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

Nanostructured PbTe-based compounds, including n-type LAST- m (AgPb m SbTe m +2 ) and p-type LASTT- m (Ag(Pb 1− x Sn x ) m SbTe m +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 Ag 0.9 (Pb 0.5 Sn 0.5 ) 10 Sb 0.8 Te 12 (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 (Ag 1− x Sb x ) 1+ y Te 2 ( 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, (Ag 1− x Sb x ) 1+ y Te 2 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/cm 2 and conversion efficiency ∼4.3% at Δ T ∼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.