Relating Local Structure to Thermoelectric Properties in Pb_1-xGe_xBi₂Te₄

Layered compounds have garnered widespread interest owing to their nontrivial physical properties, particularly their potential as thermoelectric materials. We systematically investigated PbBi₂Te₄, a compound derived from Bi₂Te₃ and PbTe. Synchrotron X-ray diffraction and transmission electron microscopy revealed that PbBi₂Te₄ adopts and maintains the R3̅m phase from 300 to 723 K, without any phase transition. Moreover, neutron pair distribution function analysis confirmed that the short-range local structure was consistent with the high-symmetry R3̅m structure. PbBi₂Te₄ exhibits a negative Seebeck coefficient, indicating electron-dominated transport. It has a low lattice thermal conductivity (ca. 0.6 Wm^-1K^-1) and a ZT value of 0.4 at 573 K. The effects of GeBi₂Te₄ alloying in PbBi₂Te₄ (Pb_1-xGe_xBi₂Te₄, where x ranges from 0.0 to 0.6) were also investigated. Due to alloying-induced point defect scattering and the off-centering effects of Ge²⁺, the room-temperature lattice thermal conductivity decreased to 0.55 Wm^-1K^-1 when x = 0.5. Combined with a maintained weighted mobility (ca. 60 cm²V^-1s^-2), the room-temperature ZT increased to 0.28. This value could further increase to 0.65 with a reduction in lattice thermal conductivity to its lower-limit value. A high ZT of 1.0 is also predicted for pristine PbBi₂Te₄ at 473 K, demonstrating its potential as a near-room-temperature thermoelectric system.