Revisiting the high-performance p-type PbTe-PbS thermoelectric materials through phase diagram design

Lead chalcogenides are regarded as highly prospective mid-temperature thermoelectric materials due to their excellent performance. Pseudo-binary PbTe-PbS compounds, a representative system, exhibit intrinsically low lattice thermal conductivities originating from phase separation. However, most previous studies have primarily relied on empirical sample fabrication, lacking a systematic thermodynamics-guided design. In this work, isothermal sections of the pseudo-binary PbTe-PbS phase diagram were constructed using the diffusion couple method. The miscibility gap of PbTe_1−xS_x at 873 K was determined to be x = 0.15–0.94. Guided by the experimental phase diagram, a series of 2 at.% Na-doped p-type samples were synthesized. A peak zT of ∼1.9 at 773 K and an average zT exceeding 1.0 were achieved in Pb_0.98Na_0.02Te_0.84S_0.16, which is located in the nucleation-and-growth region on the PbTe-rich side, owing to its ultralow lattice thermal conductivity of ∼0.55 W m⁻¹ K⁻¹ at 773 K. Microstructural analysis revealed that dislocations at the PbTe-rich and PbS-rich interfaces serve as effective phonon scattering centers, while semi-coherent phase boundaries preserve high carrier mobilities, enhancing thermoelectric performance in this equilibrium phase region.