Role of Grain Boundary Features in the Thermal Stability of LASTT-10 Thermoelectric Materials

Nanostructured PbTe-based compounds, including n-type LAST-m (AgPbmSbTem+2) and p-type LASTT-m (Ag(Pb1-xSnx)mSbTem+2), exhibit high thermoelectric performance, but their long-term thermal stability remains a major limitation for high-temperature applications. In this work, we provide detailed insights into the origin of thermal instabilities in p-type LASTT compounds by studying the Ag0.9(Pb0.5Sn0.5)10Sb0.8Te12 (LASTT-10) composition synthesized via melting, annealing, and spark plasma sintering. The prepared bulk samples exhibit a high zT of ∼1.05 ± 0.10 at 673 K. Systematic thermal cycling and isothermal annealing experiments reveal that the observed irreversible changes in transport properties stem from microstructural evolution of Te-rich (Ag1-xSbx)1+yTe2 (x = 0.12, y = 0.18) secondary phases located at the grain boundaries. Differential thermal analysis of LASTT-10 identifies an endothermic reaction ∼630 K associated with the thermodynamic instability of the grain boundary phases. Beyond 610 K, (Ag1-xSbx)1+yTe2 undergoes multiple phase transformations, which lead to macroscopic thermal instability. Cyclic power generation tests indicate a stable performance for LASTT-10 below 610 K, with maximum power density ∼0.51 W/cm2 and conversion efficiency ∼4.3% at ΔT ∼316 K. These findings clarify the mechanisms underlying the thermal degradation in LASTT-10 and highlight critical parameters for improving the thermal robustness of PbTe-based thermoelectric materials for high-temperature power generation.