in the low-temperature range.<br/><br/>We further demonstrate the effectiveness of entropy engineering in tailoring the microstructure and transport properties through controlling crystal symmetry. By alloying CdTe, the increase in configurational entropy of PbGeSnCd_xTe_3+x results in a progressive reduction of the phase transition temperature and stabilizes PbGeSnCd_0.2Te_3.2 in a cubic structure (<i>Fm</i>-3<i>m</i>) at room temperature. The increase in crystal symmetry eliminates the symmetry-breaking process during synthesis, preventing the formation of domains and consequently eliminating abnormal thermal conductivity behavior. Additionally, the Seebeck coefficient exhibits significant improvement, attributed to an increased density-of-states effective mass from 1.36 <i>m</i>₀ (free electron mass) to 2.63 <i>m</i>₀. This enhancement compensates for the degradation in charge carrier mobility, resulting in an improved thermoelectric performance with a maximum figure-of-merit (<i>ZT</i>) of 1.63 achieved at 875 K.<br/><br/>This study reveals the presence of structural inhomogeneity in high-entropy thermoelectrics, emphasizing the need for a comprehensive investigation at nanoscale to establish a robust structure-property relationship. Furthermore, we demonstrate that entropy engineering offers a promising avenue for tailoring microstructures and transport properties by modifying crystal structures. This finding opens new possibilities for the development of next-generation thermoelectric materials.