Evolution of thermoelectric transport properties and severe bipolar effect in homologous layered Pb_1−xSn_xBi₆Te₁₀ solid solution system

Both PbBi₆Te₁₀ and SnBi₆Te₁₀ are layered homologous tellurides that share an identical structural phase. Herein, the thermoelectric transport properties of a series of (Pb_1−xSn_x)Bi₆Te₁₀ (x = 0, 0.15, 0.3, 0.5, 0.7, 0.85, and 1) compositions were systematically investigated to clarify how Sn alloying modifies carrier transport and band structure. With increasing Sn content, the electrical transport exhibits a crossover from metallic-like to semiconducting-like behavior, accompanied by a strong suppression of the Seebeck coefficient at high temperatures for x = 0.7–0.85. As a result, the power factor decreases monotonically despite the partial recovery of electrical conductivity at elevated temperatures. Single-parabolic-band analysis shows that the Hall carrier concentration shifts toward the SPB-predicted optimization regime with Sn alloying; however, the maximum attainable power factor decreases systematically, indicating that the performance degradation originates from an intrinsic reduction of transport quality rather than off-optimal carrier concentration. This interpretation is supported by the pronounced reduction in weighted mobility and thermoelectric quality factor. Furthermore, two band analysis reveals progressive bandgap narrowing by Sn alloying, which enhances high-temperature minority-carrier contributions and leads to severe bipolar transport. Consistently, the thermal conductivity shows a crossover behavior, where Sn-rich compositions exhibit increased high-temperature thermal conductivity due to additional bipolar heat transport. Consequently, the thermoelectric figure of merit zT exhibits a maximum value of 0.37 at 550 K for PbBi₆Te₁₀ and it gradually suppressed as the Sn content increases. Ultimately, this study reveals that unlike conventional alloying strategies, isovalent Sn substitution in this narrow-gap homologous system inherently triggers a detrimental trade-off: it severely degrades the intrinsic mobility and narrows the band gap to induce catastrophic bipolar effects.