Relating Local Structure to Thermoelectric Properties in Pb1-x Ge x Bi2Te4

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 R3m 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 R3m 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 (Pb1-x Ge x Bi2Te4, 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.