High Entropy Bi–Ti Alloying for Improved Sb₂Si₂Te₆ Thermoelectrics

A₂B₂Te₆ type materials attract significant interest due to their layered structures and inherently low thermal conductivity, but their low electrical conductivity limits their thermoelectric performance. High‐entropy alloying has emerged as an effective strategy to enhance electrical conductivity while preserving low thermal conductivity. This study introduces a two‐step high‐entropy alloying approach in Sb₂Si₂Te₆ where Bi/Ti‐coalloying optimizes the carrier concentration and reduces thermal conductivity. Pisarenko plot analysis reveals a substantial increase in effective mass for Sb_1.5Bi_0.5Si_1.6Ti_0.3Te₆, resulting in a stable Seebeck coefficient of 210 μVK⁻¹, and a power factor of 700 μWm⁻¹ K⁻² between 773–873 K. At 673 K, lattice thermal conductivity drops to 0.26 Wm⁻¹ K⁻¹ at 673 K, approaching the amorphous limit, due to enhanced alloy scattering, point defect scattering, and dislocations as confirmed by scanning electron microscopy (SEM) and transmission electron microscope (TEM) analyses. Jonker plot analysis demonstrates minimal degradation in electrical properties despite the significant reduction in lattice thermal conductivity, improving the quality factor. The combination of optimized Fermi level and reduced thermal conductivity leads to a zT (thermoelectric figure of merit) plateau of 1.11 between 623 and 723 K with an average zT of 0.68. These results highlight the potential of high‐entropy alloying in advancing layered thermoelectric materials.