Lattice Mismatch Induced Lattice Thermal Conductivity Reduction and Negative Thermal Expansion of GeTe-Based Thermoelectric Bulk Alloys.

Lattice mismatch is an intriguing concept that gives rise to various fascinating physical phenomena in emerging research fields such as twistronics and heterostructure devices, which are based on thin films. Although lattice mismatch generates defects and dislocations that significantly affect lattice thermal conductivity, studies on the effects of lattice mismatch in bulk materials remain insufficient. This study investigates the effects of lattice mismatch on the thermoelectric properties of GeTe-based bulk alloys, known for their excellent thermoelectric performance. XRD results of the GeTe-based alloys show the enhanced internal lattice strain (ε), increased dislocation density (δ), and negative thermal expansion, accompanied by phase separations of PbTe or PbSe. The higher internal lattice strain (ε), greater dislocation density (δ), and lower characteristic temperature for negative thermal expansion in the Ge(Ag)Te-PbSe alloy indicate stronger lattice interaction due to an appropriate lattice mismatch compared to the larger lattice mismatch of the Ge(Ag)Te-PbTe alloy. Consequently, the thermoelectric figure of merit (ZT) for Ge(Ag)Te-PbSe is significantly enhanced to 2.0, compared to 1.6 for Ge(Ag)Te-PbTe near 750 K. These results suggest that an appropriate lattice mismatch in bulk alloys can effectively decrease lattice thermal conductivity and generate negative thermal expansion, thereby contributing to thermal stability of thermoelectric modules at high temperatures. The lattice mismatch concept could be a promising approach to reduce lattice thermal conductivity and enhance thermal stability in thermoelectric materials and other high-temperature applications.